Water-soluble chitosan enhances phytoremediation efficiency of cadmium by Hylotelephium spectabile in contaminated soils

Differential immobilization of cadmium and changes in soil surface charge in acidic Ultisol by chitosan and citric acid: effect of their functional groups

Soil organic matter plays an important role in cadmium adsorption and immobilization. Since different organic matter components affect cadmium adsorption processes differently, selecting the right organic substrate and knowing how to apply it could improve cadmium remediation. This study compares the effects of two contrasting organic molecules; chitosan and citric acid, on cadmium adsorption and speciation in acidic Ultisol. The adsorption of chitosan to Ultisol significantly increased the soil positive charge while adsorption of citric acid increased the soil negative charge. At pH 5.0, the maximum amount of cadmium adsorbed in excess chitosan was 341% greater than that in excess citric acid. About 73-89% and 60-62% of adsorbed cadmium were bound to Fe/Mn oxides and organic matter/sulfide at pH 4.0 while this fraction was 77-100% and 57-58% for citric acid and chitosan at pH 5.0, respectively. This decrease in the complexing ability of chitosan was related to the destabilizing effect of high pH on chitosan's structure. Also, the sequence through which chitosan, citric acid, and cadmium were added into the adsorption system influenced the adsorption profile and this was different along a pH gradient. Specifically, adding chitosan and cadmium together increased adsorption compared to when chitosan was pre-adsorbed within pH 3.0-6.5. However, for citric acid, the addition sequence had no significant effect on cadmium adsorption between pH 3.0-4.0 compared to pH 6.5 and 7.5, with excess citric acid generally inhibiting adsorption. Given that the action of citric acid is short-lived in soil, chitosan could be a good soil amendment material for immobilizing cadmium.

Prevalence of antibiotic and metal resistance genes in phytoremediated cadmium and zinc contaminated soil assisted by chitosan and Trichoderma harzianum

Heavy metal in soil have been shown to be toxic with high concentrations and acts as selective pressure on both bacterial metal and antibiotic resistance determinants, posing a serious risk to public health. In cadmium (Cd) and zinc (Zn) contaminated soil, chitosan (Chi) and Trichoderma harzianum (Tri) were applied alone and in combination to assist phytoremediation by Amaranthus hypochondriacus L. Prevalence of antibiotic and metal resistance genes (ARGs and MRGs) in the soil was also evaluated using metagenomic approach. Results indicated that the phytoremediation of Cd and Zn contaminated soil was promoted by Chi, and Tri further reinforced this effect, along with the increased availability of Cd and Zn in soil. Meanwhile, combination of Chi and Tri enhanced the prevalence of ARGs (e.g., multidrug and β-lactam resistance genes) and maintained a high level of MRGs (e.g., chromium, copper) in soil. Soil available Zn and Cd fractions were the main factors contributing to ARGs profile by co-selection, while boosted bacterial hosts (e.g., Mitsuaria, Solirubrobacter, Ramlibacter) contributed to prevalence of most MRGs (e.g., Cd). These findings indicate the potential risk of ARGs and MRGs propagation in phytoremediation of metal contaminated soils assisted by organic and biological agents.

Intercropped Amygdalus persica and Pteris vittata applied with additives presents a safe utilization and remediation mode for arsenic-contaminated orchard soil

Intercropping the arsenic (As) hyperaccumulator Pteris vittata with fruit trees can safely yield peaches in As-polluted orchards in South China. However, the soil As remediation effects and the related mechanisms of P. vittata intercropped with peach trees with additives in the north temperate zone have rarely been reported. A field experiment was conducted to systematically study the intercropping of peach (Amygdalus persica) with P. vittata with three additives [calcium magnesium phosphate (CMP), ammonium dihydrogen phosphate (ADP), and Stevia rebaudiana Bertoni residue (SR)] in a typical As-contaminated peach orchard surrounding a historical gold mine in Pinggu County, Beijing City. The results showed that compared with monoculture (PM) and intercropping without addition (LP), the remediation efficiency of P. vittata intercropping was significantly increased by 100.9 % (CMP) to 293.5 % (ADP). CMP and ADP mainly compete with available As (A-As) adsorbed to the surface of Fe-Al oxide through PO₄^3-, while SR might activate A-As by enhancing dissolved organic carbon (DOC) in P. vittata rhizospheres. The photosynthetic rates (G_s) of intercropped P. vittata were significantly positively correlated with pinna As. The intercropping mode applied with the three additives did not obviously affect fruit quality, and the net profit of the intercropping mode (ADP) reached 415,800 yuan·ha^-1·a^-1. The As content in peaches was lower than the national standard in the intercropping systems. Comprehensive analysis showed that A. persica intercropped with P. vittata applied with ADP is better than other treatments in improving risk reduction and agricultural sustainability. In this study, a theoretical and practical basis is provided for the safe utilization and remediation of As-contaminated orchard soil in the north temperate zone.

