Review of interactions between phosphorus and arsenic in soils from four case studies

Redox conditions and biochar pyrolysis temperature affecting As and Pb biogeochemical cycles and bacterial community of sediment from mining tailings

Despite the widespread use of biochar for soil and sediment remediation, little is known about the impact of pyrolysis temperature on the biogeochemistry of arsenic (As) and lead (Pb) and microorganisms in sediment under reducing conditions. In this study, we investigated the effects of pyrolysis temperature and the addition of glucose on the release and transformation of As and Pb, as well as their potential effects on the bacterial community in contaminated sediments. The addition of biochar altered the geochemical cycle of As, as it favors specific bacterial groups capable of changing species from As(V) to As(III) through fermentation, sulfate respiration and nitrate reduction. The carbon quality and content of N and S in solution shaped the pH and redox potential in a way that changed the microbial community, favoring Firmicutes and reducing Proteobacteria. This change played a fundamental role in the reductive dissolution of As and Pb minerals. The addition of biochar was the only efficient way to remove Pb, possibly as a function of its sorption and precipitation mechanisms. Such insights could contribute to the production or choice of high-efficiency biochar for the remediation of sediments subjected to redox conditions.

Modifying humus-phosphorus-arsenic interactions in sludge composting: The strengthening of phosphorus availability and arsenic efflux detoxification mechanisms

Arsenic (As) in sewage sludge poses a significant threat to environmental and human health, which has attracted widespread attention. This study investigated the value of adding sodium percarbonate (SP) on phosphorus (P) availability and As efflux detoxification through HS-P-As interactions. Due to the unique structure of humus (HS) and the similar chemical properties of P and As, the conditions for HS-P-As interaction are provided. This study discussed the content, morphology and microbial communities of HS, P and As by using metagenomic and correlation analysis. The results showed that the humification index in the experiment group (SPC) was 2.34 times higher than that in the control group (CK). The available phosphorus (AP) content of SPC increased from 71.09 mg/kg to 126.14 mg/kg, and SPC was 1.11 times that of CK. The relative abundance of ACR3/ArsB increased. Pst, Actinomyces and Bacillus commonly participated in P and As conversion. The correlation analysis revealed that the humification process was enhanced, the AP was strengthened, and the As was efflux detoxified after SP amendment. All in all, this study elucidated the key mechanism of HS-P-As interaction and put forward a new strategy for sewage sludge resource utilization and detoxification.

An evaluation of arsenic contamination status and its potential health risk assessment in villages of Nadia and North 24 Parganas, West Bengal, India

The present study was conducted to evaluate the arsenic (As) contamination and possible associated health hazards to exposed population in four villages of two districts (Nadia and North 24 Parganas) of West Bengal, India. The study included two villages each from Nadia (Jaguli and Kugacchi) and North 24 Parganas (Chamta and Byaspur) districts. Groundwater, surface water, soil, rice grains and rice-based food samples were collected from these villages. The results revealed the presence of As in high concentrations in groundwater (35.00 to 186.00 µg L-1), surface water (30.00 to 61.00 µg L-1), soil (46.17 to 66.00 mg kg-1), rice grains (0.017 to 1.27 µg g-1) and rice-based food products (0.012 to 0.40 µg g-1). The maximum As levels were recorded in all types of samples collected from Kugacchi village. The rice grain samples included high-yielding and local varieties, and the level of As in high-yielding varieties was found to be higher (0.72 to 1.27 µg g-1) than in local varieties (0.25 to 1.06 µg g-1). The data of As concentrations was used for understanding the hazard quotient (HQ) and incremental lifetime cancer risk (ILCR) to the As-exposed population, and significant non-carcinogenic and carcinogenic risks were revealed considering consumption of rice grains at 400 g per day. The study demonstrates the severity of As contamination in the surveyed villages, which may pose a hindrance to attainment of sustainable development goals (SDGs) by 2030 and proposes the implementation of requisite safety measures.

Nutrient and mycoremediation of a global menace 'arsenic': exploring the prospects of phosphorus and Serendipita indica-based mitigation strategies in rice and other crops

KEY MESSAGE

Serendipita indica induced metabolic reprogramming in colonized plants complements phosphorus-management in improving their tolerance to arsenic stress on multifaceted biological fronts. Restoration of the anthropic damage done to our environment is inextricably linked to devising strategies that are not only economically sound but are self-renewing and ecologically conscious. The dilemma of heavy metal (HM) dietary ingestion, especially arsenic (As), faced by humans and animals alike, necessitates the exploitation of such technologies and the cultivation of healthy and abundant crops. The remarkable symbiotic alliance between plants and 'mycorrhizas' has evolved across eons, benefiting growth/yield aspects as well as imparting abiotic/biotic stress tolerance. The intricate interdependence of Serendipita indica (S. indica) and rice plant reportedly reduce As accumulation, accentuating the interest of microbiologists, agriculturists, and ecotoxicological scientists apropos of the remediation mechanisms of As in the soil-AMF-rice system. Nutrient management, particularly of phosphorus (P), is also praised for mitigating As phytotoxicity by deterring the uptake of As molecules due to the rhizospheric cationic competition. Taking into consideration the reasonable prospects of success in minimizing As acquisition by rice plants, this review focuses on the physiological, metabolic, and transcriptional alterations underlying S. indica symbiosis, recuperation of As stress together with nutritional management of P by gathering case studies and presenting successful paradigms. Weaving together a volume of literature, we assess the chemical forms of As and related transport pathways, discuss As-P-rice interaction and the significance of fungi in As toxicity mitigation, predominantly the role of mycorrhiza, as well as survey of the multifaceted impacts of S. indica on plants. A potential strategy for simultaneous S. indica + P administration in paddy fields is proposed, followed by future research orientation to expand theoretic comprehension and encourage field-based implementation.

Assessment of co-contaminated soil amended by graphene oxide: Effects on pollutants, microbial communities and soil health

Graphene oxide (GOx) is a nanomaterial with demonstrated capacity to remove metals from water. However, its effects on organic pollutants and metal(loid)s present in polluted soils when used for remediation purposes have not been extensively addressed. Likewise, few studies describe the effects of GOx on edaphic properties and soil biology. In this context, here we assessed the potential of GOx for remediating polluted soil focusing also on different unexplored effects of GOx in soil. To achieve this, we treated soil contaminated with concurrent inorganic (As and metals) and organic pollution (TPH and PAHs), using GOx alone and in combination with nutrients (N and P sources). In both cases increased availability of As and Zn was observed after 90 days, whereas Cu and Hg availability was reduced and the availability of Pb and the concentration of organic pollutants were not significantly affected. The application of GOx on the soil induced a significant and rapid change (within 1 week) in microbial populations, leading to a transient reduction in biodiversity, consistent with the alteration of several soil properties. Concurrently, the combination with nutrients exhibited a distinct behaviour, manifesting a more pronounced and persistent shift in microbial populations without a decrease in biodiversity. On the basis of these findings, GOx emerges as a versatile amendment for soil remediation approaches.

Remediation options to reduce bioaccessible and bioavailable lead and arsenic at a smelter impacted site - consideration of treatment efficacy

In this study, smelter contaminated soil was treated with various soil amendments (ferric sulfate [Fe2(SO4)3], triple superphosphate [TSP] and biochar) to determine their efficacy in immobilizing soil lead (Pb) and arsenic (As). In soils incubated with ferric sulfate (0.6M), gastric phase Pb bioaccessibility was reduced from 1939 ± 17 mg kg-1 to 245 ± 4.7 mg kg-1, while intestinal phase bioaccessibility was reduced from 194 ± 25 mg kg-1 to 11.9 ± 3.5 mg kg-1, driven by the formation of plumbojarosite. In TSP treated soils, there were minor reductions in gastric phase Pb bioaccessibility (to 1631 ± 14 mg kg-1) at the highest TSP concentration (6000 mg kg-1) although greater reductions were observed in the intestinal phase, with bioaccessibility reduced to 9.3 ± 2.2 mg kg-1. Speciation analysis showed that this was primarily driven by the formation of chloropyromorphite in the intestinal phase following Pb and phosphate solubilization in the low pH gastric fluid. At the highest concentration (10% w/w), biochar treated soils showed negligible decreases in Pb bioaccessibility in both gastric and intestinal phases. Validation of bioaccessibility outcomes using an in vivo mouse assay led to similar results, with treatment effect ratios (TER) of 0.20 ± 0.01, 0.76 ± 0.11 and 1.03 ± 0.10 for ferric sulfate (0.6M), TSP (6000 mg kg-1) and biochar (10% w/w) treatments. Results of in vitro and in vivo assays showed that only ferric sulfate treatments were able to significantly reduce As bioaccessibility and bioavailability with TER at the highest application of 0.06 ± 0.00 and 0.14 ± 0.04 respectively. This study highlights the potential application of ferric sulfate treatment for the immobilization of Pb and As in co-contaminated soils.

