Hg(II) adsorption by bacteria: a surface complexation model and its application to shallow acidic lakes and wetlands in Kejimkujik National Park, Nova Scotia, Canada

Effective reducing the mobility and health risk of mercury in soil under thiol-modified biochar amendment

Soil mercury contamination poses health risks. However, effective immobilization techniques are lacking with challenges including low efficiency, limited long-term stability, susceptibility to multi-medium interference, and difficulty in controlling health risks. This study confirmed the feasibility of thiol-modified biochar, and elucidated the underlying mechanisms. Within 32 days of treatment, the leachable mercury decreased from 184.7 μg/L to below the hazardous waste threshold (100 μg/L, HJ/T299-2007, China). After 198 days of treatment, the soil achieved a safe ecological state with a mercury immobilization rate of 79.8-98.2% and a 50% reduction in available methylmercury. Thiol-modified biochar facilitated the conversion of active mercury species (exchangeable, carbonate, and oxide) into stable forms (organic and residual) through complexation and precipitation (e.g., HgS). Soil quality improvements were observed, including enhanced cation exchange capacity, available nitrogen, and total organic carbon. Thiol-modified biochar exhibits long-term effectiveness. After one and two years of treatment, the leachable mercury remained within acceptable health risk limit (hazard quotient < 1) for adults and children, respectively. After three years, the leachable mercury met the Level III groundwater quality standard (< 1 μg/L, GB 14848-2017, China). This study demonstrates an effective strategy for long-term diminishing mercury mobility and health risks in soil.

Investigation of cadmium and nickel biosorption by Pseudomonas sp. via response surface methodology

The environmental contamination of heavy metals has grown over the last several decades along with global industrialization and now constitutes a serious threat to human health. In this research, high cadmium (Cd) and nickel (Ni) resistant bacteria that were chosen for heavy metal biosorption were isolated from the Industries Factory in Sari, Mazandaran, Iran. Pseudomonas aeruginosa was recognized as the isolated bacterium based on its morphological, physiological, biochemical, antibiotic resistance testing, and 16S rRNA sequences. The bacteria had the highest concentrations of resistance to Cd (up to 1600 ppm) and Ni, according to the Minimum Inhibition Concentration (MIC) test (up to 2000 ppm). Single-factor studies in single and binary systems were used to examine the effects of temperature, contact duration, pH value, starting Cd and Ni concentration, and biomass dose on the Cd and Ni adsorption by P. aeruginosa. The Cd and Ni biosorption in binary solutions was optimized using the response surface methodology (RSM) based on Central Composite Design (CCD). The investigation revealed that at pH 7.0, 45 °C, and 1.5 gL^-1 biomass dose, the greatest biosorption efficiency for Cd and Ni was 92.43 percent and 88.45 percent, respectively. According to the adsorption of Cd and Ni in urban water, under these similarly extreme conditions, Cd adsorption drops to 54% and Ni to 60%. Analysis Potential functional groups involving interactions between cells and metal ions were identified using Fourier transform infrared spectroscopy (FTIR). Different compounds and heavy metal ions were found to have been adsorbed to the surface of the biosorbent by scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDS). It was determined that P. aeruginosa had a high rate of Cd and Ni adsorption and that variations in pH level had a greater impact than other parameters. The findings imply that P. aeruginosa biomass may be an effective, economical, and environmentally friendly method for removing Cd and Ni from contaminated settings. It also has a reasonable capacity for biosorption in both a natural environment and a laboratory environment.

Influence of Carbon Sources on the Phenolic Compound Production by Euglena gracilis Using an Untargeted Metabolomic Approach

Industrial development and urbanization has led to the diverse presence of metals in wastewater that are often improperly treated. The microalgae Euglena gracilis can tolerate high concentrations of metal via the excretion of organic metabolites, including phenolics. This study aims to evaluate how carbon amendment stimulates phenolic compound production by E. gracilis. The number, relative intensity and molecular composition of the phenolic compounds were significantly different between each of four carbon amended cultures (i.e., glutamic acid, malic acid, glucose, reduced glutathione) during the log phase. Phenolic compounds were mainly produced during the minimum growth rate, likely a response to stressful conditions. A better understanding of phenolic compounds production by E. gracilis and the impact of growth conditions will help identify conditions that favor certain phenolic compounds for dietary and metal chelation applications.