Cadmium accumulation responses in Hylotelephium spectabile: The role of photosynthetic characteristics under different nitrogen, moisture, and light conditions

The influence of environmental factors on Cd accumulation by Hylotelephium spectabile and its physiological mechanisms are unclear. A field trial was conducted to investigate the effects of nitrogen, soil moisture, and light regulation on plant growth, Cd absorption and translocation, and the photosynthetic characteristics of two H. spectabile populations (LN with high Cd accumulation capacity and HB1 with relatively low Cd accumulation capacity). The results showed that Cd accumulation in LN was 59.6% higher than that in HB1 which may partly be explained by the inherent high transpiration rate of LN, especially at the terminal stage. In addition, the photosynthetic rate of LN responded more positively to nitrogen than HB1, which further amplified its advantages on plant growth and Cd accumulation. Moderate drought significantly stimulated root growth of LN, indicating that LN possesses stronger resistance to drought. Shade inhibited Cd distribution, rather than directly affecting Cd concentrations in H. spectabile. The combined stress of shade and drought had a synergistic effect on Cd translocation in H. spectabile. Moreover, LN achieved 17.3%∼444.5% higher transpiration levels than HB1 under environmental stress, which ensured a more efficient Cd transport capacity of LN. Therefore, the investigation of photosynthetic characteristics further revealed the physiological mechanism by which LN accumulated Cd superior to HB1 under environmental stress and responded more positively to nitrogen nutrition.

Modified biochar/humic substance/fertiliser compound soil conditioner for highly efficient improvement of soil fertility and heavy metals remediation in acidic soils

Fertile and uncontaminated soil with appropriate pH is crucial in terms of the agricultural sustainable development. Herein, a compound soil conditioner containing chitosan modified straw biochar (CBC), kitchen waste compost product-derived humic substance (HS), NPK compound fertiliser (NPK-CF) was prepared to simultaneously adjust acidic soil pH, improve fertility, and immobilize heavy metal. The results exhibited that the best Pb and NH₄⁺ adsorption performance was obtained in CBC with chitosan:biochar of 1:5. Then, the acid soil pH was improved from 5.03 to 6.66 in the presence of CBC/HS (5:5) with 3% addition weight (the mass ratio of conditioner to soil). Meanwhile, compared with the control, the contents of organic matter, available nitrogen, and available phosphorus significantly increased by 52.4%, 92.6%, and 136.3%, respectively. Moreover, Pb was highly efficient immobilised by CBC, and the concentration of Pb in the soil was decreased by 55.2%. The optimal growth trend of ryegrass was obtained in the presence of 3% addition weight (the mass ratio of conditioner to soil) CBC/HS (CBC:HS = 5:5) combined with 60% of the recommended NPK-CF application weight, which was mainly contributed by the improvement of the soil microbial abundance and community structure diversity. The addition of CBC/HS could effectively reduce the addition of NPK-CF and contribute to simultaneous controlling nitrogen loss, releasing phosphorus, immobilising Pb, adjusting pH, improving soil quality and controlling nonpoint pollution.

Assessing the suitable regions and the key factors for three Cd-accumulating plants (Sedum alfredii, Phytolacca americana, and Hylotelephium spectabile) in China using MaxEnt model

Phytoremediation is an effective way to remove metals from contaminated soil, and selecting remediation plants suitable for climate conditions is a prerequisite for effective phytoremediation. In this study, a MaxEnt model was applied to investigate the potential distribution and habitat suitability of three Cd-accumulating plants in China- Sedum alfredii, Phytolacca americana, and Hylotelephium spectabile and explore the key environmental factors that affect their habitat suitability. A total of 44 environmental parameters, including bioclimatic variables, altitude, and soil property parameters were used. The results showed that: (1) For S. alfredii, suitable areas account for 14.9 % of the area of China, which are mainly distributed in the middle and lower reaches of the Yangtze River. (2) The suitable areas of P. americana account for 22.7 % of China and are mainly located in the regions of the Qinling Mountains and the south of China. (3) While that for H. spectabile are mainly located in the regions of northeastern China and certain regions of central China, with suitable areas account for 8.3 % of the area of China. (4) The distribution of these three plants is significantly affected by precipitation; specifically, solar radiation is an influential factor for the distribution of S. alfredii and H. spectabile, and temperature limits the distribution of P. americana. The selection and agronomic management of hyperaccumulators for phytoremediation requires multifactor consideration (e.g., climate, soil conditions and planting patterns). The results can provide guidance for identifying suitable areas for planting these three accumulating plants, which could not only prevent the unscientific cultivation of them in unsuitable habitats but also enhance the efficiency of phytoremediation. Meanwhile, these findings are expected to contribute to agronomic management for improved phytoremediation effects in different Cd-contaminated regions of China.