Evidence for the accumulation of toxic metal(loid)s in agricultural soils impacted from long-term application of phosphate fertilizer

Phosphate fertilizers may contain elevated concentrations of toxic metals and metalloids and therefore, their excessive application can result in the accumulation of both phosphorus (P) and metal(loid)s in agricultural soils. This study aims to investigate the occurrence, distribution, and potential plant-availability of metal(loid)s originating from phosphate fertilizer in a long-term experimental field at the Tidewater Research Station in North Carolina, where topsoil (10-20 cm deep) and subsoil (up to 150 cm deep) samples were collected from five plots with consistent and individually different application rates of P-fertilizer since 1966. We conducted systematic analyses of P and metal(loid)s in bulk soils, in the plant available fraction, and in four sequentially extracted soil fractions (exchangeable, reducible, oxidizable, and residual). The results show that P content in topsoils were directly associated with the rate of P-fertilizer application (ρ = 1, p < 0.05). Furthermore, P concentrations were highly correlated with concentrations of Cd, U, Cr, V, and As in the bulk topsoil (ρ > 0.58, p < 0.05), as well as the potential plant-available fraction (ρ > 0.67, p < 0.01), indicating the accumulation of the fertilizer-derived toxic metal(loid)s in the topsoil. Significant correlations (p < 0.001) of metal(loid)s concentrations between the bulk soil and the potential plant-available fraction raises the possibility that P-fertilizer application could increase the accumulation of toxic metal(loid)s in plants, which could increase human exposure. Results from sequential leaching experiments revealed that large portions of the trace elements, in particular Cd, occur in the soluble (exchangeable and reducing) fractions of topsoil with higher P-fertilizer input, whereas the levels of redox-sensitive elements (As, V, U, Cr) were higher in the reducible and oxidizable fractions of the soils. Overall, the data presented in this study demonstrate the effect of long-term P-fertilizer application on the occurrence and accumulation of a wide range of toxic metal(loid)s in agricultural topsoil.

Disentangling a metabolic cross-feeding in a halophilic archaea-bacteria consortium

Microbial syntrophy, a cooperative metabolic interaction among prokaryotes, serves a critical role in shaping communities, due to the auxotrophic nature of many microorganisms. Syntrophy played a key role in the evolution of life, including the hypothesized origin of eukaryotes. In a recent exploration of the microbial mats within the exceptional and uniquely extreme Cuatro Cienegas Basin (CCB), a halophilic isolate, designated as AD140, emerged as a standout due to its distinct growth pattern. Subsequent genome sequencing revealed AD140 to be a co-culture of a halophilic archaeon from the Halorubrum genus and a marine halophilic bacterium, Marinococcus luteus, both occupying the same ecological niche. This intriguing coexistence hints at an early-stage symbiotic relationship that thrives on adaptability. By delving into their metabolic interdependence through genomic analysis, this study aims to uncover shared characteristics that enhance their symbiotic association, offering insights into the evolution of halophilic microorganisms and their remarkable adaptations to high-salinity environments.

Validation of the efficiency of arsenic mitigation strategies in southwestern region of Bangladesh and development of a cost-effective adsorbent to mitigate arsenic levels

World's highest arsenic (As) contamination is well-documented for the groundwater system of southwestern region (mainly Jashore district) of Bangladesh, where the majority of inhabitants are underprivileged. To mitigate As poisoning in southwestern Bangladesh, numerous steps have been taken so far by the government and non-governmental organizations (NGOs). Among them, digging deep tube wells and As removal by naturally deposited Fe(OH)3 species are being widely practiced in the contaminated areas. However, these actions have been left unmonitored for decades, making people unaware of this naturally occurring deadly poison in their drinking water. Hence, water samples (n = 63, both treated and untreated) and soil samples (n = 4) were collected from different spots in Jashore district to assess the safety level of drinking water and to understand the probable reasons for high As(III) contamination. About 93.7% of samples were found to contain As(III) above 10 μg/L; among them, 38% contained above 50 μg/L. The study shows that current As(III) removal strategies in the study area are ineffective. In this connection, a simple low-cost As(III) removal adsorbent is proposed that can be prepared with very cheap and locally available materials like iron sludge and charcoal. The adsorbent was characterized in terms of SEM, EDX, and XPS. The optimal dosage of the adsorbent was investigated for real-life application concerning several vital water quality parameters. The Fe-C adsorbent exhibited a maximum As(III) removal efficiency of 92% in real groundwater samples. The study will allow policymakers for informed decision-making regarding water body management as well as enable the local people to avail As-safe water in a way that aligns with their economic factors.

Risk assessment of toxic and hazardous metals in paddy agroecosystem by biochar-for bio-membrane applications

Toxic and carcinogenic metal (loid)s, such arsenic (As) and cadmium (Cd), found in contaminated paddy soils pose a serious danger to environmental sustainability. Their geochemical activities are complex, making it difficult to manage their contamination. Rice grown in Cd and As-polluted soils ends up in people's bellies, where it can cause cancer, anemia, and the deadly itai sickness. Solving this issue calls for research into eco-friendly and cost-effective remediation technology to lower rice's As and Cd levels. This research delves deeply into the origins of As and Cd in paddy soils, as well as their mobility, bioavailability, and uptake mechanisms by rice plants. It also examines the current methods and reactors used to lower As and Cd contamination in rice. Iron-modified biochar (Fe-BC) is a promising technology for reducing As and Cd toxicity in rice, improving soil health, and boosting rice's nutritional value. Biochar's physiochemical characteristics are enhanced by the addition of iron, making it a potent adsorbent for As and Cd ions. In conclusion, Fe-BC's biomembrane properties make them an attractive option for remediating As- and Cd-contaminated paddy soils. More efficient mitigation measures, including the use of biomembrane technology, can be developed when sustainable agriculture practices are combined with these technologies.

Improved efficiency of Sedum lineare (Crassulaceae) in remediation of arsenic-contaminated soil by phosphate-dissolving strain P-1 in association with phosphate rock

The selection of appropriate plants and growth strategies is a key factor in improving the efficiency and universal applicability of phytoremediation. Sedum lineare grows rapidly and tolerates multiple adversities. The effects of inoculation of Acinetobacter sp. phosphate solubilizing bacteria P-1 and application of phosphate rock (PR) as additives on the remediation efficiency of As-contaminated soil by S. lineare were investigated. Compared with the control, both the single treatment and the combination of inoculation with strain P-1 and application of PR improved the biomass by 30.7-395.5%, chlorophyll content by 48.1-134.8%, total protein content by 12.5-92.4% and total As accumulation by 45.1-177.5%, and reduced the As-induced oxidative damage. Inoculation with strain P-1 increased the activities of superoxide dismutases and catalases of S. lineare under As stress, decreased the accumulation of reactive oxygen species in plant tissues and promoted the accumulation of As in roots. In contrast, simultaneous application of PR decreased As concentration in S. lineare tissues, attenuated As-induced lipid peroxidation and improved As transport to shoots. In addition, the combined application showed the best performance in improving resistance and biomass, which significantly increased root length by 149.1%, shoot length by 33%, fresh weight by 395.5% and total arsenic accumulation by 159.2%, but decreased the malondialdehyde content by 89.1%. Our results indicate that the combined application of strain P-1 and PR with S. lineare is a promising bioremediation strategy to accelerate phytoremediation of As-contaminated soils.

Arsenic fractionation and speciation in different textured soils supplied with farmyard manure and accumulation by sunflower under alkaline calcareous conditions

Arsenic (As) is a naturally occurring element that is found in soil, water, and rocks. However, it can also be released into the environment through human activities. Arsenic is considered an environmental hazard because it is toxic to humans and animals and can cause serious health problems. Additionally, As-contaminated soil can limit plant growth and reduce crop yields, leading to economic losses for farmers. So, decreasing metal/metalloid solubility in soil by synthetic and organic amendments leads to better crop productivity on contaminated soils. The current study aimed to evaluate farmyard manure (FYM)-mediated changes in soil arsenic (As) behavior, and subsequent effects on achene yield of sunflower. Treatment plan comprised of two As levels, i.e., As-60 (60 mg kg-1) and As-120 (120 mg kg-1), four FYM levels (0, 20, 35, and 50 g kg-1), three textural types (sandy, loamy and clayey), and replicated thrice. Seven As fractions including water soluble-As (WS-As), labile-As (L-As), calcium-bound As (Ca-As), aluminum-bound As (Al-As), iron-bound As (Fe-As), organic-matter-bound As (OM-As), and residual-As (R-As) were determined which differed significantly (P ≤ 0.05) with FYM and soil texture. FYM supplementation decreased WS-As, L-As, Ca-As, and Al-As while increased Fe-As, OM-As, and R-As. The immobilizing effect of FYM increased with increasing its rate of application, and maximum effect was found in clayey soil. As speciation in soil also significantly (P ≤ 0.05) affected by FYM and soil texture, with a reduction in arsenate while increase in arsenite, mono-methyl arsenate, and di-methyl arsenate with increasing the rate of FYM supplementation. Bioaccumulation factor reduced with FYM addition, and highest reduction of 38.65 and 42.13% in sandy, 34.24 and 36.26% in loamy while 29.16 and 35.10% in clayey soils at As-60 and As-120, respectively, by 50 g kg-1 FYM compared with respective As treatments without FYM. As accumulation in plant parts was significantly (P ≤ 0.05) reduced by FYM with the subsequent improvement in achene yield.

