Speciation and Possible Origins of Organosulfur Compounds in Rice Paddy Soils Affected by Acid Mine Drainage.

Although sulfur cycling in acid mine drainage (AMD)-contaminated rice paddy soils is critical to understanding and mitigating the environmental consequences of AMD, potential sources and transformations of organosulfur compounds in such soils are poorly understood. We used sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy to quantify organosulfur compounds in paddy soils from five AMD-contaminated sites and one AMD-uncontaminated reference site near the Dabaoshan sulfide mining area in South China. We also determined the sulfur stable isotope compositions of water-soluble sulfate (δ34SWS), adsorbed sulfate (δ34SAS), fulvic acid sulfur (δ34SFAS), and humic acid sulfur (δ34SHAS) in these samples. Organosulfate was the dominant functional group in humic acid sulfur (HAS) in both AMD-contaminated (46%) and AMD-uncontaminated paddy soils (42%). Thiol/organic monosulfide contributed a significantly lower proportion of HAS in AMD-contaminated paddy soils (8%) compared to that in AMD-uncontaminated paddy soils (21%). Within contaminated soils, the concentration of thiol/organic monosulfide was positively correlated with cation exchange capacity (CEC), moisture content (MC), and total Fe (TFe). δ34SFAS ranged from -6.3 to 2.7‰, similar to δ34SWS (-6.9 to 8.9‰), indicating that fulvic acid sulfur (FAS) was mainly derived from biogenic S-bearing organic compounds produced by assimilatory sulfate reduction. δ34SHAS (-11.0 to -1.6‰) were more negative compared to δ34SWS, indicating that dissimilatory sulfate reduction and abiotic sulfurization of organic matter were the main processes in the formation of HAS.

Facet-Dependent Competitive Adsorption Mechanisms of Chromate and Oxalic Acid on γ-FeO(OH) Nanocrystals.

Facet-dependent toxic metal adsorption of iron oxides widely occurred in natural environments. It is known that organic acids can alter the adsorption behaviors of trace elements by cooperative or competitive effects. However, the coadsorption mechanisms of the specific facets are still not fully understood. In the current investigation, Cr(VI) adsorption onto the lepidocrocite (γ-FeO(OH))-exposed facets in the presence of oxalic acid (OA) was studied using macroexperiments, in situ attenuated total reflectance Fourier transform infrared spectroscopy, X-ray adsorption fine structure, and density functional theory calculations. Rod-like lepidocrocite (R-LEP) with a high ratio of {001}/{010} facet showed excellent Cr(VI) adsorption capacity than that of plate-like lepidocrocite (P-LEP, the dominant facet is {010}) in the absence/presence of OA. Interestingly, OA reacted with R-LEP would be easier to diminish Cr(VI) adsorption than with P-LEP. The competitive adsorption occurred on the {001} facet due to the formation of inner-sphere OA configurations (monodentate mononuclear and bidentate mononuclear structures) and a bidentate binuclear Cr(VI) complex. However, OA coordinated with {010} facets via the outer-sphere complexes, while Cr(VI) could form a protonated monodentate binuclear configuration. These observations suggest that the competitive adsorption processes between OA and Cr(VI) exhibit facet dependence. Furthermore, lepidocrocite-exposed facets determine the interfacial interactions and geochemical behaviors of Cr(VI) in polluted environments.

Fe(II)-mediated transformation of schwertmannite associated with calcium from acid mine drainage treatment.