Dissolved organic matter-assisted phytoremediation potential of cotton for Cd-contaminated soil: a relationship between dosage and phytoremediation efficiency

Dissolved organic matter (DOM) is a novel Cd-contaminated soils amendment for phytoremediation. However, the phytoremediation efficiency for different DOM doses has been insufficiently investigated. In this study, we investigated the effect of five DOM doses (v/w, 0%, 1%, 2%, 4% and 8%) on the phytoremediation efficiency of cotton in Cd-contaminated soil through pot experiment. The results showed that bioavailable Cd concentrations and organic matter in the soil increased with the increased of DOM dosage. The DOM dose increased the chlorophyll content, photosynthesis, and the total biomass of cotton. In addition, the DOM application increased the Cd content in cotton roots and changed the Cd uptake in cotton shoots, increasing shoot Cd extraction efficiency by 8.53-20%. Simultaneously, soil Cd phytoextraction efficiency significantly increased. Furthermore, applying a 1% DOM dose resulted in safeguarding fibre biomass and maximising the efficiency of shoot extraction. Redundancy analysis showed that the Mn content in leaves is critical for increasing cotton biomass, anti-oxidation competence and phytoremediation efficiency under 1% DOM dose. In conclusion, DOM enhanced cotton remediation in Cd-contaminated soils and applying DOM at 1% was a suitable choice for Cd-contaminated soils.

Phyto-Fenton remediation of a dichloro-diphenyl-trichloroethane contaminated site in Ha Tinh Province, Vietnam

A field trial was conducted at a site in Cam Binh commune, Ha Tinh province, Vietnam, highly contaminated with organo-pesticides. The phyto-Fenton process was applied to remove pesticide residues in soils. In addition to magnetite (Fe₃O₄) materials added to the soils, fertilizers and elicitors for oxidative burst were also added in the different experimental treatments. Dichloro-diphenyl-trichloroethane (DDT) and isomers were removed in all experimental lots. The removal efficiency was highest in lot B1, a site where only iron materials were added. The removal efficiency and the final content of DDTs in B1 were 98.4% and 0.009 mg kg⁻¹, respectively. In the presence of elicitors, the conversion of DDT to dichloro-diphenyl-dichloroethylene was more favorable. Analysis of soil properties indicated that the phyto-Fenton process can occur at neutral soil pH, and when there are only small changes in soil organic carbon content and cation exchange capacities. Shifts in the composition of the microbial communities were observed. Further studies on the interactions between materials added to soil, plants, and the soil microbiome are needed to understand the mechanism of action of the phyto-Fenton process during soil remediation.

Effects of water-soluble chitosan on Hylotelephium spectabile and soybean growth, as well as Cd uptake and phytoextraction efficiency in a co-planting cultivation system

Intercropping a Cd-accumulator with economically valuable crops is common in slightly or moderately Cd-polluted farmland soils. A field experiment was conducted to evaluate the effects of water-soluble chitosan (WSC) on the growth and Cd uptake of the Cd-accumulator Hylotelephium spectabile and soybean (Glycine max) during a co-cultivation in Cd-contaminated agricultural soil (WSC, 0 and 10 g·m-2). The results indicated that soybean yields were highest in response to the intercropping and WSC treatment. The results from the field trials generally showed that intercropping and WSC treatments significantly decreased Cd concentrations in inedible parts of soybean by 42.9-72.1% (except for stems), in the meantime, increased 95.8%-334.6% in shoot and root tissues of H. spectabile compared with the control (p < 0.05). The data revealed that Cd uptake was highest for H. spectabile during the intercropping and WSC treatment. The application of WSC in the intercropping system significantly increased the uptake of Cd by H. spectabile, but not by soybean. The findings of this study suggest that combining an intercropping system with a WSC treatment may be better for remediating Cd-contaminated soils than other methods involving the growth of a single hyperaccumulator.

Purification and Characterization of Gum-Derived Polysaccharides of Moringa oleifera and Azadirachta indica and Their Applications as Plant Stimulants and Bio-Pesticidal Agents

Plant gums are bio-organic substances that are derived from the barks of trees. They are biodegradable and non-adverse complex polysaccharides that have been gaining usage in recent years due to a number of advantages they contribute to various applications. In this study, gum was collected from Moringa oleifera and Azadirachta indica trees, then dried and powdered. Characterizations of gum polysaccharides were performed using TLC, GC-MS, NMR, etc., and sugar molecules such as glucose and xylose were found to be present. Effects of the gums on Abelmoschus esculentus growth were observed through root growth, shoot growth, and biomass content. The exposure of the seeds to the plant gums led to bio stimulation in the growth of the plants. Poor quality soil was exposed to the gum polysaccharide, where the polysaccharide was found to improve soil quality, which was observed through soil analysis and SEM analysis of soil porosity and structure. Furthermore, the plant gums were also found to have bio-pesticidal activity against mealybugs, which showed certain interstitial damage evident through histopathological analysis.

