The migrated behavior and bioavailability of arsenic in mangrove sediments affected by pH and organic acids

Effect mechanism of low-molecular-weight organic acids during sulfidation of As(V)-bearing ferrihydrite

Sulfide induces the reductive dissolution of iron (oxyhydr) oxides, the primary host phases for arsenic (As), thereby triggering As release. We investigates the physicochemical mechanisms of three types of low molecular weight organic acids (LMWOAs) on sulfide-mediated reductive dissolution of As(V)-ferrihydrite and As release using batch experiments combined with hydro-chemical, spectroscopic, and microscopic analyses. Arsenate dominated the aqueous (97.2-100 %) and solid phases throughout the experiment. LMWOAs accelerated S(-II) consumption and As release by inhibiting FeS formation, with rates ordered as citric acid (CA) > oxalic acid (OA) > malic acid (MA) > control (Kb). At S(-II): Fe = 0.5, maximum As release was 11.78 % (Kb) and 14.60 % (CA); at S(-II): Fe = 1, it was 27.58 % (Kb) and 30.71 % (OA). LMWOAs enhanced As release via non-reductive ligand dissolution of As(V)-ferrihydrite. Secondary mineral formation in later stages re-immobilized As, with mineral layers ≥50 nm thick. LMWOAs interacted differently with secondary minerals: CA primarily adsorbed on surfaces, while MA integrated into the matrix. LMWOAs influenced As redistribution in secondary minerals, increasing contamination risks. Thus, the complex effects of organic matter (OM) on Fe, S, and As biogeochemistry must be considered in risk assessments and remediation strategies for As-contaminated sites in sulfidic environments.

Interactions between iron mineral and low-molecular-weight organic acids accelerated nitrogen conversion and release in lake sediments

Endogenous nitrogen (N) release from lake sediments is one of main causes affecting water quality, which can be affected by the presence of iron (Fe) minerals and organic matter, especially low-molecular-weight organic acids (LMWOAs). Although these substances always coexist in sediments, their interaction effect on N fate is not yet clear. In this study, the role and mechanisms of the coexistence of iron mineral (ferrihydrite, Fh) and LMWOAs, i.e. citric acid (CA) and galacturonic acid (GA) on the release and transformation of N in lake sediments were systematically evaluated via microcosm cultivation for 45 d Results showed that the addition of Fh+LMWOAs significantly accelerated N mineralization and conversion in lake sediments, accompanied by increasing ferrous iron content and decreasing redox potential. Biotic pathways played more critical roles than abiotic oxidation pathways during this process, and Fh+LMWOAs strengthened cooperation among microbial species by forming complex topologies and higher positive correlations. Correspondingly, cellular functions, iron respiration, and N metabolism modules were increased. CA with high carboxyl content showed greater total nitrogen removal and metabolic abundance. The present findings facilitate understanding the synergies of iron minerals and organic matter on N fate and N biogeochemical cycling in lake sediments.

The redistribution process of As(Ⅲ) and Fe(Ⅱ) caused by As/Fe ratio, organic matter, and co-existing ions: Co-precipitation and co-oxidation

The contamination of arsenic (As) in aqueous environments has drawn widespread attention, and iron compounds may largely alter the migration ability of As. However, the stability of As(III) in Fe-As system with the intervention of organic matter (OM) remains unclear. Herein, we had explored the co-precipitation and co-oxidation processes of As-Fe system by using batch experiments combined with Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) in this research. The precipitation quantity of As(III) increased (28.85-92.41 %) when the As/Fe ratio decreased, and increased (24.20-64.20 %) with pH increased. The main active substance for oxidizing As(III) was H2O2, which was produced in the As-Fe system. FTIR and XPS revealed that As(III) was first oxidized in neutral, and then absorbed and enteredthe interior of Fe(OH)3 colloids. But under alkaline conditions, As(III) was adsorbed by Fe (Oxyhydr) oxides firstly, and then oxidized. The intervention of OM would inhibit the redistribution process of As(III) in aqueous environments. Functional groups and unsaturation of the carbon chain were the dominant factors that affected the precipitation and oxidation processes of As(III), respectively. Co-existing ions (especially PO43-) also signally affected the precipitation quantity of As(Ⅲ) in the system and, when coexisting with OM, could exacerbate this process. The influence of co-existing ions on the redistributive process of As(III) in the As-Fe system with/without OM were as follows: PO43- > SO42- > mixed ions > SiO32-. Moreover, high concentration of OM and PO43- might lead to morphological alterations of As, acting as a threat to aqueous environments. In summary, the present findings were to further understand and appreciate the changes of As toxicity in the aqueous environments. Particularly, the coexistence of OM and As can potentially increase the risk to drinking water safety.