Schwertmannite is an important Fe(III)-oxyhydroxysulfate in acid mine drainage (AMD) polluted areas and its stability depends on surrounding environmental factors and previously bound elements. The treatment and neutralization of AMD normally involve the use of lime, which leads to the discharge of abundant Ca in the mining area. Such an environmental disturbance brings up an important and less considered problem of how the reductive transformation of schwertmannite associated with coexisting Ca occurred. Here, the Fe(II)-mediated transformation of Ca-adsorbed schwertmannite and subsequent Ca repartitioning behaviors were investigated. Results showed that adsorbed Ca had a weak inhibitory effect on Fe(II)-mediated schwertmannite transformation. Release of SO42- and SEM images both indicated that transformation rates of schwertmannite decreased under the influence of adsorbed Ca. XRD patterns indicated that adsorbed Ca altered schwertmannite transformation pathways and product compositions upon treatment with 0.4 mmol/L Fe(II). The end products of Sch notably contained both goethite and lepidocrocite; however, transformation products of SchCa only contained goethite all along. Approximately 33.5% of the surface adsorbed-Ca was released into solution within 6 hr after Fe(II) injection. Aqueous Ca behaved in a "first release and then im-mobilization" manner, which indicated dissolution and secondary mineralization drove Ca migration during the Fe(II)-mediated transformation of SchCa. Adsorbed Ca blocked the surface sites for subsequent Fe(II) adsorption, limited the Fe(II)-Fe(III) ETAE, and decreased the transformation rates. This work sheds light on the complex geochemical behavior of schwertmannite under the influences of environmental perturbations in AMD environments.

Transcriptome Analysis of the Acid Stress Response of Desulfovibrio vulgaris ATCC 7757.

The application of sulfate-reducing bacteria (SRB) shows great potential in the anaerobic biological treatment of acid mine wastewater; therefore, it has attracted much attention. The low pH in acidic wastewater affects the growth and reducing power of SRB. To uncover the mechanism underlying the reduction efficiency of SRB under acidic conditions, in this study, transcriptomic analysis was performed with Desulfovibrio vulgaris ATCC 7757 under three different pH conditions (pH 4.0, 5.5 and 7.0) and in the initial inoculation, logarithmic growth and plateau phases. Our results showed that ATCC 7757 still had biological activity at pH 4.0 and exhibited gene expression patterns at pH 4.0 that were different from those at pH 5.5 and pH 7. Importantly, the gene expression pattern was similar between pH 5.5 and pH 7. Transcriptomic analysis identified differentially expressed genes that affected the growth of ATCC 7757 under pH 7.0 at 22 h compared to 15 h; 196 of these genes were upregulated and 575 were downregulated. These differentially expressed genes were mainly enriched in genetic information processing and metabolism. Additionally, we identified 57 candidate genes associated with low-pH tolerance. Adaptation to low pH was reflected by an increase in the expression of genes involved in cell membrane structure and proton transport. The expression of genes involved in the reduction process decreased, including the genes DVU0499 and sat, which encode proteins that affect the sulfate reduction process. Both gene activities were validated by qPCR. Our results will contribute to further promoting the reducing power of SRB in acid mine wastewater and the development of successful bioremediation strategies.

Effects of adsorbed phosphate on jarosite reduction by a sulfate reducing bacterium and associated mineralogical transformation.

Jarosite is one of the iron oxyhydroxysulfate minerals that are commonly found in acid mine drainage (AMD) systems. In natural environments, phosphate and sulfate reducing bacteria (SRB) may be coupled to jarosite reduction and transformation. In this research, the effect of phosphate on jarosite reduction by SRB and the associated secondary mineral formation was studied using batch experiments. The results indicated that Fe3+ is mainly reduced by biogenic S2- in this experiment. The effect of PO43- on jarosite reduction by SRB involved not only a physico-chemical factor but also a microbial factor. Phosphate is an essential nutrient, which can support the activity of SRB. In the low PO43- treatment, the production of total Fe2+ was found to be slightly larger than that in the zero PO43- treatment. Sorption of PO43- effectively elevated jarosite stability via the formation of inner sphere complexes, which, therefore, inhibited the reductive dissolution of jarosite. At the end of the experiment, the amounts of total Fe2+ accumulation were determined to be 4.54 ± 0.17a mM, 4.66 ± 0.22a mM, 3.91 ± 0.04b mM and 2.51 ± 0.10c mM (p < 0.05) in the zero, low, medium and high PO43- treatments, respectively, following the order of low PO43- treatment > zero PO43- treatment > medium PO43- treatment > high PO43- treatment. PO43- loading modified the transformation pathways for the jarosite mineral, as well. In the zero PO43- treatment, the jarosite diffraction lines disappeared, and mackinawite dominated at the end of the experiment. Compared to PO43--free conditions, vivianite was found to become increasingly important at higher PO43- loading conditions. These findings indicate that PO43- loading can influence the broader biogeochemical functioning of AMD systems by impacting the reactivity and mineralization of jarosite mineral.