Unraveling the multifaceted resilience of arsenic resistant bacterium Deinococcus indicus

Arsenic (As) is a toxic heavy metal widely found in the environment that severely undermines the integrity of water resources. Bioremediation of toxic compounds is an appellative sustainable technology with a balanced cost-effective setup. To pave the way for the potential use of Deinococcus indicus, an arsenic resistant bacterium, as a platform for arsenic bioremediation, an extensive characterization of its resistance to cellular insults is paramount. A comparative analysis of D. indicus cells grown in two rich nutrient media conditions (M53 and TGY) revealed distinct resistance patterns when cells are subjected to stress via UV-C and methyl viologen (MV). Cells grown in M53 demonstrated higher resistance to both UV-C and MV. Moreover, cells grow to higher density upon exposure to 25 mM As(V) in M53 in comparison with TGY. This analysis is pivotal for the culture of microbial species in batch culture bioreactors for bioremediation purposes. We also demonstrate for the first time the presence of polyphosphate granules in D. indicus which are also found in a few Deinococcus species. To extend our analysis, we also characterized DiArsC2 (arsenate reductase) involved in arsenic detoxification and structurally determined different states, revealing the structural evidence for a catalytic cysteine triple redox system. These results contribute for our understanding into the D. indicus resistance mechanism against stress conditions.

Wheat PHT1;9 acts as one candidate arsenate absorption transporter for phytoremediation

Arsenate (AsV) is one of the most common forms of arsenic (As) in environment and plant high-affinity phosphate transporters (PHT1s) are the primary plant AsV transporters. However, few PHT1s involved in AsV absorption have been identified in crops. In our previous study, TaPHT1;3, TaPHT1;6 and TaPHT1;9 were identified to function in phosphate absorption. Here, their AsV absorption capacities were evaluated using several experiments. Ectopic expression in yeast mutants indicated that TaPHT1;9 had the highest AsV absorption rates, followed by TaPHT1;6, while not for TaPHT1;3. Under AsV stress, further, BSMV-VIGS-mediated TaPHT1;9-silencing wheat plants exhibited higher AsV tolerance and lower As concentrations than TaPHT1;6-silenced plants, whereas TaPHT1;3-silencing plants had similar phenotype and AsV concentrations to control. These suggested that TaPHT1;9 and TaPHT1;6 possessed AsV absorption capacity with the former showing higher activities. Under hydroponic condition, furthermore, CRISPR-edited TaPHT1;9 wheat mutants showed the enhanced tolerance to AsV with decreased As distributions and concentrations, whereas TaPHT1;9 ectopic expression transgenic rice plants had the opposite results. Also, under AsV-contaminated soil condition, TaPHT1;9 transgenic rice plants exhibited depressed AsV tolerance with increased As concentrations in roots, straws and grains. Moreover, Pi addition alleviated the AsV toxicity. These suggested that TaPHT1;9 should be a candidate target gene for AsV phytoremediation.

Negative Impacts of Arsenic on Plants and Mitigation Strategies

Arsenic (As) is a metalloid prevalent mainly in soil and water. The presence of As above permissible levels becomes toxic and detrimental to living organisms, therefore, making it a significant global concern. Humans can absorb As through drinking polluted water and consuming As-contaminated food material grown in soil having As problems. Since human beings are mobile organisms, they can use clean uncontaminated water and food found through various channels or switch from an As-contaminated area to a clean area; but plants are sessile and obtain As along with essential minerals and water through roots that make them more susceptible to arsenic poisoning and consequent stress. Arsenic and phosphorus have many similarities in terms of their physical and chemical characteristics, and they commonly compete to cause physiological anomalies in biological systems that contribute to further stress. Initial indicators of arsenic's propensity to induce toxicity in plants are a decrease in yield and a loss in plant biomass. This is accompanied by considerable physiological alterations; including instant oxidative surge; followed by essential biomolecule oxidation. These variables ultimately result in cell permeability and an electrolyte imbalance. In addition, arsenic disturbs the nucleic acids, the transcription process, and the essential enzymes engaged with the plant system's primary metabolic pathways. To lessen As absorption by plants, a variety of mitigation strategies have been proposed which include agronomic practices, plant breeding, genetic manipulation, computer-aided modeling, biochemical techniques, and the altering of human approaches regarding consumption and pollution, and in these ways, increased awareness may be generated. These mitigation strategies will further help in ensuring good health, food security, and environmental sustainability. This article summarises the nature of the impact of arsenic on plants, the physio-biochemical mechanisms evolved to cope with As stress, and the mitigation measures that can be employed to eliminate the negative effects of As.

Mitigation of arsenic toxicity in rice by the co-inoculation of arsenate reducer yeast with multifunctional arsenite oxidizing bacteria

The study aimed to explicate the role of microbial co-inoculants for the mitigation of arsenic (As) toxicity in rice. Arsenate (AsV) reducer yeast Debaryomyces hansenii NBRI-Sh2.11 (Sh2.11) with bacterial strains of different biotransformation potential was attempted to develop microbial co-inoculants. An experiment to test their efficacy (yeast and bacterial strains) on plant growth and As uptake was conducted under a stressed condition of 20 mg kg-1 of arsenite (AsIII). A combination of Sh2.11 with an As(III)-oxidizer, Citrobacter sp. NBRI-B5.12 (B5.12), resulted in ∼90% decrease in grain As content as compared to Sh2.11 alone (∼40%). Reduced As accumulation in rice roots under co-treated condition was validated with SEM-EDS analysis. Enhanced As expulsion in the selected combination under in vitro conditions was found to be correlated with higher As content in the soil during their interaction with plants. Selected co-inoculant mediated enhanced nutrient uptake in association with better production of indole acetic acid (IAA) and gibberellic acid (GA) in shoot, support microbial co-inoculant mediated better biomass under stressful condition. Boosted defense response in association with enhanced glutathione-S-transferase (GST) and glutathione reductase (GR), activities under in vitro and in vivo conditions were observed. These results indicated that the As(III) oxidizer-B5.12 accelerated the As detoxification property of the As(V) reducer-Sh2.11. Henceforth, the results confer that the coupled reduction-oxidation process of the co-inoculant reduces the accumulation of As in rice grain. These co-inoculants can be further developed for field trials to achieve higher biomass with alleviated As toxicity in rice.

Arsenic-induced toxicity and the ameliorative role of antioxidants and natural compounds

Arsenic (As) poisoning has proven to be a major threat worldwide because of its toxic effects on the human body. As toxicity through drinking water is a global health concern. The toxicity of As is known to affect the liver, kidney, lungs, muscles, cardiovascular system, and nervous system and can even induce diabetes. Further As can cause skin lesions leading to notable diseases in the skin like Bowen's disease. Chronic exposure to As has caused many tragedies in Eastern, and several Southeast Asian and Latin American countries. Long-term exposure to As makes it an immediate threat that should be dealt with as a priority, and one of the ways to handle it may be with the use of antioxidants. In this review, we have discussed the natural and anthropogenic sources of As, its metabolism, pathophysiology, and mechanism of toxicity. Besides, we have also discussed some of the synthetic chelators and the ameliorative role of antioxidants and natural compounds in reducing As toxicity.

Transferring network analysis to the study of potential biogeochemical interactions of phosphorus-relevant elements in floodplain subsoils - A new use case for the Soilscape Network Approach (SNAp)

Subsurface phosphorus (P) loss from deep P stocks in floodplain subsoils can contribute to eutrophication of freshwaters. To date, knowledge on the complex biogeochemical interactions of P in floodplain subsoils is too scarce to enable targeted P management to mitigate subsurface P loss from deep P stocks. We propose using graph theory and the Soilscape Network Approach (SNAp) based on correlations between P-relevant elements to study these complex biogeochemical interactions in the soilscape. Complex interactions of several elements in soils are difficult to investigate from a holistic perspective with conventional data analysis. We translated soil element data from topsoils and subsoils of terrestrial sites, proximal and distal floodplain sites into relational data and analyzed network structure, centrality, and modularity. The results indicate that a higher frequency of groundwater level fluctuations in distal subsoils and proximal topsoils could result in 24-44 % less biogeochemical interaction compared to sites with stable conditions. Impeded microbial processes on the frequently disturbed sites may explain this finding. Our analyses suggest biogeochemical differences between floodplain topsoils and subsoils expressed in 24 % lower and 75 % higher network connectivity in distal and proximal subsoils (respectively). We also found 22 % lower network connectivity in distal than proximal floodplain subsoils, suggesting biogeochemical differences between both soil sections. These findings imply that floodplain P management should not take a whole-floodplain approach but a 3D-approach, which differentiates laterally between floodplain zones and vertically between soil sections. In addition, SNAp indicated that Fe(II) oxides are important in P biogeochemistry of floodplain subsoils but are not the key element. Instead, labile P forms are suggested to have different major associations in distal (Al_ox, Fe_ox) versus proximal deep P stocks (Al_ox, Mn, Ca). Our study provides new insights into the biogeochemistry of deep P stocks in floodplain subsoils which require targeted validation by other methods.