Application of surface complexation modeling on adsorption of uranium at water-solid interface: A review

Precise prediction of uranium adsorption at water-mineral interface is of great significance for the safe disposal of radionuclides in geologic environments. Surface complexation modeling (SCM) as a very useful tool has been extensively investigated for simulating adsorption behavior of metals/metalloids at water-mineral interface. Numerous studies concerning the fitting of uranium adsorption on various adsorbents using SCM are well documented, but the systematic and comprehensive review of uranium adsorption using various SCM is not available. In this review, we briefly summarized the rationale of SCM, including constant-capacitance-model (CCM), diffuse-layer-model (DLM), triple-layer-model (TLM); The recent progress in the application of SCM on the fitting of uranium adsorption towards metal (hydr)oxides, clay minerals and soil/sediments was reviewed in details. This review hopefully provides the beneficial guidelines for predicting the transport and fate of uranium in geologic environments beyond laboratory timescales.

Rotatable central composite design versus artificial neural network for modeling biosorption of Cr6+ by the immobilized Pseudomonas alcaliphila NEWG-2

Heavy metals, including chromium, are associated with developed industrialization and technological processes, causing imbalanced ecosystems and severe health concerns. The current study is of supreme priority because there is no previous work that dealt with the modeling of the optimization of the biosorption process by the immobilized cells. The significant parameters (immobilized bacterial cells, contact time, and initial Cr⁶⁺ concentrations), affecting Cr⁶⁺ biosorption by immobilized Pseudomonas alcaliphila, was verified, using the Plackett-Burman matrix. For modeling the maximization of Cr⁶⁺ biosorption, a comparative approach was created between rotatable central composite design (RCCD) and artificial neural network (ANN) to choose the most fitted model that accurately predicts Cr⁶⁺ removal percent by immobilized cells. Experimental data of RCCD was employed to train a feed-forward multilayered perceptron ANN algorithm. The predictive competence of the ANN model was more precise than RCCD when forecasting the best appropriate wastewater treatment. After the biosorption, a new shiny large particle on the bead surface was noticed by the scanning electron microscopy, and an additional peak of Cr⁶⁺ was appeared by the energy dispersive X-ray analysis, confirming the role of the immobilized bacteria in the biosorption of Cr⁶⁺ ions.

Toward an Internally Consistent Model for Hg(II) Chemical Speciation Calculations in Bacterium-Natural Organic Matter-Low Molecular Mass Thiol Systems

To advance the scientific understanding of bacteria-driven mercury (Hg) transformation processes in natural environments, thermodynamics and kinetics of divalent mercury Hg(II) chemical speciation need to be understood. Based on Hg L_III-edge extended X-ray absorption fine structure (EXAFS) spectroscopic information, combined with competitive ligand exchange (CLE) experiments, we determined Hg(II) structures and thermodynamic constants for Hg(II) complexes formed with thiol functional groups in bacterial cell membranes of two extensively studied Hg(II) methylating bacteria: Geobacter sulfurreducens PCA and Desulfovibrio desulfuricans ND132. The Hg EXAFS data suggest that 5% of the total number of membranethiol functionalities (Mem-RS_tot = 380 ± 50 μmol g^-1 C) are situated closely enough to be involved in a 2-coordinated Hg(Mem-RS)₂ structure in Geobacter. The remaining 95% of Mem-RSH is involved in mixed-ligation Hg(II)-complexes, combining either with low molecular mass (LMM) thiols like Cys, Hg(Cys)(Mem-RS), or with neighboring O/N membrane functionalities, Hg(Mem-RSRO). We report log K values for the formation of the structures Hg(Mem-RS)₂, Hg(Cys)(Mem-RS), and Hg(Mem-RSRO) to be 39.1 ± 0.2, 38.1 ± 0.1, and 25.6 ± 0.1, respectively, for Geobacter and 39.2 ± 0.2, 38.2 ± 0.1, and 25.7 ± 0.1, respectively, for ND132. Combined with results obtained from previous studies using the same methodology to determine chemical speciation of Hg(II) in the presence of natural organic matter (NOM; Suwannee River DOM) and 15 LMM thiols, an internally consistent thermodynamic data set is created, which we recommend to be used in studies of Hg transformation processes in bacterium-NOM-LMM thiol systems.

Magnesium isotope fractionation during microbially enhanced forsterite dissolution