Viability and distribution of bacteria immobilized on Sawdust@silica: The removal mechanism of phenanthrene in soil.

Immobilized cells (ICs) have been widely used to enhance the remediation of organic-contaminated soil (e.g., polycyclic aromatic hydrocarbons, PAHs). Once ICs are added to the heterogeneous soil, degradation hotspots are immediately formed near the carrier, leaving the remaining soil lack of degrading bacteria. Therefore, it remains unclear how ICs efficiently utilize PAHs in soil. In this study, the viability of Silica-IC (Cells@Sawdust@Silica) and the distribution of inoculated ICs and phenanthrene (Phe) in a slurry system (soil to water ratio 1:2) were investigated to explore the removal mechanism of PAHs by the ICs. Results showed that the Silica-IC maintained (i) good reproductive ability (displayed by the growth curve in soil and water phase), (ii) excellent stability, which was identified by the ratio of colony forming units in the soil phase to the water phase, the difference between the colony number and the DNA copies, and characteristics of the biomaterial observed by the FESEM, and (iii) high metabolic activity (the removal percentages of Phe in soil by the ICs were more than 95% after 48 h). Finally, the possible pathways for the ICs to efficiently utilize Phe in soil are proposed based on the distribution and correlation of Phe and ICs between the soil and water phase. The adsorption-degradation process was dominant, i.e., the enhanced degradation occurred between the ICs and carrier-adsorbed Phe. This study provided new insights on developing a bio-material for efficient bio-remediation of PAHs-contaminated soil.

Bioaccumulation and distribution of cadmium by Burkholderia cepacia GYP1 under oligotrophic condition and mechanism analysis at proteome level.

Bacteria have been applied for the bioremediation of cadmium-contaminated environment. Less is known about the bioaccumulation of high concentration of Cd over time under the oligotrophic environment. Burkholderia cepacia GYP1, which was isolated from multiple heavy metal contaminated farmland, was studied for its bioaccumulation mechanism of Cd under oligotrophic condition. GYP1 possessed highly accumulation capacity for cadmium reaching 116 mg Cd/g biomass (dry weight). ATR-FTIR, electron microscopy, flow cytometry along with subcellular fraction demonstrated that the uptake and distribution of cadmium varied with the increased amount of cadmium of GYP1 cell during the 7-day treatment time: the accumulation of cadmium was mainly on the outer membrane at the beginning (within 1 day), and the intracellular cadmium kept increased and held stable after 2 days, after that, the increased amount of cadmium mainly located extracellularly, related to the secreted EPS. Further mechanism analysis of bioaccumulation of Cd by GYP1 based on iTRAQ-based proteomics showed that Cd(II) could trigger the up-regulation of the Cd2+/Zn2+-exporting ATPase, type VI protein secretion systems, and glutathione-S-transferase that are related to cadmium response, which may contribute to maintain the intracellular cadmium homeostasis. In summary, the immobilization of Cd(II) by B. cepacia GYP1 contains three steps:(1) fast immobilization of Cd(II) on the cell surface coordinated with functional groups, (2) transport of Cd(II) to cells and accumulation in cytoplasm, and (3) efflux of intracellular Cd(II) depended on energy and the entrapped or adsorbed of extracellular Cd(II) by EPS. Our study provided the understanding of the cadmium accumulation process of B. cepacia GYP1 under oligotrophic condition, which would be helpful in bioremediation of natural cadmium contaminated environment.

Thiocyanate-induced labilization of schwertmannite: Impacts and mechanisms.