Trigonella foenum-graecum (fenugreek) differentially regulates antioxidant potential, photosynthetic, and metabolic activities under arsenic stress

Arsenic (As) contamination is continuously increasing in the groundwaters and soils around the world causing toxicity in the plants with a detrimental effect on physiology, growth, and yield. In a hydroponic system, thirty-day-old plants of Trigonella foenum-graecum were subjected to 0, 50, or 100 µM NaHAsO₄0.7 H₂O for 10 days. The magnitude of oxidative stress increased, whereas growth indices and photosynthetic parameters decreased in a dose-dependent manner. The efficiency of photosystem II in terms of Hill reaction activity (HRA) or chlorophyll-a was adversely affected by As stress. The antioxidant potential of plants regarding ferric reducing antioxidant power (FRAP) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) assays was enhanced, indicating the augmented resistance mechanism in plants to counter As stress. The metabolite analysis of leaf extracts revealed many As responsive metabolites including amino acids, organic acids, sugars/polyols, and others. Phenylalanine and citrulline were highly accumulated at 50 or 100 µM As, salicylic acid accumulated more at 50 µM of As while ascorbic acid notably increased at 100 µM of As. At 50 or 100 µM As, the glucose and fructose contents increased while the sucrose content decreased. At both As doses, tagatose and glucitol contents were 13 times higher than controls. Varied accumulation of metabolites could be associated with the different As doses that represent the range of tolerance in T. foenum-graecum towards As toxicity. Pathway analysis of metabolites revealed that amino acid and carbohydrate metabolism and the citrate cycle play important roles under As stress. This study helps in a better metabolomic understanding of the dose-dependent toxicity and response of As in T. foenum-graecum.

Occurrence and behavior of arsenic in groundwater-aquifer system of irrigated areas

Groundwater arsenic pollution has received much attention worldwide for decades as a serious threat to public health, but the mechanisms responsible for arsenic mobilization are not fully understood. Groundwater and bore drilling sediment samples from Qiji county, a small geographical agricultural area with endemic arsenicosis, are collected for demonstrating the occurrence and speciation of arsenic in groundwater and sediments, and arsenic release between solid-liquid phase influenced by human activities. Results show that arsenic concentrations in groundwater vary from 5 μg/L to 19.6 μg/L, with 80% exceeding the maximum permissible limits required by WHO (10 μg/L) for drinking water and therefore constituting a health risk for humans. In a weak oxidizing environment (oxidation-reduction potential (ORP): 12.9 mV-151 mV), inorganic As(V) accounts for 85% of total dissolved As, which to some extent alleviates the harm of As pollution on humans. Total As content in the sediments is in the range of 6.98 mg/kg and 14.34 mg/kg (median of 10.71 mg/kg), three times higher than the average value of many countries. Sequential chemical leaching indicates that 11% of arsenic in sediments is labile bound and may be closely related to the arsenic in groundwater. Additionally, irrigation intensity contributes to arsenic release with diverse As³⁺/As⁵⁺ by dissolving weakly bound arsenic rapidly. Subsequently part of As(III) is oxidized to As(V). Competitive and/or alkaline desorption of As(V), which had been adsorbed by FeMn (hydrous)-oxides and carbonates in the unsaturated zone and the aquifer, exerts a significant role in releasing arsenic into the groundwater. Our study indicates that systematic management and regulation of irrigation intensity are required to prevent further deterioration of groundwater resources.

Effects of Phosphate, Red Mud, and Biochar on As, Cd, and Cu Immobilization and Enzymatic Activity in a Co-Contaminated Soil

Arsenic (As), cadmium (Cd), and copper (Cu) are the primary inorganic pollutants commonly found in contaminated soils. The simultaneous stabilization of the three elements is a preferred approach for mixture-contaminated soils which has received extensive research attention. However, few studies have focused on the immobilization efficiency of a single amendment on the three elements. In this study, phosphate, red mud, and biochar were used to remediate As (237.8 mg kg−1), Cd (28.72 mg kg−1), and Cu (366.5 mg kg−1) co-contaminated soil using a 180-day incubation study. The BCR (European Community Bureau of Reference) extraction method, NH4H2PO4–extractable As, and diethylenetriamine penta-acetic acid (DTPA)–extractable Cd and Cu were analyzed at different time intervals. The results indicated that the application of red mud and biochar significantly reduced soil DTPA–Cd and Cu concentrations during the incubation, while the decrease in soil NH4H2PO4–As was much less than that of soil DTPA–Cd and Cu. After 180 days of incubation, the concentrations of NH4H2PO4–As in red mud and biochar treatments decreased by 2.15~7.89% and 3.01~9.63%, respectively. Unlike red mud and biochar, phosphate significantly reduced the concentration of soil DTPA–Cd and Cu, but failed to lower that of As. The BCR extraction method confirmed that red mud and biochar addition increased the reducible fraction of As due to the surface complexes of As with Fe oxide. Canonical correspondence analysis (CCA) demonstrated that soil pH in addition to available As, Cd, and Cu concentrations were the primary factors in driving the changes in soil enzymatic activity. Soil pH showed positive correlation with soil urease and catalase activities, while negative correlation was observed between soil-available As, Cd, and Cu, and soil enzyme activities. This study revealed that it is difficult to simultaneously and significantly reduce the bioavailabilities of soil As, Cd, and Cu using one amendment. Further research on modifying these amendments or applying combined amendments will be conducted, in order to develop an efficient method for simultaneously immobilizing As, Cd, and Cu.

The effects of biochar and redox conditions on soil Pb bioaccessibility to people and waterfowl

Biochar can reduce lead (Pb) bioavailability to plants in metal-contaminated soil, but the ability of biochar to reduce the bioavailability of soil Pb to people and wildlife remains unknown. In this study, 17 biochars were evaluated as in situ amendments for three soils with distinct sources of Pb contamination (smelter emissions, ceramics waste, mining waste), hydrology (upland, well-drained soil vs submerged wetland soil), and biological receptors (human vs waterfowl). Biochars were made from blends of 30% manure (poultry litter or dairy manure) and 70% lignocellulosic material (wheat straw or grand fir shavings) and pyrolyzed at 300, 500, 700, and 900 °C. Soils were amended with 2% biochar (w/w) and incubated for 6 months. A suite of standard (e.g., EPA Method 1340) and experimental soil Pb bioaccessibility assays were used to assess the impact of the treatments. The results showed that biochar amendments to upland soils resulted in modest reductions in gastrointestinal Pb bioaccessibility (maximum reduction from 78 to 68% bioaccessibility as a percent of total, EPA Method 1340 at pH 2.5). In the wetland soil, sample redox status had a greater impact on Pb bioaccessibility than any amendment. Low-solubility Pb sulfides in this soil oxidized over the course of the study and no treatment was able to offset the increase in Pb bioaccessibility caused by this oxidation. The impact of redox status on Pb bioaccessibility was only evident when soil bioaccessibility assays were adapted to preserve sample redox status. This result highlights the importance of maintaining in situ redox conditions when processing/analyzing samples from low-oxygen environments and that soil remediation efforts should consider the role of redox conditions on Pb bioaccessibility.

Arsenic Release from Soil Induced by Microorganisms and Environmental Factors

In rhizospheric soil, arsenic can be activated by both biological and abiotic reactions with plant exudates or phosphates, but little is known about the relative contributions of these two pathways. The effects of microorganisms, low-molecular-weight organic acid salts (LMWOASs), and phosphates on the migration of As in unrestored and nano zero-valent iron (nZVI)-restored soil were studied in batch experiments. The results show that As released by microbial action accounted for 17.73%, 7.04%, 92.40%, 92.55%, and 96.68% of the total As released in unrestored soil with citrate, phytate, malate, lactate, and acetate, respectively. It was only suppressed in unrestored soil with oxalate. In restored soil, As was still released in the presence of oxalate, citrate, and phytate, but the magnitude of As release was inhibited by microorganisms. The application of excess nZVI can completely inhibited As release processes induced by phosphate in the presence of microorganisms. Microbial iron reduction is a possible mechanism of arsenic release induced by microorganisms. Microorganisms and most environmental factors promoted As release in unrestored soil, but the phenomenon was suppressed in restored soil. This study helps to provide an effective strategy for reducing the secondary release of As from soils due to replanting after restoration.

Field observations to establish the impact of fluvial flooding on potentially toxic element (PTE) mobility in floodplain soils

Inundation of river water during flooding deposits contaminated sediments onto floodplain topsoil. Historically, floodplains were considered an important sink for potentially toxic elements (PTEs). With increasing flood frequency and duration, due to climate change and land use change, it is important to understand the impact that further flooding may have on this legacy contamination. In this study a field-based approach was taken, extracting soil pore waters by centrifugation of soils sampled on multiple occasions from multiple locations across a floodplain site, which lies adjacent to the River Loddon in southeast England. Flooding generally decreased pore water PTE concentrations and significantly lower pore water concentrations of Cd, Cu, and Cr were found post-flood compared to pre-flood. The dominant process responsible for this observation was precipitation with sulphides resulting in PTE removal from the pore water post-flood. The changes in pH were found to be associated with the decreased pore water concentration of Cu, which suggests the pH rise may have aided adsorption mechanisms or precipitation with phosphates. The impact of flooding on the release and retention of PTEs in floodplain soils is the net effect of several key processes occurring concurrently. It is important to understand the dominant processes that drive mobility of individual PTEs on specific floodplains so that site-specific predictions can determine the impact of future floods on the environmental fate of legacy contaminants.