Review on arsenic environment behaviors in aqueous solution and soil

Arsenic pollution in environment has always been an important environmental problem that has attracted wide attention in recent years. Adsorption is one of the main methods of treatment for arsenic in the aqueous solution and soil because of the advantages of high efficiency, low cost and wide application. Firstly, this report summarizes the commonly and widely used adsorbent materials such as metal-organic frameworks, layered bimetallic hydroxides, chitosan, biochar and their derivatives. The adsorption effects and mechanisms of these materials are further discussed, and the application prospects of these adsorbents are considered. Meanwhile, the gaps and deficiencies in the study of adsorption mechanism was pointed out. Then, this study comprehensively evaluated the effects of various factors on arsenic transport, including (i) the effects of pH and redox potential on the existing form of As; (ii) complexation mechanism of dissolved organic matter and As; (iii) factors affecting the plant enrichment of As. Finally, the latest scientific researches on microbial remediation of arsenic and the mechanisms were summarized. The review finally enlightens the subsequent development of more efficient and practical adsorption material.

Stimulation of oxalate root exudate in arsenic speciation and fluctuation with phosphate and iron in anoxic mangrove sediment

Mutual transformations of rhizospheric arsenic (As) in pollution-prone mangrove sediments affected by root exudate oxalate were simulated. This study focuses on the effect of oxalate on As release, mobilization, and phase speciation associated with P and Fe was examined under anoxic conditions in time-dependent changes. Results showed that oxalate addition significantly facilitated As-Fe-P release from As-contaminated mangrove sediments. Sediment As formed the adsorptive and the carbonate-binding fractionations, facilitating the re-adsorption processes. Solution As and As⁵⁺ correlated with NaOH-P positively but with NaHCO₃-P and HCl-P negatively. Dominant Fe³⁺ (>84 %) from the amorphous Fe regulated suspension changes and then time-dependent co-precipitation with As and P. Sediment P formed strong complexes with Fe oxides and could be substituted for As via STEM analysis. Oxalate ligand exchange, competitive adsorption of oxalate, and Fe-reduced dissolution are confirmed to involve, allowing for an insight As/P/Fe mobilization and fate in mangrove wetland.

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.

Dynamics of low-molecular-weight organic acids for the extraction and sequestration of arsenic species and heavy metals using mangrove sediments

Mangrove wetlands are subjected to pollution due to anthropogenic activities. Mangrove fitness is mainly determined by root exudates and microorganisms activities belowground, but the mechanisms are not yet well known. Rhizospheric interactions among mangrove sediments, microorganisms and root exudates were simulated. In particular, low-molecular-weight organic acids (LMWOA), were examined to explore the metal(loid)s rhizospheric dynamics via batch experiments. Using a combination of comparative sterilised and unsterilised sediments, LMWOA extracts and sediments constituents were examined. Factors such as the solution pH, dissolved organic carbon (DOC), arsenic and iron species and metal(loid)s in the aqueous phase were evaluated. The results show that on an average, the As decreased by 68.3 % and 42.1 % under citric and malic acid treatments, respectively, after sterilisation. In contrast, the As content increased by 29.6 % under oxalic acid treatment. Microorganisms probably facilitate sediment As release in the presence of citric and malic acids but suppress As mobilisation in the presence of oxalic acid. Fe, Mn and Al were significantly (p < 0.05) positively correlated with the trace metal(loid)s (Zn, Pb, Ni, Cu, Cr, Co, Ba, Cd and As). The solution pH was negatively correlated with the solution As. Both DOC and pH reach the peaks at the end of all treatments. The As absorption-desorption dynamics are closely linked to proton consumption, Fe-Mn-Al sedimentation of ageing performance and organic ligand complexation. The study provides an insight into the rhizospheric processes of microbial involvement and gives an enlightening understanding of the metal(loid)s redeployment for plant adaptation in mangrove wetlands.