Schwertmannite is an amorphous iron(III)-oxyhydroxysulfate that forms in acid mine drainage (AMD) environments. The characteristic of high heavy metal adsorption capability makes schwertmannite a potentially useful, environmentally friendly material in wastewater treatment. Unstable schwertmannite is prone to recrystallization. Understanding the mechanisms that induce schwertmannite labilization and affect its capacity to remove heavy metals are of great environmental and geochemical significance. Thiocyanate (SCN¯) is a hazardous pseudohalide that is also normally found in AMD. However, little is known about the impact of Fe(III)-binding ligand SCN¯ on schwertmannite stability and its subsequent capacity to bind trace elements. Here, we investigated the adsorption of SCN¯ on schwertmannite and subsequent mineral transformation to characterize this little-known process. The appearance of Fe2+ indicated that the interactions between schwertmannite and SCN¯ may involve complexation and reduction reactions. Results showed that the majority of the adsorbed-SCN¯ was immobilized on schwertmannite during the 60-days transformation. The transformation rates of schwertmannite increased with increasing concentrations of SCN¯. Goethite was detected as the dominant transformation product with or without SCN¯. The mechanisms of SCN¯-promoted dissolution of schwertmannite can be described as follows: (1) formation of Fe(III)-NCS complexes on the schwertmannite surface and in solution, a process which increases the reactivity of solid phase Fe(III); (2) the extraction of Fe(III) from schwertmannite by SCN¯ and subsequent schwertmannite dissolution; and (3) the formation of secondary minerals from extracted Fe(III). These findings may improve AMD treatment strategies and provide insight into the use and potential reuse of schwertmannite as a trace element sorbent.

Comparative proteomics reveal the mechanism of Tween80 enhanced phenanthrene biodegradation by Sphingomonas sp. GY2B.

Previous study concerning the effects of surfactants on phenanthrene biodegradation focused on observing the changes of cell characteristics of Sphingomonas sp. GY2B. However, the impact of surfactants on the expression of bacterial proteins, controlling phenanthrene transport and catabolism, remains obscure. To overcome the knowledge gap, comparative proteomic approaches were used to investigate protein expressions of Sphingomonas sp. GY2B during phenanthrene biodegradation in the presence and absence of a nonionic surfactant, Tween80. A total of 23 up-regulated and 19 down-regulated proteins were detected upon Tween80 treatment. Tween80 could regulate ion transport (e.g. H+) in cell membrane to provide driving force (ATP) for the transmembrane transport of phenanthrene thus increasing its uptake and biodegradation by GY2B. Moreover, Tween80 probably increased GY2B vitality and growth by inducing the expression of peptidylprolyl isomerase to stabilize cell membrane, increasing the abundances of proteins involved in intracellular metabolic pathways (e.g. TCA cycle), as well as decreasing the abundances of translation/transcription-related proteins and cysteine desulfurase, thereby facilitating phenanthrene biodegradation. This study may facilitate a better understanding of the mechanisms that regulate surfactants-enhanced biodegradation of PAHs at the proteomic level.

Pyrene biodegradation with layer-by-layer assembly bio-microcapsules.

Biotechnology is considered as a promising technology for the removal of polycyclic aromatic hydrocarbons from the environment. Free bacteria are often sensitive to some biotic and abiotic factors in the environment to the extent that their ability to effect biodegradation of organic pollutants, such as polycyclic aromatic hydrocarbons, is hampered. Consequently, it is imperative to carry out investigations into biological systems that will obviate or aid tolerance of bacteria to harsh environmental conditions. Chitosan/alginate bio-microcapsules produced using layer-by-layer (LBL) assembly method were tested for pyrene (PYR) biodegradation under harsh environmental conditions. Morphology observation indicated that the flake bio-microcapsules could be successfully prepared through LBL assembly method. Surface analysis showed that the bio-microcapsules had large fractions of mesopores. The results of the biodegradation experiments revealed that the 95% of 10mgL-1 PYR could be removed by the bacteria encapsulated chitosan/alginate bio-microcapsules in 3 days, which was higher than that of the free bacteria (59%). Compared to the free cells, the bacteria encapsulated chitosan/alginate bio-microcapsules produced 1-6 times higher PYR biodegradation rates at a high initial PYR concentration (50mgL-1) and extremely low pH values (pH =3) or temperatures (10°C or 40°C), as well as high salt stress. The results indicated that bacteria in microcapsules treatment gained a much higher tolerance to environmental stress and LBL bio-microcapsule could be promising candidate for remediating the organic pollutants.