Bacillus subtilis endospore-mediated forsterite dissolution experiments were performed to assess the effects of cell surface reactivity on Mg isotope fractionation during chemical weathering. Endospores present a unique opportunity to study the isolated impact of cell surface reactivity because they exhibit extremely low metabolic activity. In abiotic control assays, ²⁴ Mg was preferentially released into solution during forsterite dissolution, producing an isotopically light liquid phase (δ²⁶ Mg = -0.39 ± 0.06 to -0.26 ± 0.09‰) relative to the initial mineral composition (δ²⁶ Mg = -0.24 ± 0.03‰). The presence of endospores did not have an apparent effect on Mg isotope fractionation associated with the release of Mg from the solid into the aqueous phase. However, the endospore surfaces preferentially adsorbed ²⁴ Mg from the dissolution products, which resulted in relatively heavy aqueous Mg isotope compositions. These aqueous Mg isotope compositions increased proportional to the fraction of dissolved Mg that was adsorbed, with the highest measured δ²⁶ Mg (-0.08 ± 0.07‰) corresponding to the highest degree of adsorption (~76%). The Mg isotope composition of the adsorbed fraction was correspondingly light, at an average δ²⁶ Mg of -0.49‰. Secondary mineral precipitation and Mg adsorption onto secondary minerals had a minimal effect on Mg isotopes at these experimental conditions. Results demonstrate the isolated effects of cell surface reactivity on Mg isotope fractionation separate from other common biological processes, such as metabolism and organic acid production. With further study, Mg isotopes could be used to elucidate the role of the biosphere on Mg cycling in the environment.

In situ remediation of mercury-contaminated soil using thiol-functionalized graphene oxide/Fe-Mn composite

Mercury (Hg) contamination in soil is a paramount concern to the environment and public health. Yet, effective in situ remediation technologies have been lacking. In this study, a novel thiol-functionalized graphene oxide/Fe-Mn (SGO/Fe-Mn) composite was prepared and investigated for in situ immobilization of Hg in contaminated soil. Batch tests showed that application of SGO/Fe-Mn at doses of 0.4% and 0.8% effectively reduced H₂O, H₂SO₄ and HNO₃, CH₃COOH, and CaCl₂-extractable Hg by 90.3-98.9% and 96.5-98.9%, respectively, upon equilibrium after 72 d. An increasing of soil moisture content from 0 to 12.5% significantly enhanced the immobilization efficiency from 75.0% to 97.6%. XRD, FTIR, and XPS analyses suggested that the composite mainly immobilized Hg through surface complexation and chemical precipitation. Sequential extraction procedure demonstrated that the composite promoted the conversion of more accessible Hg (exchangeable and carbonate fractions) into the less accessible forms, i.e., oxides, organic matter, and residual fractions, resulting in substantially reduced environmental risk of Hg. The application of SGO/Fe-Mn enhanced soil cation exchange capacity, available N and K, and total organic carbon, and can be used to effectively improve soil properties. Moreover, immobilized Hg in soil by this composite remained stable over one year. The present study demonstrates the potential and viability of SGO/Fe-Mn for enhanced immobilization of Hg in soil and sediment.

Sustainable treatment of municipal landfill leachate by combined association of air stripping, Fenton oxidation, and enhanced coagulation

The present world has been facing the problem of municipal solid waste disposal with the generation of highly complex and toxic landfill leachate. Thus, in this research work, treatability of landfill leachate had been investigated by the combined approach of air stripping, Fenton oxidation, and enhanced coagulation to comply with discharge standard. At the initial stage of treatment, air stripping of raw leachate was implemented which removes around 51.50% of COD, 74.60% of BOD₅, and 97.60% of NH₃-N within 36 h of optimum retention time. Following air stripping, Fenton oxidation was applied with an optimum molar ratio of 1.9 of H₂O₂/Fe⁺² which register a maximal removal of 67.70% of COD, 92.30% of BOD₅, and 14.90% of Hg. Finally, enhanced coagulation (EC) with in situ formed Mn-Fe hydr(oxides) was employed and optimized by central composite design (CCD) of response surface methodology (RSM). Response surface plots denote an optimum condition of 0.13 M ratio of Mn/Fe, 22.67 mM of coagulant dose, and 7.78 of pH which corresponds to a maximum removal of 55.98% of COD and 77.68% of Hg. FTIR analysis of the precipitates of EC explained that the hydroxyl groups are primarily involved in the process of Hg removal. Moreover, EDAX spectrum also assured the removal of Hg by its existence with Mn-Fe complexes. Thus, the present line of treatment record an overall removal of 90.80% of COD, 98.0% of BOD₅, 97.60% of NH₃-N, and 82.68% of Hg which proves to be effective for the removal of leachate pollutants.

Biosorption of multi-heavy metals by coral associated phosphate solubilising bacteria Cronobacter muytjensii KSCAS2