Impact of Chitosan on the Mechanical Stability of Soils

Chitosan is becoming increasingly applied in agriculture, mostly as a powder, however little is known about its effect on soil mechanical properties. Uniaxial compression test was performed for cylindrical soil aggregates prepared from four soils of various properties (very acidic Podzol, acidic Arenosol, neutral Fluvisol and alkaline Umbrisol) containing different proportions of two kinds of chitosan (CS1 of higher molecular mass and lower deacetylation degree, and CS2 of lower molecular mass and higher deacetylation degree), pretreated with 1 and 10 wetting–drying cycles. In most cases increasing chitosan rates successively decreased the mechanical stability of soils that was accompanied by a tendential increase in soil porosity. In one case (Fluvisol treated with CS2) the porosity decreased and mechanical stability increased with increasing chitosan dose. The behavior of acidic soils (Podzol and Arenosol) treated with CS2, differed from the other soils: after an initial decrease, the strength of aggregates increased with increasing chitosan amendment, despite the porosity consequently decreasing. After 10 wetting–drying cycles, the strength of the aggregates of acidic soils appeared to increase while it decreased for neutral and alkaline soils. Possible mechanisms of soil–chitosan interactions affecting mechanical strength are discussed and linked with soil water stability and wettability.

Water-soluble chitosan and phytoremediation efficiency of two Brassica napus L. cultivars in cadmium-contaminated farmland soils

Pot and field trials were conducted to investigate Cd uptake and phytoremediation efficiency of two Brassica napus cultivars (QY-1 and SYH) with applied water-soluble chitosan (WSC, Pot: 0, 2% and 4%; Field: 0 and 10 g·m^-2) grown in Cd-contaminated soils. The results from the pot and field trials generally showed that WSC treatments significantly increased Cd concentrations in shoot and root tissues by 33.77-159.71% (except for SYH/JY) and 7.42-168.71% of two B. napus cultivars compared with the control (p < 0.05). The uptake of Cd by shoots of SYH was obviously higher than by shoots of QY-1 treated with WSC under pot and field conditions, which was 1.54-2.22 times than that of QY-1 (p < 0.05). The results indicated that 2% WSC treatment significantly increased the water-soluble and acid extractable Cd in rhizosphere soils of both B. napus cultivars. Furthermore, Cd concentrations in the oils of two B. napus cultivars with applied WSC (10 g·m^-2) grown under field conditions were not significantly different from commercial rapeseed oils. Rapeseed oil of B. napus is not only an edible oil with high nutritional value, but it can also be converted into biomass diesel that can be used as a substitute for petroleum diesel.

Water-soluble polymers in agriculture: xanthan gum as eco-friendly alternative to synthetics

Water-soluble polymers (WSPs) are a versatile group of chemicals used across industries for different purposes such as thickening, stabilizing, adhesion and gelation. Synthetic polymers have tailored characteristics and are chemically homogeneous, whereas plant-derived biopolymers vary more widely in their specifications and are chemically heterogeneous. Between both sources, microbial polysaccharides are an advantageous compromise. They combine naturalness with defined material properties, precisely controlled by optimizing strain selection, fermentation operational parameters and downstream processes. The relevance of such bio-based and biodegradable materials is rising due to increasing environmental awareness of consumers and a tightening regulatory framework, causing both solid and water-soluble synthetic polymers, also termed 'microplastics', to have come under scrutiny. Xanthan gum is the most important microbial polysaccharide in terms of production volume and diversity of applications, and available as different grades with specific properties. In this review, we will focus on the applicability of xanthan gum in agriculture (drift control, encapsulation and soil improvement), considering its potential to replace traditionally used synthetic WSPs. As a spray adjuvant, xanthan gum prevents the formation of driftable fine droplets and shows particular resistance to mechanical shear. Xanthan gum as a component in encapsulated formulations modifies release properties or provides additional protection to encapsulated agents. In geotechnical engineering, soil amended with xanthan gum has proven to increase water retention, reduce water evaporation, percolation and soil erosion - topics of high relevance in the agriculture of the 21st century. Finally, hands-on formulation tips are provided to facilitate exploiting the full potential of xanthan gum in diverse agricultural applications and thus providing sustainable solutions.