Removal of heavy metals from acid mine drainage using chicken eggshells in column mode.

Chicken eggshells (ES) as alkaline sorbent were immobilized in a fixed bed to remove typical heavy metals from acid mine drainage (AMD). The obtained breakthrough curves showed that the breakthrough time increased with increasing bed height, but decreased with increasing flow rate and increasing particle size. The Thomas model and bed depth service time model could accurately predict the bed dynamic behavior. At a bed height of 10 cm, a flow rate of 10 mL/min, and with ES particle sizes of 0.18-0.425 mm, for a multi-component heavy metal solution containing Cd2+, Pb2+ and Cu2+, the ES capacities were found to be 1.57, 146.44 and 387.51 mg/g, respectively. The acidity of AMD effluent clearly decreased. The ES fixed-bed showed the highest removal efficiency for Pb with a better adsorption potential. Because of the high concentration in AMD and high removal efficiency in ES fixed-bed of iron ions, iron floccules (Fe2(OH)2CO3) formed and obstructed the bed to develop the overall effectiveness. The removal process was dominated by precipitation under the alkaline reaction of ES, and the co-precipitation of heavy metals with iron ions. The findings of this work will aid in guiding and optimizing pilot-scale application of ES to AMD treatment.

Biosurfactant rhamnolipid enhanced modification of corn stalk and its application for sorption of phenanthrene.

The application of modified agricultural wastes for removing polycyclic aromatic hydrocarbons (PAHs) from water is gaining a growing interest. However, most modified methods using synthetic chemicals may cause secondary pollution. To overcome this limitation, in this study, a rhamnolipid modified corn stalk (RL-CS) for the removal of phenanthrene (PHE) from aqueous solution was prepared using a rhamnolipid-enhanced acid modification method. RL-CS with higher surface area and lower polarity exhibited higher PHE removal efficiency than that of raw corn stalk (RCS). The adsorption kinetics of RL-CS fitted well with pseudo-second-order kinetics (R2 > 0.999). Sorption coefficients and carbon-normalized sorption coefficient of RL-CS were 4.68 and 2.86 times higher than that of RCS. Sorption process of RL-CS was nonlinear. Meanwhile, the sorption was an exothermic process and could occur spontaneously. The present study demonstrated that biosurfactant-modified biosorbent RL-CS may be of great potential for the removal of low concentrations of PAHs from the contaminated waters.

Atomistic Simulation of Solubilization of Polycyclic Aromatic Hydrocarbons in a Sodium Dodecyl Sulfate Micelle.

Solubilization of two polycyclic aromatic hydrocarbons (PAHs), naphthalene (NAP, 2-benzene-ring PAH) and pyrene (PYR, 4-benzene-ring PAH), into a sodium dodecyl sulfate (SDS) micelle was studied through all-atom molecular dynamics (MD) simulations. We find that NAP as well as PYR could move between the micelle shell and core regions, contributing to their distribution in both regions of the micelle at any PAH concentration. Moreover, both NAP and PYR prefer to stay in the micelle shell region, which may arise from the greater volume of the micelle shell, the formation of hydrogen bonds between NAP and water, and the larger molecular volume of PYR. The PAHs are able to form occasional clusters (from dimer to octamer) inside the micelle during the simulation time depending on the PAH concentration in the solubilization systems. Furthermore, the micelle properties (i.e., size, shape, micelle internal structure, alkyl chain conformation and orientation, and micelle internal dynamics) are found to be nearly unaffected by the solubilized PAHs, which is irrespective of the properties and concentrations of PAHs.

Nonionic surfactants induced changes in cell characteristics and phenanthrene degradation ability of Sphingomonas sp. GY2B.