This paper examines the potential detoxification efficiency of heavy metals by phosphate solubilising bacteria (PSB) that were isolated from coral, sea grass and mangrove environment. Initially, four potential bacterial isolates were selected based on their phosphate solubilisation index from 42 strains and were used for the metal tolerance test. Among the four isolates, KSCAS2 exhibited maximum tolerance to heavy metals and the phenotype indicated the production of extra polymeric substances. In a multi-heavy metal experimental setup at two concentrations (100 and 200 mg L^-l), it has been demonstrated that the bacteria have extracellularly sequestered metal ions in amorphous deposits and this has been confirmed by scanning electron microscopy. In experiments with a 100 mg L^-1 initial metal concentration, the percentages of metal removal by bacteria were 55.23% of Cd, 72.45% of Cr, 76.51% of Cu and 61.51% of Zn, respectively. In subsequent experiments, when the metal concentration was increased up to 200 mg L^-l, the metal removal capacity decreased as follows: 44.62%, 63.1%, 67% and 52.80% for Cd, Cr, Cu and Zn, respectively. In addition, the biosorption of heavy metals was confirmed by the Fourier transform infrared Spectroscopy (FT-IR) and scanning electron microscopy (SEM) analysis. The heavy metal concentrations in a broth culture were analysed by inductively coupled plasma-optical emission spectroscopy (ICP-OES). The study suggests that PSB Cronobacter muytjensii KSCAS2 can efficiently remove the heavy metals and these bacteria could be used for the metal removal from the agricultural soils.

Enhanced Removal of Dissolved Hg(II), Cd(II), and Au(III) from Water by Bacillus subtilis Bacterial Biomass Containing an Elevated Concentration of Sulfhydryl Sites

In this study, the sorption of Hg(II), Cd(II), and Au(III) onto Bacillus subtilis biomass with an elevated concentration of sulfhydryl sites, induced by adding excess glucose to the growth medium (termed 'High Sulfhydryl Bacillus subtilis' or HSBS) was compared to that onto B. subtilis biomass with a low concentration of sulfhydryl sites (termed 'Low Sulfhydryl Bacillus subtilis' or LSBS) and to sorption onto a commercially available cation exchange resin. Our results show that HSBS exhibits sorption capacities for the three studied metals that are two to five times greater than the sorption capacities of LSBS for these metals. After blocking the bacterial cell envelope sulfhydryl sites using a qBBr treatment, the sorption of the metals onto HSBS was significantly inhibited, indicating that the enhanced sorption onto HSBS was mainly due to the elevated concentration of sulfhydryl sites on the bacteria. A direct comparison of the removal capacity of the HSBS and that of the cation exchange resin for the three metals demonstrates that HSBS, compared to this commercially available resin, exhibits superior sorption capacity and selectivity for the removal of Hg(II), Cd(II), and Au(III), especially in systems with dilute metal concentrations. These results suggest that bacterial sulfhydryl sites control the sorption behavior of these three metals, and therefore biomass with induced high concentrations of sulfhydryl sites represents a promising and low cost biosorbent for the effective removal and recovery of chalcophile heavy metals from aqueous media.

Preparation of a novel graphene oxide/Fe-Mn composite and its application for aqueous Hg(II) removal

A novel graphene oxide/Fe-Mn (GO/Fe-Mn) composite was synthesized (molar ratio of Fe/Mn=3/1 and mass ratio of Fe/GO=1/7.5) and investigated for the sorption characteristics and mechanisms of aqueous mercury (Hg(2+)) as well as the biological effects to wheat and rice. Characterization tests showed that Fe-Mn oxides were impregnated onto GO sheets in an amorphous form through oxygen-containing functional groups (i.e., CO, epoxy COC, carboxyl OCO, and CO) and π-π interactions. GO/Fe-Mn possessed large surface area, surface enhanced Raman scattering with more sp(3) defects, and greater thermal stability than GO. XPS analysis revealed that Fe2O3, FeOOH, MnO2, MnOOH, and MnO were the dominant metal oxides in GO/Fe-Mn. Pseudo-second-order kinetic model and Sips isotherm model fitted well with the sorption kinetic and isotherm data. The maximum sorption capacity for mercury was 32.9mg/g. Ligand exchange and surface complexation were the dominant mechanisms for mercury removal. GO/Fe-Mn greatly reduced the bioavailability of mercury to wheat and rice, even promoted the seedling growth. This work suggests that GO/Fe-Mn can be used as an effective and environmental-friendly adsorbent in heavy metal remediation.

Removal of trace mercury (II) from aqueous solution by in situ MnO(x) combined with poly-aluminum chloride

Removal of trace mercury from aqueous solution by Mn (hydr)oxides formed in situ during coagulation with poly-aluminum chloride (PAC) (in situ MnO(x) combined with PAC) was investigated. The efficiency of trace mercury removal was evaluated under the experimental conditions of reaction time, Mn dosage, pH, and temperature. In addition, the ionic strength and the initial mercury concentration were examined to evaluate trace mercury removal for different water qualities. The results clearly demonstrated that in situ MnO(x) combined with PAC was effective for trace mercury removal from aqueous solution. A mercury removal ratio of 9.7 μg Hg/mg Mn was obtained at pH 3. Furthermore, at an initial mercury concentration of 30 μg/L and pH levels of both 3 and 5, a Mn dosage of 4 mg/L was able to lower the mercury concentration to meet the standards for drinking water quality at less than 1 μg/L. Analysis by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy suggests that the hydroxyls on the surface of Mn (hydr)oxides are the active sites for adsorption of trace mercury from aqueous solution.