Alteration in soil arsenic dynamics and toxicity to sunflower (Helianthus annuus L.) in response to phosphorus in different textured soils

Being analogue to arsenic (As), phosphorus (P) may affect As dynamics in soil and toxicity to plants depending upon many soil and plant factors. Two sets of experiments were conducted to determine the effect of P on As fractionation in soils, its accumulation by plants and subsequent impact on growth, yield and physiological characteristics of sunflower (Helianthus annuus L.). Experimental plan comprised of two As levels (60 and 120 mg As kg^-1 soil), four P (0-5-10-20 g phosphate rock kg^-1 soil) and three textural types (sandy, loamy and clayey) with three replications. Among different As fractions determined, labile, calcium-bound, organic matter-bound and residual As increased while iron-bound and aluminum-bound As decreased with increasing P in all the three textural types. Labile-As percentage increased in the presence of P by 16.9-48.0% at As60 while 36.0-68.1% at As120 in sandy, 19.1-64.0% at As60 while 11.5-52.3% at As120 in loamy, and 21.8-58.2% at As60 while 22.3-70.0% at As120 in clayey soil compared to respective As treatment without P. Arsenic accumulation in plant tissues at both contamination levels declined with P addition as evidenced by lower bioconcentration factor. Phosphorus mitigated the As-induced oxidative stress expressed in term of reduced hydrogen peroxide, malondialdehyde while increased glutathione, and consequently improved the achene yield. Although, P increased As solubility in soil but restricted its translocation to plant, leading to reversal of oxidative damage, and improved sunflower growth and yield in all the three soil textural types, more profound effect at highest P level and in sandy texture.

Phosphorus-arsenic interaction in the 'soil-plant-microbe' system and its influence on arsenic pollution

Elevated arsenic (As) in soil is of public concern due to the carcinogenicity. Phosphorus (P) strongly influences the adsorption, absorption, transport, and transformation of As in the soil and in organisms due to the similarity of the chemical properties of P and As. In soil, P, particularly inorganic P, can release soil-retained As (mostly arsenate) by competing for adsorption sites. In plant and microbial systems, P usually reduces As (mainly arsenate) uptake and affects As biotransformation by competing for As transporters. The intensity and pattern of PAs interaction are highly dependent on the forms of As and P, and strongly influenced by various biotic and abiotic factors. An understanding of the PAs interaction in 'soil-plant-microbe' systems is of great value to prevent soil As from entering the human food chain. Here, we review PAs interactions and the main influential factors in soil, plant, and microbial subsystems and their effects on the As release, absorption, transformation, and transport in the 'soil-plant-microbe' system. We also analyze the application potential of P fertilization as a control for As pollution and suggest the research directions that need to be followed in the future.

Establishing the state-of-the-art on the adsorption of coexisting pnictogens in water: A literature review

The occurrence of pnictogens, namely phosphorus, arsenic, and antimony, can be observed in soils, sediments and mining areas, and their coexistence requires a multifaceted approach to the design of adsorption systems to maximize their simultaneous removal efficiency. Therefore, this work aims to provide an extensive literature review of P, As, and Sb adsorption in multicomponent systems and the statistical treatment of the quantitative results. Binary As-P systems have been the most studied in the literature. The oxidation state did not significantly affect the P influence in As adsorbed amount (p = 0.955), but this influence was correlated with the As:P ratio (p < 0.05). A few works have explored As-Sb and Sb-P systems, demonstrating that effective treatments for As do not always reveal a good removal efficiency of the other pnictogens. The Sb adsorbed amount was significantly less affected in the trivalent than in the pentavalent state in both As-Sb and Sb-P systems (p < 0.05). Most of the interactions were competitive, with a few studies reporting synergistic effects for Sb due to the presence of the other elements. Many topics have been identified as lacking in-depth research: ternary As-Sb-P systems, the effect of concentration ratios, pH, and redox conditions (namely those that lead to trivalent species' prevalence), the surface interactions with materials other than iron oxides, and the influence of other aqueous components. This review provides a first step in gathering the relevant literature and approaching the study of adsorption treatment methodology as a complex subject involving many factors.

Temporal changes in arsenic and lead pools in a contaminated sediment amended with biochar pyrolyzed at different temperatures

Globally, tons of soils and sediments are experiencing degradation due to the presence of high concentrations of potentially toxic elements (PTEs), such as arsenic (As) and lead (Pb), in areas in the vicinity of metal mining activities. The addition of biochar to contaminated sediments is a promising in situ remediation approach, and the effects of pyrolysis temperature and biochar aging are important factors for the immobilization and fate of PTEs. In this study, we evaluated the temporal changes in pools of As and Pb in sediment amended with biochars produced from sugarcane (Saccharum officinarum) pyrolyzed at 350 (BC350), 550 (BC550), and 750 °C (BC750). Biochars were aged by natural process (without additional acid or heat), and changes in As and Pb pools were evaluated every 45 days until completing 180 days of incubation. Changes in the As and Pb pools were extracted with water (bioavailable), magnesium chloride (exchangeable), nitric acid (active geochemical fraction), and exchangeable Mehlich-3 (associated with organic matter). As and Pb available contents have increased over time. BC750 was more effective in reducing the bioavailable and exchangeable As contents, while BC550 and BC350 were more effective in reducing the contents of bioavailable and exchangeable Pb.

Effect of P/As molar ratio in soil porewater on competitive uptake of As and P in As sensitive and tolerant rice genotypes

Phosphorus (P) can affect the bioavailability and mobility of arsenic (As) in paddy soil-plant system, but it is not clear how different forms of phosphorus fertilizers affect P/As molar ratio in soil and how the ratio in turn affects the competitive uptake of P and As in two genotypes. Different P fertilizers, i.e., calcium-magnesium phosphate (CMP), superphosphate (SP) and potassium phosphate monobasic (PPM), were used to investigate the difference in competitive uptake between As sensitive (IIY3301) and As tolerant (SY9519) rice genotypes. Our results indicated that the contents of total As in brown rice of PPM and SP treatments (II-PPM and II-SP) were 15.4% and 26.9% lower than that of CMP treatment (II-CMP) for IIY3301 genotype, but their P contents were 27.0% and 17.8% higher than that of II-CMP treatment. However, the As content in brown rice showed no significant difference between PPM and CMP treatments for SY9519 genotype (S-PPM and S-CMP). The net As accumulation in shoots of II-PPM during the tillering stage was significantly lower than that of II-CMP, but the difference of net As accumulation between S-PPM and S-CMP was not significant. The As translocation factor in II-PPM and II-SP were 16.7% and 22.2% lower than that in II-CMP, but the difference of As translocation factor between S-PPM and S-CMP was not significant. In addition, the contents of total As in porewater showed no significant difference between PPM and CMP. Conversely, the P/As molar ratio in porewater of PPM during tillering stage was 10.9% higher than that of CMP. In summary, PPM led to a higher P/As molar ratio in porewater, which promoted the competitive uptake of As and P by IIY3301 genotype; and the competitive uptake of As and P was more likely to occur in As sensitive rice genotype than in As tolerant rice genotype.

Comparative risk assessment of phosphorus loss from "deep phosphorus stocks" in floodplain subsoils to surface waters

Phosphorus (P) loss from soil may trigger freshwater eutrophication and endanger supply with drinking water regionally. The present paper aims at encouraging discussion and development of sophisticated strategies for risk assessment of P loss from soils of riparian buffer zones (RBZ) as a prerequisite for targeted and effective mitigation of such P losses and their effects on freshwater eutrophication. We use data from a case study on RBZ soils in Germany to compare the performance of different environmental indicators of a risk for P loss from soil. Our data suggest that RBZ soils are temporarily sinks or sources for P. The spatial hotspots of P loss are the topsoils and the deep P stocks (labile P enriched in RBZ subsoils below on average 87.5 cm depth). We discuss four aspects to be considered conceptually and methodologically in the assessment of a risk for P loss from RBZ soils: (1) spatial heterogeneity and spatial bias; (2) temporal heterogeneity and temporal bias; (3) conceptual bias caused by different dynamics of individual P fractions; and (4) adequacy of threshold values. To minimize bias, we propose to assess risk for P loss from RBZ soils using a geospatial, temporally resolved sampling strategy, site-specific or regional threshold values, and a P fractionation approach. For this purpose, we introduce P_dHCl as a risk indicator, which is not susceptible to very short-term dynamics (in contrast to water-soluble P).