Surfactant-mediated bioremediation has been widely applied in decontaminating PAH-polluted sites. However, the impacts of surfactants on the biodegradation of PAHs have been controversial in the past years. To gain a clear insight into the influencing mechanisms, three nonionic surfactants (Tween80, TritonX-100 and Brij30) were selected to systematically investigate their effects on cell surface properties (membrane permeability, functional groups and elements), cell vitality as well as subsequent phenanthrene degradation ability of Sphingomonas sp. GY2B. Results showed that biodegradation of phenanthrene was stimulated by Tween80, slightly inhibited by TritonX-100 and severely inhibited by Brij30, respectively. Positive effect of Tween80 may arise from its role as the additional carbon source for GY2B to increase bacterial growth and activity, as demonstrated by the increasing viable cells in Tween80 amended degradation systems determined by flow cytometry. Although TritonX-100 could inhibit bacterial growth and disrupt cell membrane, its adverse impacts on microbial cells were weaker than Brij30, which may result in its weaker inhibitive extent. Results from this study can provide a rational basis on selecting surfactants for enhancing bioremediation of PAHs.

Effects of nano bamboo charcoal on PAHs-degrading strain Sphingomonas sp. GY2B.

Nano bamboo charcoal (NBC) has been commonly used in the production of textiles, plastics, paint, etc. However, little is known regarding their effects towards the microorganisms. The effects of NBC on phenanthrene degrading strain Sphingomonas sp. GY2B were investigated in the present study. Results showed that the addition of NBC could improve the phenanthrene removal by Sphingomonas sp. GY2B, with removal efficiencies increased by 10.29-18.56% in comparison to the control at 24h, and phenanthrene was almost completely removed at 48h. With the presence of low dose of NBC (20 and 50mgL(-1)), strain GY2B displayed a better growth at 6h, suggesting that NBC was beneficial to the growth of GY2B and thus resulting in the quick removal of phenanthrene from water. However, the growth of strain GY2B in high dose of NBC (200mgL(-1)) was inhibited at 6h, and the inhibition could be attenuated and eliminated after 12h. NBC-effected phenanthrene solubility experiment suggested that NBC makes a negligible contribution to the solubilization of phenanthrene in water. Results of electronic microscopy analysis (SEM and TEM) indicated NBC may interact with the cell membrane, causing the enhanced membrane permeability and then NBC adsorbed on the membrane would enter into the cells. The findings of this work would provide important information for the future usage and long-term environmental risk assessment of NBC.

Accumulation of Hydrocarbons by Maize (Zea mays L.) in Remediation of Soils Contaminated with Crude Oil.

This study has investigated the use of screened maize for remediation of soil contaminated with crude oil. Pots experiment was carried out for 60 days by transplanting maize seedlings into spiked soils. The results showed that certain amount of crude oil in soil (≤2 147 mg·kg(-1)) could enhance the production of shoot biomass of maize. Higher concentration (6 373 mg·kg(-1)) did not significantly inhibit the growth of plant maize (including shoot and root). Analysis of plant shoot by GC-MS showed that low molecular weight polycyclic aromatic hydrocarbons (PAHs) were detected in maize tissues, but PAHs concentration in the plant did not increase with higher concentration of crude oil in soil. The reduction of total petroleum hydrocarbon in planted soil was up to 52.21-72.84%, while that of the corresponding controls was only 25.85-34.22% in two months. In addition, data from physiological and biochemical indexes demonstrated a favorable adaptability of maize to crude oil pollution stress. This study suggested that the use of maize (Zea mays L.) was a good choice for remediation of soil contaminated with petroleum within a certain range of concentrations.

The Effect of Pollination on Cd Phytoextraction From Soil by Maize (Zea mays L.).

A pot experiment was conducted to investigate the effects of pollination on cadmium (Cd) phytoextraction from soil by mature maize plants. The results showed that the unpollinated maize plants accumulated 50% more Cd than that of the pollinated plants, even though the dry weight of the former plants was 15% less than that of the latter plants. The Cd accumulation in root and leaf of the unpollinated maize plant was 0.47 and 0.89 times higher than that of the pollinated plant, respectively. The Cd concentration in the cob was significantly decreased because of pollination. Preventing pollination is a promising approach for enhancing the effectiveness of phytoextraction in Cd-contaminated soils by maize. This study suggested that in low Cd-contaminated soil pollination should be encouraged because accumulation of Cd in maize grains is very little and maize seeds can bring farmers economic benefits, while in high Cd-contaminated soil, inhibition of pollination can be applied to enhance phytoextraction of Cd from soil by maize plant.