Preparation and characterization of exfoliated graphene oxide–l-cystine as an effective adsorbent of Hg(ii) adsorption

Hg(ii) adsorption involves,l-cystine bears amino group (–NH₂) could interact with GO hydroxyl and carboxyl groups through covalent bond interaction.

Strong enhancement of trace mercury removal from aqueous solution with sodium thiosulfate by in situ formed Mn-(hydr)oxides

The effect of sodium thiosulfate (Na2S2O3) on trace mercury removal from aqueous solution by in situ MnOx was investigated. Removal efficiency was studied at different molar ratios of Na2S2O3/Mn (0, 0.264, 0.593 and 1.582) and under changes in Mn dosage, reaction time and pH conditions. Additionally, the ionic strength and the mercury removal amount were examined to evaluate the efficiency of trace mercury removal. The results indicated that the presence of thiosulfate clearly improved removal of mercury from solution, and that increases in the ionic strength enhanced removal in a certain range of thiosulfate concentration. At neutral conditions, the mercury removal amount reached to maximum of 64 μg/mg. It is proposed that the ability of thiosulfate to reduce some MnOx to Mn(2+) as well as transfer the uncharged mercury species to a negatively charged species [Formula: see text] improved trace mercury removal. The mechanism analysis revealed that ternary complexes or large aggregations may be formed because of surface complexation or electrostatic attraction.

Removal of trace mercury(II) from aqueous solution by in situ formed Mn-Fe (hydr)oxides

The efficiency and mechanism of trace mercury (Hg(II)) removal by in situ formed manganese-ferric (hydr)oxides (in situ Mn-Fe) were investigated by reacting KMnO4 with Fe(II) in simulated solutions and natural water. In the simulated solutions, the impact of coagulant dosage, pH, and temperature on mercury removal was studied. Experimental results showed that in situ Mn-Fe more effectively removed mercury compared with polyaluminum chloride (PAC) and iron(III) chloride (FeCl3), and that mercury existed in the form of uncharged species, namely Hg(OH)2, HgClOH(aq), and HgCl2(aq). Fourier transform infrared spectroscopy demonstrated that in situ Mn-Fe contained hydroxyl groups as the surface active sites, while X-ray photoelectron spectroscopy (XPS) measurements revealed that MnO2 or MnOOH and FeOOH were the dominant species in the precipitates. XPS analysis indicated that an Hg-Mn-Fe mixture was formed in the precipitates, suggesting that mercury was removed from solutions via transfer from the liquid phase to solid phase. These results indicated that the primary mercury removal mechanisms in in situ Mn-Fe were surface complexation and flocculation-precipitation processes. Satisfactory removal efficiency of mercury was also observed following in situ Mn-Fe in natural waters.

Effects of salinity and humic acid on the sorption of Hg on Fe and Mn hydroxides

The objective of this study was to investigate the influence of humic acid (HA) and salinity on adsorption of Hg on the amorphous and crystalline of iron and manganese hydroxides. The results show that the adsorption of Hg(2+) on Fe and Mn hydroxides was inhibited in marine system due to the formation of stable, nonsorbing aqueous HgCl(2) complexes in solution. Moreover, Cl(-) inhibited the Hg(2+) adsorption more severely on amorphous than crystalline hydroxides. The addition of HA inhibited Hg(2+) adsorption on Fe and Mn hydroxides in freshwater system might be attributed to the competition between Hg(2+) and HA on adsorption to Fe and Mn hydroxides. In contrast, the addition of HA promoted Hg(2+) adsorption on Fe and Mn hydroxides in the marine system, which might be due to the addition of humic acid resulted in the reaction between Cl(-) and HA, and therefore the reducing of Cl(-) promoted more Hg(2+) on Fe and Mn hydroxides. In addition, the influence of HA on Hg(2+) adsorption on Fe and Mn hydroxides are more visible for crystalline than amorphous hydroxides.

Adsorption of mercury on lignin: combined surface complexation modeling and X-ray absorption spectroscopy studies

Adsorption of mercury (Hg) on lignin was studied at a range of pH values using a combination of batch adsorption experiments, a surface complexation model (SCM) and synchrotron X-ray absorption spectroscopy (XAS). Surface complexation modeling indicates that three types of acid sites on lignin surfaces, namely aliphatic carboxylic-, aromatic carboxylic- and phenolic-type surface groups, contributed to Hg(II) adsorption. The bond distance and coordination number of Hg(II) adsorption samples at pH 3.0, 4.0 and 5.5 were obtained from extended X-ray absorption fine structure (EXAFS) spectroscopy analysis. The results of SCM and XAS combined reveal that the predominant adsorption species of Hg(II) on lignin changes from HgCl(2)(0) to monodentate complex -C-O-HgCl and then bidentate complex -C-O-Hg-O-C- with increasing pH value from 2.0 to 6.0. The good agreement between SCM and XAS results provides new insight into understanding the mechanisms of Hg(II) adsorption on lignin.