Low-level arsenite boosts rhizospheric exudation of low-molecular-weight organic acids from mangrove seedlings (Avicennia marina): Arsenic phytoextraction, removal, and detoxification

Arsenic (As) contamination in mangrove wetlands has become a major concern. However, the impact of As on mangroves and the rhizospheric mechanism remains unclarified. In this study, various properties and responses of mangrove seedlings were investigated after exposure to arsenite (As³⁺). The results indicate that low-level As promoted the secretion of Low-molecular-weight organic acids (LMWOA, 4.5-6.59 mg/kg root in dry weight) and Fe plaque formation in their rhizospheres. Citric, oxalic, and malic acid were the three main components (84.3%-86.8%). Low-level As (5 and 10 μmol/L) also inhibited the rate of radial oxygen loss (ROL) but increased the accumulation of plant As (stem > leaf > root) and plaque As (0.23-1.13 mg/kg root in dry weight). We selected model LMWOAs to further examine As migration and speciation over time in As-enriched sediments (0, 20 and 40 mg/kg). The results reveal that LMWOAs promoted sediment As mobilisation and followed the order of citric acid > malic acid > oxalic acid. The hydrolysis and precipitation of Fe³⁺ and the complexation with organic ligand led to aqueous As and Fe sedimentation and, conversely, increased solution pH and re-translocated free As. The tolerance mechanisms include lowering ROL, translocating As and releasing LMWOAs to reduce its toxicity, and facilitating the fixation in sediment of oxidised As. The present study highlights the fact that mangroves are potentially favourable for As phytoextraction, removal and detoxification.

Dispose waste liquor of fresh biomass of a hyperaccumulator Sedum plumbizincicola in phytoextraction process

Sedum plumbizincicola has been widely employed to remove cadmium (Cd) and zinc (Zn) from contaminated soils and harvested biomass is used to recover valuable metals. While chopping and compacting are efficient methods to rapidly reduce the volume and moisture of fresh biomass, the resulting waste liquor containing metals needs treatment. Two types of contaminated soils were cropped with S. plumbizincicola and top-dressed with this liquor to study metals migration in soil profile and their uptake by plants. There were three treatments: planting and adding liquor (PL), planting without liquor (P) and adding liquor without planting (L). The results showed that Cd and Zn from liquor were mainly retained at top soil 0-10 cm under L treatment. Compared with L treatment, soil Cd and Zn under PL treatment decreased significantly in soil profile due to the extraction of S. plumbizincicola. Moreover, the amount of Cd and Zn extracted by plants was greater than that applied in soils. The metal removal rate by S. plumbizincicola in acid clay loam soil was higher than that in neutral sandy soil. To sum up, metal retaining in soil and uptake by S. plumbizincicola can be used to treat waste liquor from its fresh biomass. Novelty StatementRapid and efficient treatment of harvested fresh biomass is still a challenge although phytoextraction using hyperaccumulator Sedum plumbizincicola has been widely employed. Chopping and compacting fresh biomass are efficient methods for rapid dehydration, however, a large amount of waste liquor that contains of Cd and Zn is produced and needs treatment. In the present study, a simple and low-cost method was tested to dispose the liquor, i. e. irrigating it onto the surface of contaminated soils where grown S. plumbizincicola. It was found that Cd and Zn applied in soils from liquor were mainly retained at top 0-10 cm soil depth where S. plumbizincicola root system was widespread, and the amount of Cd and Zn extracted by plants was greater than that applied in soils. Therefore, it is technically feasible to dispose the waste liquor dewatering from fresh biomass of S. plumbizincicola in its phytoextraction process. This study is helpful for the rapid, efficient and low-cost treatment of harvested fresh biomass in the large-scale application of phytoremediation.