Influence of flocculation conditioning on environmental risk of heavy metals in dredged sediment

This study comprehensively analyzes the environmental risk of heavy metals (HMs) in the dewatering process of dredged sediment. First, the toxicity leaching capacity, total content, and chemical speciation of the HMs (As, Cd, Co, Cr, Cu, Hg, Mo, Pb, Sb, Sr, Tl, Zn) in dewatered sediment were determined using toxicity characteristic leaching procedure and modified Community Bureau of Reference sequential extraction procedure. The ecotoxicity and environmental risk of the HMs were then evaluated based on sediment quality guidelines, geo-accumulation index, enrichment factor, potential ecological risk, and risk assessment code. The results showed that flocculants reduced the ecological risk of Hg and Mo in sediment, and promoted the transformation of Mo, Sb, and Tl from the biologically active fraction to the more stable fraction. The transformation percentages of Mo, Sb, and Ti were 45.15%, 50.59% and 76.44%, respectively, after chitosan (CTS) treatment, and 64.55%, 31.75% and 99.90%, respectively, after cationic polyacrylamide (CPAM) treatment. CTS reduced the potential risks of bioavailable As, Cr, Cu, Mo, Sb, and Hg by (at most) 46.28%, 45.92%, 43.01%, 100.00%, 44.45%, and 39.69%, respectively, whereas CPAM decreased the ecotoxicity of bioavailable Cd, Co, and Zn by (at most) 27.49%, 16.10%, and 20.89%, respectively. According to the result of principal component analysis, the main factors affecting the environmental risk of HMs in sediment dewatering were nitrogenous organic compounds (mainly protein substances), fulvic acid substances, and minerals. The most essential factor was nitrogenous organic compounds, which accounted for 89.52% of the total variance. Chemical speciation was apparently more suitable for environmental risk assessment of sediment dewatering than total content. This study provides an important basis for controlling the environmental risk of HMs caused by sediment dewatering.

Contaminant metal concentrations in three species of aquatic macrophytes from the Coeur d'Alene Lake basin, USA

The Coeur d'Alene Lake basin in Northwestern USA has extensive contamination from legacy mining waste, which overlaps with aquatic macrophyte habitat. We examined concentrations of arsenic (As), cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn) in three macrophytes: Elodea canadensis (submerged), Myriophyllum spicatum (submerged), and Sagittaria latifolia (emergent). We collected macrophyte tissues from five contaminated sites and one uncontaminated site. Tissue concentrations were compared to sediment quality guidelines to assess potential toxicity from metal(loid)s to macrophyte-associated biota. We used threshold and probable effect concentrations to screen for potential toxicity. For the submerged species, the highest site means ± SD (analyte mg/kg dry mass) were 96 ± 61 (As), 18 ± 1.7 (Cd), 24 ± 15 (Cu), 610 ± 392 (Pb), and 1425 ± 222 (Zn). For contaminated sites, the probable effect threshold was exceeded in 38% (As), 45% (Cd), 0% (Cu), 74% (Pb), and 67% (Zn) of submerged species concentrations. Metal concentrations in S. latifolia tubers were lower than the submerged species leaves and shoots. Tuber concentrations did not exceed the probable effect threshold for any metal. Spatial differences in concentrations were most distinct for the submerged species. Our work shows significant amounts of metals are accumulating in some macrophytes of the study area and that biota associated with this vegetation may experience toxicity based upon guideline exceedances. Additionally, managers of invasive plants (e.g., M. spicatum) should consider the ramifications of control efforts given the high metal content of some plants (e.g., disposal issue).

Usage of Si, P, Se, and Ca Decrease Arsenic Concentration/Toxicity in Rice, a Review

Rice is one of the most important routes for arsenic to enter the human food chain and threatens more than half of the world’s population. In addition, arsenic-contaminated soils and waters increase the concentration of this element in various tissues of rice plants. Thus, direct or indirect—infecting livestock and poultry—increase diseases such as respiratory diseases, gastrointestinal tract, liver, and cardiovascular diseases, cancer, and ultimately death in the long term. Therefore, finding different ways to reduce the uptake and transfer of arsenic by rice would reduce the contamination of rice plants with this dangerous element and improve animal and human nutrition and ultimately disease and mortality. In this article, we aim to take a small step in improving sustainable life on earth by referring to the various methods that researchers have taken to reduce rice contamination by arsenic in recent years. Adding micronutrients and macronutrients as fertilizer for rice is one way to improve this plant’s growth and health. In this study, by examining two types of macronutrients and two types of micronutrients, their role in reducing arsenic toxicity and absorption was investigated. Therefore, both calcium and phosphorus were selected from the macronutrients, and selenium and silicon were selected from the micronutrients, whose roles in previous studies had been investigated.

A Soilscape Network Approach (SNAp) to investigate subsurface phosphorus translocation along slopes

Subsurface phosphorus (P) translocation along slopes may contribute to P enrichment in the subsoils of riparian buffer zones. Such "deep P stocks" might contribute to P concentrations and eutrophication of freshwaters. Better understanding of subsurface P translocation through the soilscape is required to understand the build-up of deep P stocks and to develop targeted mitigation strategies against it. However, such soilscape P dynamics are difficult to tackle due to logistical limitations of common field sampling strategies. Here, we introduce the Soilscape Network Approach (SNAp) as a solution to this problem: It enables to study soilscape P dynamics from a new analytical perspective but on the basis of common field sampling strategies. For this purpose, we are using the graph visualization platform Gephi with field data from a study on subsurface P translocation in Germany. The application of SNAp corroborated prior results regarding deep P stocks in riparian buffer zones, and it enabled the identification of major P sink and source sites as well as dominant P translocation pathways. Our SNAp analysis suggests that subsurface P translocation from topslopes and middle slopes is relevant for the build-up of deep P stocks in the studied toeslope subsoils, especially with shallow basalt or agricultural fertilizer inputs on the top- and middle slopes. Besides, the data imply that lateral P translocation along the studied slopes is small on short slopes, increases until a maximum is achieved, then decreases again when slopes are too long. The SNAp analysis offers new findings which gave valuable insights for the mitigation of subsurface P translocation along slopes.

Occurrence of arsenite in surface and groundwater associated with a perennial stream located in Western Nebraska, USA

Dissolved arsenic typically results from chemical weathering of arsenic rich sediments and is most often found in oxidized forms in surface water. The mobility of arsenic is controlled by its valence state and also by its association with iron oxides minerals, the forms of which are both influenced by abiotic and biotic processes in aqueous environment. In this study, speciation methods were used to measure and confirm the presence of reduced arsenic species in the surface water of Frenchman creek, a gaining stream that crosses the Colorado-Nebraska border. Selective extraction analysis of aquifer and stream bed sediments shows that the bulk of the arsenic occurs with labile iron-rich oxy(hydroxide) minerals. Total dissolved arsenic in surface and groundwater ranged from ~3-18 µg L^-1, and reduced arsenic species comprise about 41% of the total dissolved arsenic (16.0 µg L^-1) in Frenchman creek. Leachable arsenic in the aquifer sediment samples ranged up to 1553 µg kg^-1, while samples from Frenchman creek bed sediments contained 4218 µg kg^-1. Dynamic surface and groundwater interaction sustains arsenite in iron-rich surface headwaters, and the implied toxicity of reduced arsenic in this hydrogeological setting, which can be important in surface water environments around the globe.

Influences on tidal channel and aquaculture shrimp pond water chemical composition in Southwest Bangladesh

Detailed geochemical studies of both major and minor elements in Bangladesh surface waters are sparse, particularly in shrimp aquaculture pond environments. Therefore, water samples from shrimp aquaculture ponds and tidal channels were collected in high precipitation (July) and low precipitation (May) months from 2018-2019 in Southwest Bangladesh and analyzed for complete water chemistry. Selenium (Se) and arsenic (As) were elevated above WHO guidelines in 50% and ~ 87% of samples, respectively, but do not show any recognizable spatial patterns. Shrimp pond and tidal channel water compositions in the dry season (May) are similar, illustrating their connectivity and minimal endogenous effects within shrimp ponds. Tidal channels are less saline in July than shrimp ponds still irrigated by tidal channels, suggesting that either farmers limit irrigation to continue farming saltwater shrimp, or the irrigation flux is low and leads to a lag in aquaculture-tidal channel compositional homogenization. δ¹⁸O and δ²H isotopic compositions from samples in May of 2019 reveal tidal channel samples are closer to the local meteoric water line (LMWL) than shrimp pond samples, because of less evaporation. However, evaporation in May shrimp ponds has a minimal effect on water composition, likely because of regular drainage/exchange of pond waters. Dissolved organic carbon (DOC) is positively correlated with both δ¹⁸O and δ²H in shrimp ponds, suggesting that as evaporation increases, DOC becomes enriched. Multiple linear regression reveals that As and Se can be moderately predicted (adjusted R² values between 0.4 and 0.7, p < 0.01) in surface waters of our study with only 3-4 independent predictor variables (e.g., Ni, V and DOC for Se prediction; Cu, V, Ni and P for As prediction). Thus, this general approach should be followed in other regions throughout the world when measurements for certain hazardous trace elements such as Se and As may be lacking in several samples from a dataset.

Multicomponent adsorption of pentavalent As, Sb and P onto iron-coated cork granulates

The assessment of multicomponent adsorption of pentavalent metalloids is important since they are often found together in groundwaters and mining runoff. For this purpose, adsorption of As(V), Sb(V) and P(V) onto iron-coated cork granulates was studied in binary and ternary systems. Data from equilibrium and kinetic studies revealed that uptake of these contaminants is a multilayer, heterogeneous process well described by Freundlich, extended Freundlich and Elovich models. Most of the observed interactions are competitive and were related to the chemical structure and aqueous behaviour of each anion. Sb(V) adsorption was found to be most impaired and P(V) uptake the least affected by the presence of other pentavalent anions. The aggravation in the reduction of adsorbed amount from binary to ternary solution was more prominent for As(V) than Sb(V). Sb(V) adsorption outweighed that of the other pnictogens in acidic solutions, but in neutral conditions As(V) or P(V) adsorption may predominate instead. P(V) adsorption was the most sensitive to electrolyte addition, namely Ca salts, which may promote precipitation of calcium phosphates. This work provides useful insights regarding the design of adequate adsorption treatment systems for the simultaneous treatment of pentavalent metalloids.