Discrete site surface complexation constants for lanthanide adsorption to bacteria as determined by experiments and linear free energy relationships

Bacteria are abundant in many natural and engineered environments where they are thought to exert important controls on the cycling, mobility, bioavailability, and toxicity of metal contaminants. In order to probe their role in moderating the behavior of lanthanides, pH-dependent adsorption edges of 13 individual lanthanides and yttrium to the Gram-negative bacterium Pantoea agglomerans were used to generate discrete site surface complexation constants. The calculated surface complexation constants were compared with stability constants estimated using linear free energy relationships based on a number of hydroxyl-containing ligands. The experimental data suggests that lanthanide adsorption edges below pH 6.5 are consistent with adsorption to phosphate groups for the light and some of the middle lanthanides (La to Gd), whereas some of the middle and heavy lanthanides appear to favor carboxyl co-ordination (Tb to Yb), although exceptions occur in each grouping. The experimentally derived surface complexation constants for carboxyl coordination were of similar magnitude to stability constants estimated from linear free energy correlations using fulvic acid stability constants. The implication is that the adsorption of lanthanides to bacterial surfaces could be modeled reasonably well using lanthanide stability constants for natural organic matter, except perhaps at low pH where phosphate binding dominates.

Removal of aqueous Hg(II) by polyaniline: sorption characteristics and mechanisms

A polyaniline (PAN) prepared by chemical oxidation method was studied for Hg(II) removal from aqueous solutions. Batch adsorption results showed solution pH values had a major impact on mercury adsorption by this sorbent with optimal removal observed around pH 4-6. At both acidic and alkaline solutions beyond this optimal pH window, sorption capacity of PAN was substantially lowered, with the impact less pronounced at pH above 6. Among the water constituents tested, only chloride and humic acid had significant inhibition on mercury removal due to competitive complexation. In the range of 0.02-0.2 M, ionic strength had less impact on Hg(II) removal by PAN while further increase in background electrolyte concentration to 1.0 M substantially decreased mercury removal. An adsorption mechanism was proposed by analyzing the XPS spectra of the key elements (N(1s), Cl(2p) and Hg(4f)) on polyaniline surfaces and the change of its electrokinetic properties, both before and after Hg(II) adsorption. Specifically, at pH 5.5, it is likely that all the nitrogen-containing functional groups on the polymer matrix including imine, protonated imine and amine could be responsible for mercury adsorption, with imine having the highest affinity while the remaining two having similar strength to complex mercury.

The role of sorption and bacteria in mercury partitioning and bioavailability in artificial sediments

This study compared the relative importance of three types of sorption (organic matter-particle, mercury-organic matter and mercury-particle) in controlling the overall mercury partitioning and bioavailability in sediments. We found that all three types of sorption were important for both inorganic mercury (Hg) and methylated mercury (MeHg). Mercury-particle sorption was more important than mercury-fulvic acid (FA) sorption in increasing the mercury concentrations with increasing aging. Bioavailability (quantified by gut juice extraction from sipunculans) was mainly controlled by mercury-particle sorption, while FA-particle and mercury-FA sorption were not as important, especially for MeHg. Bacterial activity also increased the partitioning of Hg or MeHg in the sediments and was further facilitated by the presence of organic matter. The bioavailability of Hg or MeHg from sediments was only slightly influenced by bacterial activity. This study highlights the importance of sorption from various sources (especially mercury-particle sorption) as well as bacteria in controlling the partitioning and bioavailability of Hg or MeHg in sediments.

Mercury sorption to sediments: dependence on grain size, dissolved organic carbon, and suspended bacteria