Using machine learning to explore oxyanion adsorption ability of goethite with different specific surface area.

In this study, we developed prediction models for the adsorption of divalent and trivalent oxyanions on goethite based on machine learning algorithms. After verifying the reliability of the models, the importance of goethite specific surface area (SSA) and the average oxyanion adsorption capacities of goethite with different SSAs were calculated by shapley additive explanations (SHAP) importance analysis and partial dependence (PD) analysis. Despite there were differences in the feature importance of divalent and trivalent oxyanions, the contribution of goethite's SSA to the adsorption amount ranked the fourth based on SHAP importance, indicating SSA played the important role in oxyanion adsorption. Meanwhile, the PD values of SSA and the optimized complexation constants from surface complexation modeling (SCM) both indicated a non-monotonic relationship between the goethite with different SSA and its oxyanions binding capacity. When the total site concentration and crystal face composition were used as the machine learning model input features, the SHAP importance values of crystal faces and the PD decomposition results indicated that the (001) face showed the crucial influence on oxyanions adsorption amount. These findings demonstrated the important role of crystal face composition in goethite's adsorption ability, and provided a theoretical explanation for the variations of oxyanions adsorption amount on different SSA goethite.

Prediction of adsorption of metal cations by clay minerals using machine learning.

Adsorption of heavy metals by clay minerals occurs widely at the solid-liquid interface in natural environments, and in this paper, the phenomenon of adsorption of Cd2+, Cu2+, Pb2+, Zn2+, Ni2+ and Co2+ by montmorillonite, kaolinite and illite was simulated using machine learning. We firstly used six machine learning models including Random Forest(R), Extremely Forest(E), Gradient Boosting Decision Tree(G), Extreme Gradient Boosting(X), Light Gradient Boosting(LGB) and Category Boosting(CAT) to feature engineer the metal cations and the parameters of the minerals, and based on the feature engineering results, we determined the first order hydrolysis constant(log K), solubility product constant(SPC), and higher hydrolysis constant (HHC) as the descriptors of the metal cations, and site density(SD) and cation exchange capacity(CEC) as the descriptors of the clay minerals. After comparing the predictive effects of different data cleaning methods (pH50 method, Box method and pH50-Box method) and six model combinations, it was finally concluded that the best simulation results could be achieved by using the pH 50-Box method for data cleaning and Extreme Gradient Boosting for modelling (RMSE = 4.158 %, R2 = 0.977). Finally, model interpretation was carried out using Shapley explanation plot (SHAP) and partial dependence plot(PDP) to analyse the potential connection between each input variable and the output results. This study combines machine learning with geochemical analysis of the mechanism of heavy metal adsorption by clay minerals, which provides a different research perspective from the traditional surface complexation model.

Prediction of heavy metal removal performance of sulfate-reducing bacteria using machine learning.

A robust modeling approach for predicting heavy metal removal by sulfate-reducing bacteria (SRB) is currently missing. In this study, four machine learning models were constructed and compared to predict the removal of Cd, Cu, Pb, and Zn as individual ions by SRB. The CatBoost model exhibited the best predictive performance across the four subsets, achieving R2 values of 0.83, 0.91, 0.92, and 0.83 for the Cd, Cu, Pb, and Zn models, respectively. Feature analysis revealed that temperature, pH, sulfate concentration, and C/S (the mass ratio of chemical oxygen demand to sulfate) had significant impacts on the outcomes. These features exhibited the most effective metal removal at 35 °C and sulfate concentrations of 1000-1200 mg/L, with variations observed in pH and C/S ratios. This study introduced a new modeling approach for predicting the treatment of metal-containing wastewater by SRB, offering guidance for optimizing operational parameters in the biological sulfidogenic process.