Developing a biosurfactant to attenuate arsenic contamination in mining tailings

The present study aimed to investigate the ability of a microbial consortium to produce biosurfactant in the presence of two carbon sources and also to evaluate the efficiency of the cell-free supernatant cultures to mobilize As from naturally contaminated soil. Pseudomonas and Stenotrophomonas were the main microorganisms in the microbial consortium. The pH, the incubation time, the temperature, and the glucose and glycerol ratios in the culture medium are the main factors influencing biosurfactant production. The lowest surface tension, 30 mN.m-1, and the higher emulsification index, 58%, were achieved at the optimum production conditions (OPC), i.e., pH 9.5, a 2.5 glucose/glycerol ratio, after three days of incubation at 25 °C. The cell-free extracts containing biosurfactants were more efficient in mobilizing As than distilled water, CaCl2 0.1 mol.L-1; saponin, 0.1%; or sodium dodecyl sulfate, 1% during a sequential soil-flushing procedure. The As mobilization using the supernatants containing biosurfactant was sensitive to pH. The use of OPC cell-free supernatant under alkaline conditions leads to the best-obtained results: 24.6% of As removal (678 mg.kg-1) during sequential extractions. The toxicity reduction of the column eluted solution from the first to the seventh cycle evaluated by the germination index (GI) and morphological structures of Allium cepa in the ecotoxicological assessment confirmed the efficiency of the proposed treatment. Allium cepa seeds and seedlings were sensitive in detecting As in soil and eluted solutions with short time responses to the contaminant. Seeds development increased gradually with arsenic removal. The germination index rose from 0 to 55% after soil decontamination through the proposed soil-flushing procedure.

Phosphorus enrichment in floodplain subsoils as a potential source of freshwater eutrophication

Despite decades of management efforts, freshwater eutrophication has not been effectively mitigated in each affected ecosystem. This might be due to insufficient knowledge of the sources of phosphorus (P) inputs into surface waters. We sampled 2 m-deep soil profiles in four floodplain areas under differently managed grassland in Germany under dry and moist conditions regarding soil moisture and precipitation. Four soil P fractions of decreasing solubility were determined. We found systematic enrichment of easily soluble P forms in the floodplain subsoils (average: from 87.5 cm depth). Water-soluble P in these "deep P stocks" was positively correlated with total phosphorus concentrations in the adjacent surface waters. Our data cautiously suggest increased P mobilization from deep P stocks under moist conditions. Drier conditions coincided with increased P retention, resulting in relatively large amounts of easily soluble P which could readily be desorbed and lost at the next stronger precipitation event. We found no effects of grassland management on deep P stock features and dynamics. Deep P stocks might be considered a new source of diffuse P losses from soils. To effectively mitigate freshwater eutrophication, best management practices need to be developed to minimize P transfer from deep P stocks.

Microbial Oxidation of Arsenite: Regulation, Chemotaxis, Phosphate Metabolism and Energy Generation

Arsenic (As) is a metalloid that occurs widely in the environment. The biological oxidation of arsenite [As(III)] to arsenate [As(V)] is considered a strategy to reduce arsenic toxicity and provide energy. In recent years, research interests in microbial As(III) oxidation have been growing, and related new achievements have been revealed. This review focuses on the highlighting of the novel regulatory mechanisms of bacterial As(III) oxidation, the physiological relevance of different arsenic sensing systems and functional relationship between microbial As(III) oxidation and those of chemotaxis, phosphate uptake, carbon metabolism and energy generation. The implication to environmental bioremediation applications of As(III)-oxidizing strains, the knowledge gaps and perspectives are also discussed.

Hausmannite as potential As(V) filter. Macroscopic and spectroscopic study of As(V) adsorption and desorption by citric acid

Arsenic (As) is a toxic element that leads the list of human health threats and is one of the priority contaminants in soil and water. In order to remove As(V) and/or reduce its mobility, filters and amendments with high affinity for As(V) adsorption are used in drinking water treatment or directly applied to the soil, thereby promoting its immobilization. Hausmannite and hematite were compared by in-situ Attenuated Total Reflection - Fourier Transform Infrared (ATR-FTIR) spectroscopy and batch experiments for evaluating As(V) adsorption and sequential desorption by citrate. The pH and contact time were used as variables. Hausmanite adsorbed more As(V) than hematite. As(V) was adsorbed on the mineral surface of simultaneously inner- and outer-sphere species. Inner-sphere bidentate complex form preferentially at high pH, early adsorption time and low surface loading, while the monodentate species should be responsible to increase total As(V) adsorption at low pH, later adsorption kinetics and higher As(V) surface loading. Citrate was effective in causing As(V) desorption at higher citric acid concentrations and higher pH values. After a long time of incubation, the neogenesis of a manganite by hausmnannite oxidation was observed. Concomitantly, less As(V) was desorbed by citrate desorption, even in the presence of high citric acid concentrations. Hausmannite was an efficient mineral for As(V) removal and immobilization.

Phytoremediation Potential, Photosynthetic and Antioxidant Response to Arsenic-Induced Stress of Dactylis glomerata L. Sown on Fly Ash Deposits

Arsenic (As) from coal fly ash can be released into soil/groundwater, presenting a global threat to the environment and human health. To overcome this environmental problem, phytoremediation represents an urgent need, providing ‘green’ cleanup of contaminated lands. The present study focused on As concentrations in fly ash and plants, evaluation of phytoremediation potential of Dactylis glomerata sown on fly ash deposits together with its photosynthetic activity, and oxidative and antioxidative response to As stress. Field research was carried out on fly ash deposits at the thermal power plant “Nikola Tesla”, Obrenovac (TENT-A, Serbia) and the control site. Fly ash is characterized by alkaline pH reactions, small amounts of organic matter, a large amount of available phosphate, and total and available As concentrations. Results in this study indicate that phosphate application can ameliorate As toxicity, uptake and root-shoot transport. Furthermore, D. glomerata can be considered as good As phytostabilizator, because it retains more As in roots than in leaves. Excess As in leaves decreases photosynthetic efficiency (Fv/Fm) and concentrations of chlorophylls, carotenoids, and anthocyanins, whereas high content of malondialdehyde (MDA) can be a signal for biosynthesis phenolics and ascorbic acid, providing cellular redox homeostasis and recovery of photosystem II (PSII) photochemistry. In the roots, low oxidative stress under high concentrations of As is related to intense antioxidant biosynthesis. Taken together, the results in this study indicate a high adaptive potential of D. glomerata to As stress. These findings may suggest that physiological and metabolic tools can be used as a way forward in the ‘real field’ scenario, phytomanagement of fly ash and ecosystem services providing sustainable phytoremediation of As-contaminated sites around the globe.

Influence of Soil Phosphate on the Accumulation and Toxicity of Arsenic and Antimony in Choy Sum Cultivated in Individually and Co-contaminated Soils

Fertilizers containing phosphate (PO₄^3- ) are commonly used within the agricultural industry and are known to increase the bioavailability and mobility of metalloids like arsenic (As). This may increase plant uptake of As and hence pose a risk to human health. Arsenic and antimony (Sb) often co-occur in contaminated soils; however, little is known about the interactions between As and Sb with PO₄^3- on their bioavailability, accumulation, and toxicity in plants. The present study investigated individual and combined As and Sb-contaminated soils across 2 soil PO₄^3- concentrations using a commonly consumed leafy vegetable, choy sum (Brassica chinensis var. parachinensis). Increased soil PO₄^3- had no clear influence on the bioavailability of As or Sb (derived from a sequential extraction procedure). At high PO₄^3- concentration, B. chinensis accumulated higher amounts of As in the shoots and roots in both individual and co-contaminated soil, whereas Sb accumulation increased only when Sb was the only contaminant. When As was the only contaminant, the translocation of As from roots to shoots decreased as soil PO₄^3- increased. Increased soil PO₄^3- had no influence on Sb translocation from root to shoot. Although As was toxic (impaired growth) at low PO₄^3- soil concentration, no toxicity was observed in the high-PO₄^3- soil. No toxicity was observed for Sb in either low- or high-PO₄^3- soils. Increased soil PO₄^3- concentration ameliorated or masked As toxicity to plant growth and led to higher As concentration in the plant's edible parts. The addition of high soil PO₄^3- concentrations ameliorated or masked As toxicity to plant growth in both individually and As + Sb co-contaminated soil; however, the plant's edible parts accumulated higher As and Sb concentrations. Environ Toxicol Chem 2020;39:1233-1243. © 2020 SETAC.