A combination of laboratory scale derived correlations and measurements of grain size distribution, DOC (dissolved organic carbon) concentration, and density of suspended bacteria promises to be useful in estimating Hg(II) sorption in heterogeneous streambeds and groundwater environments. This was found by shaking intact sediment and fractions thereof (<63-2000microm) with solutions of HgCl2 (1.0-10.0ngml(-1)). The intact sediment was also shaken with the Hg(II) solutions separately in presence of DOC (6.5-90.2microgml(-1)) or brought in contact with suspensions of a strain of groundwater bacteria (2x10(4)-2x10(6)cellsml(-1)). Hg(II) sorption was rather weak and positively correlated with the grain size, and the sorption coefficient (Kd) varied between about 300 and 600mlg(-1). By using the relative surface areas of the fractions, Kd for the intact sediment was back calculated with 2% deviation. Kd was negatively correlated with the concentration of DOC and positively correlated with the number of bacteria. A multiple regression showed that Kd was significantly more influenced by the number of bacteria than by the grain size. The findings imply that common DOC concentrations in groundwater and streambeds, 5-20microgml(-1), will halve the Kd obtained from standard sorption assays of Hg(II), and that Kd will almost double when the cell numbers are doubled at densities that are common in aquifers. The findings suggest that simultaneous measurements of surface areas of sediment particles, DOC concentrations, and bacterial numbers are useful to predict spatial variation of Hg(II) sorption in aquifers and sandy sediments.

Manufactured nanoparticles: an overview of their chemistry, interactions and potential environmental implications

The industrial scale production and wide variety of applications of manufactured nanoparticles (NPs) and their possible release in considerable amounts into the natural aquatic environment have produced an increasing concern among the nanotechnology and environmental science community. In order to address this issue, it is important to understand NP chemistry, preparation, reactivity and possible mechanisms involved in their interaction with the naturally occurring aquatic components, particularly natural colloids and NPs present in the aquatic systems. In this review, an overview of the chemistry of both manufactured and natural aquatic NPs is outlined. This review discusses the physico-chemical aspects of both type of NPs as an essential point to assess possible routes involved in manufactured NP fate in the natural aquatic environment and their toxicity. Key advances related to the characterisation of the manufactured NPs and natural colloids.

A study on the adsorption mechanism of mercury on Aspergillus versicolor biomass

The adsorption behavior of mercury on Aspergillus versicolor biomass (AVB) has been investigated in aqueous solution to understand the physicochemical process involved and to explore the potentiality of AVB in pollution control management. This biomass has been successfully used for reducing the mercury concentration level in the effluent of chloralkali and battery industries to a permissible limit. The results establish that 75.6 mg of mercury is adsorbed per gram of biomass. The adsorption process is found to be a function of pH of the solution, with the optimum range being pH 5.0-6.0. The process obeys the Langmuir-Freundlich isotherm model. Scanning electron microscopic analysis demonstrates a conspicuous surface morphology change of the mercury-adsorbed biomass. A nearly uniform distribution of metal ions on the mycelial surface excepting a few aggregation points is revealed by X-ray elemental mapping profiles. The results of zeta potential measurement, Fourier transform infrared (FTIR) spectroscopy, and blocking of the functional groups by chemical modification reflect the binding of mercury on the biomass occurs through electrostatic and complexation reactions. The accumulation of mercury on the cell wall associated with negligible diffusion and or transportation into cytoplasm finds support from the results of adsorption kinetics and transmission electron micrographs. Mercury adsorption on biomass also leads to elongation of cells and cytoplasmic aggregation of spheroplast/protoplasts, indicating that the cell wall acts as a permeation barrier against this toxic metal.

Diel mercury-concentration variations in streams affected by mining and geothermal discharge

Diel variations of concentrations of unfiltered and filtered total Hg and filtered methyl Hg were documented during 24-h sampling episodes in water from Silver Creek, which drains a historical gold-mining district near Helena, Montana, and the Madison River, which drains the geothermal system of Yellowstone National Park. The concentrations of filtered methyl Hg had relatively large diel variations (increases of 68 and 93% from morning minima) in both streams. Unfiltered and filtered (0.1-microm filtration) total Hg in Silver Creek had diel concentration increases of 24% and 7%, respectively. In the Madison River, concentrations of unfiltered and filtered total Hg did not change during the sampling period. The concentration variation of unfiltered total Hg in Silver Creek followed the diel variation in suspended-particle concentration. The concentration variation of filtered total and methyl Hg followed the solar photocycle, with highest concentrations during the early afternoon and evening and lowest concentrations during the morning. None of the diel Hg variations correlated with diel variation in streamflow or major ion concentrations. The diel variation in filtered total Hg could have been produced by adsorption-desorption of Hg2+ or by reduction of Hg(II) to Hg0 and subsequent evasion of Hg0. The diel variation in filtered methyl Hg could have been produced by sunlight- and temperature-dependent methylation. This study is the first to examine diel Hg cycling in streams, and its results reinforce previous conclusions that diel trace-element cycling in streams is widespread but often not recognized and that parts of the biogeochemical Hg cycle respond quickly to the daily photocycle.