Cycling and total risks of multiple As fractions in the Beijing-Tianjin-Hebei area on the agricultural plain, China

The high toxicity of As can cause serious health risks for humans; therefore, understanding the behavior of As in weakly alkaline soil conditions relevant to agricultural plains is important. To investigate the mobility and total risks of multiple fractions of As, 230 pairs of soil (including soil cores) and, wheat grain, and corresponding groundwater samples and 38 atmospheric deposition samples were collected from agricultural soil in the Beijing-Tianjin-Hebei region, China, which is a typical wheat-growing area. Seven fractions of As, namely, water-soluble (As1), exchangeable (As2), carbonate-bound (As3), humic acid-bound (As4), Fe-Mn oxide-bound (As5), organic matter-bound (As6), and residual (As7) As, were analyzed using a sequential extraction procedure to better understand and confirm the relationship among these different forms. Correlation and principal components analyses showed a significant relationship among As1-As5, and As in atmospheric deposits and As in groundwater and soil samples showed a positive relationship. As found in wheat, therefore, mainly originated from the soil and atmospheric deposits, and indirectly from the groundwater. As in the soil samples was mainly controlled by Mn and Fe₂O₃ based on the vertical distribution of soil cores and correlation analysis. The health risk assessments showed that As in the multiple fractions did not form a potential non-cancer risk for children and adults. However, residents could still face the risk of developing cancer by ingesting wheat and drinking the groundwater. The findings of this study have important implications for understanding the hydrological/geochemical behavior of As and the soil and water quality in a wide range of environmental settings. Additionally, our findings provided arguments for decreasing the concentrations of As in the wheat production system and to remind residents to decrease ingestion of their staple food (wheat) and drink less of the local groundwater.

Neuropathological changes in wild muskrats (Ondatra zibethicus) and red squirrels (Tamiasciurus hudsonicus) breeding in arsenic endemic areas of Yellowknife, Northwest Territories (Canada): Arsenic and cadmium accumulation in the brain and biomarkers of oxidative stress

The brain is one of the critical organs particularly susceptible to the damaging effects of chronic arsenic poisoning and there is a growing body of evidence that suggest that oxidative stress plays a key role in the pathogenesis of neurodegenerative disorders. The aim of this present work was to comparatively assess biomarkers of oxidative stress and status of antioxidant enzyme activities in the brains of muskrats and squirrels breeding in arsenic endemic areas, specifically near the vicinity of the abandoned Giant mine site (~2 km radius), and an intermediate location approximately 20 km from the mine area and in reference locations spanning 52-105 km from the city of Yellowknife, Northwest Territories (Canada). Analysis included measurement of total arsenic and cadmium concentration in the nails, brain, and stomach content of muskrats and squirrels, in addition to biochemical evaluation of lipid peroxidation levels and antioxidant enzymes defense: catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GPx) in the brain tissues. The results revealed that arsenic concentration in the nails of muskrats collected closest to the vicinity of the mine area was in the range of 11 to 35.1 times higher than those from the reference site. The maximum concentration of arsenic in the nails of muskrats from the intermediate location was 47.6 times higher than the maximum concentration observed in the reference muskrats. Cadmium was generally undetected in the nails of muskrats and squirrels from the three sampling locations. Arsenic in the gut contents of muskrats from the arsenic affected area was 4.5 to 49.1 times higher than those from the reference site. Cadmium levels in the guts of muskrats from the mine area almost doubled those from the reference site. Arsenic accumulated in the nails of squirrels from the areas closest to the mine but was undetected in the squirrel nails from the reference location. The maximum arsenic levels in the stomach content of squirrels from the mine area was ~40 times higher than those from the reference site. Arsenic did not accumulate in the brains of muskrats, but cadmium was detected in a few brains of muskrats. Brains of squirrels from the mine area and intermediate locations accumulated both arsenic and cadmium. The brains of squirrels and muskrats from the arsenic affected area showed no evidence of increased lipid peroxidation compared to the animals from the reference site. However, SOD, CAT and GPx activities in the brains of animals from the arsenic endemic areas tended to be higher compared to the control sites. This is the first study documenting evidence of oxidative stress and altered antioxidant enzyme activities in brains of wild rodent population in arsenic endemic areas of Canada.

Mechanisms of arsenic assimilation by plants and countermeasures to attenuate its accumulation in crops other than rice

Arsenic is a ubiquitous metalloid in the biosphere, and its origin can be either geogenic or anthropic. Four oxidation states (-3, 0, +3 and + 5) characterize organic and inorganic As- compounds. Although arsenic is reportedly a toxicant, its harmful effects are closely related to its chemical form: inorganic compounds are most toxic, followed by organic ones and finally by arsine gas. Although drinking water is the primary source of arsenic exposure to humans, the metalloid enters the food chain through its uptake by crops, the extent of which is tightly dependent on its phytoavailability. Arsenate is taken up by roots via phosphate carriers, while arsenite is taken up by a subclass of aquaporins (NIP), some of which involved in silicon (Si) transport. NIP and Si transporters are also involved in the uptake of methylated forms of As. Once taken up, its distribution is regulated by the same type of transporters albeit with mobility efficiencies depending on As forms and its accumulation generally occurs in the following decreasing order: roots > stems > leaves > fruits (seeds). Besides providing a survey on the uptake and transport mechanisms in higher plants, this review reports on measures able to reducing plant uptake and the ensuing transfer into edible parts. On the one hand, these measures include a variety of plant-based approaches including breeding, genetic engineering of transport systems, graft/rootstock combinations, and mycorrhization. On the other hand, they include agronomic practices with a particular focus on the use of inorganic and organic amendments, treatment of irrigation water, and fertilization.

An assessment of arsenic hazard in groundwater-soil-rice system in two villages of Nadia district, West Bengal, India

The present study measured arsenic (As) concentrations in soil, groundwater and rice grain samples in two villages, Sarapur and Chinili, under Chakdaha block, Nadia district, West Bengal, India. This study also included a survey of the two villages to understand the knowledge among villagers about the As problem. Soil and groundwater samples were collected from fields in two villages while rice grain samples were collected from villagers' houses. The results revealed the presence of As in higher concentrations than the maximum permissible limit of As in drinking water (10 µg L^-1 and 50 µg L^-1 by WHO and Indian standard, respectively) in groundwater [124.50 ± 1.11 µg L^-1 (Sarapur) and 138.20 ± 1.34 µg L^-1 (Chinili)]. The level of As in soil was found to range from 47.7 ± 0.14 to 49.3 ± 0.19 mg Kg^-1 in Sarapur and from 57.5 ± 0.25 to 62.5 ± 0.44 mg Kg^-1 in Chinili which are also higher than European Union maximum acceptable limit in agricultural soil (i.e. 20 mg Kg^-1). The analysis of As in rice grains of five varieties, collected from residents of two villages, showed the presence of higher than recommended safe level of As in rice by FAO/WHO (0.2 mg Kg^-1). The As concentration order was Gosai (0.95 ± 0.044 mg kg^-1), Satabdi (0.79 ± 0.038 mg kg^-1), Banskathi (0.60 ± 0.026 mg kg^-1), Kunti (0.47 ± 0.018 mg kg^-1) and Ranjit (0.29 ± 0.021 mg kg^-1). Importantly, Gosai and Satabdi were the most popular varieties being consumed by local people. The data of consumption of rice per day in the survey was used for the measurement of average daily dose and hazard quotient. It was seen that the As hazard was negatively correlated to the age of residents. Therefore, children and toddlers were at higher risk of As exposure than elderly people. In addition, people with skin related As toxicity symptoms were also cited in the two villages. The study emphasized the severity of As problem in remote areas of West Bengal, India where people consume As tainted rice due to lack of awareness about the As problem and associated health issues.

Influence of amendments on metal environmental and toxicological availability in highly contaminated brownfield and agricultural soils

The immobilizing effects of wood biochar (BW2%) and iron grit (Z1%) applied alone or in combination (BW2% + Z1%) to agricultural (M750) and brownfield (MAZ) soils highly contaminated by metals were assessed in a greenhouse experiment. The results showed that Z1% and BW2% + Z1% were the most efficient amendments to reduce Cd, Cu, Pb, and Zn mobility, environmental availability, and phytoavailability in the M750 soil. The oxidation of Z1% allowed part of the Cu and Zn pools present in exchangeable or carbonate-bound forms (labile fraction) to complex in less mobile forms. In this soil, the metal chemical extractions (0.01 M CaCl₂ and 0.05 M EDTA) and the DGT (diffusive gradient in thin films) devices to assess metal in soil solution and soil pore water (SPW) also highlighted the immobilizing characteristic of Z1%. In most cases, the addition of BW2% to Z1% (BW2% + Z1%) did not improve this effect, except for the dissolved Pb and Zn concentrations in the M750 soil solution. It was also observed that Cd, Pb, and Zn passed throughout DGT mimicking the biological cell membrane were reduced by all amendments of the M750 soil corroborating metal concentrations measured in rye grass shoots. In the MAZ soil, metals were less available as shown by their low extractability rate, low capacity of metal resupply from the solid phase to pore water, and low phytoavailability. The poor metal availability could be explained by the high levels of carbonate and organic matter contents in this soil. Nevertheless, a decrease of the Cu environmental availability and the Cu concentrations in rye grass shoots grown on the MAZ soil was also observed in the soil amended with Z1% alone or in combination with BW2%. From a health point of view, the most effective amendment to reduce human exposure through ingestion of soil particles for the M750 and MAZ soils was BW2% for Cd and BW2% + Z1% for Pb. However, the presence of rye grass minimized the amendments' beneficial effects.