Surface reaction of Bacillus cereus biomass and its biosorption for lead and copper ions

In this study, the surface chemical functional groups of Bacillus cereus biomass were identified by Fourier transform infrared (FTIR) analytical technique. It had been shown that the B. cereus cells mainly contained carboxyl, hydroxyl, phosphate, amino and amide functional groups. The potentiometric titration was conducted to explain the surface acid-base properties of aqueous B. cereus biomass. The computer program FITEQL 4.0 was used to perform the model calculations. The optimization results indicated that three sites-three pKas model, which assumed the cell surface to have three distinct types of surface organic functional groups based on the IR analysis results, simulated the experimental results very well. Moreover, batch adsorption experiments were performed to investigate biosorption behavior of Cu(II) and Pb(II) ions onto the biomass. Obviously, the adsorption equilibrium data for the two ions were reasonably described by typical Langmuir isotherm.

Zinc sorption by a bacterial biofilm

Microbial biofilms are present in soils, sediments, and natural waters. They contain bioorganic metal-complexing functional groups and are thought to play an important role in metal cycling in natural and contaminated environments. In this study, the metal-complexing functional groups present within a suspension of bacterial cell aggregates embedded in extracellular polymeric substances (EPS) were identified in Zn adsorption experiments conducted at pH 6.9 with the freshwater and soil bacterium Pseudomonas putida. The adsorption data were fit with the van Bemmelen-Freundlich model. The molecular speciation of Zn within the biofilm was examined with Zn K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The Zn EXAFS data were analyzed by shell-by-shell fitting and linear least-squares fitting with reference spectra. Zinc sorption to the biofilm was attributed to predominantly Zn--phosphoryl (85 +/- 10 mol %) complexes, with a smaller contribution to sorption from carboxyl-type complexes (23 +/- 10 mol %). The results of this study spectroscopically confirm the importance of phosphoryl functional groups in Zn sorption by a bacterial biofilm at neutral pH.

Removal of inorganic mercury from aqueous solutions by biomass of the marine macroalga Cystoseira baccata

The ability of Cystoseira baccata algal biomass to remove Hg(II) from aqueous solutions is investigated. The mercury biosorption process is studied through batch experiments at 25 degrees C with regard to the influence of contact time, initial mercury concentration, solution pH, salinity and presence of several divalent cations. The acid-base properties of the alga are also studied, since they are related to the affinity for heavy metals. The studies of the pH effect on the metal uptake evidence a sharp increasing sorption up to a pH value around 7.0, which can be ascribed to changes both in the inorganic Hg(II) speciation and in the dissociation state of the acid algal sites. The sorption isotherms at constant pH show uptake values as high as 178 mg g(-1) (at pH 4.5) and 329 mg g(-1) (at pH 6.0). The studies of the salinity influence on the Hg(II) sorption capacity of the alga exhibit two opposite effects depending on the electrolyte added; an increase in concentration of nitrate salts (NaNO3, KNO3) slightly enhances the metal uptake, on the contrary, the addition of NaCl salt leads to a drop in the sorption. The addition of different divalent cations to the mercury solution, namely Ca2+, Mg2+, Zn2+, Cd2+, Pb2+ and Cu2+, reveals that their effect on the uptake process is negligible. Finally, the equilibrium sorption results are compared with predictions obtained from the application of a simple competitive chemical model, which involves a discrete proton binding constant and three additional constants for the binding of the main neutral inorganic Hg(II) complexes, Hg(Cl)2, HgOHCl and Hg(OH)2, to the algal surface sites.

Link between chemotactic response to Ni2+ and its adsorption onto the Escherichia coli cell surface

Bacterial chemotaxis is of medical, biological, and geological significance. Despite its importance, current chemotaxis measurements fail to account for the speciation of the chemical effector and the protonation state of the bacterial surface. We hypothesize that adsorption of Ni2+ onto the surface of Escherichia coli can influence its effective concentration and therefore influence its ability to induce a repellent response. By measuring repellent response at different pH values, the influence of Ni2+ adsorption on chemotaxis was assessed. In addition, we tested the effect of different Ni2+ chelating agents. Our data indicate that adsorption reactions influence the chemotactic response to Ni2+. We use potentiometric titration and Ni2+ adsorption experiments to develop and constrain a thermodynamic model capable of quantifying the concentration of Ni2+ at the bacteria/solution interface. Results from this model predict that the concentration of adsorbed Ni2+ is linearly proportional to the magnitude of the chemotactic response in E. coli. If adsorption is linked to chemotaxis in other cases, then chemotactic responses in realistic settings depend on a number of environmental factors such as pH, competing binding agents (e.g., aqueous organic acids, natural organic matter, mineral surfaces, etc.), and ionic strength. Our modeling approach quantifies adsorbed species on bacterial surfaces and may be used to predict the responses of different species to a variety of chemoeffectors. Our data suggest that specified changes in environmental conditions can be used to tune chemotactic responses in natural biological and geological settings.

Surface complexation modeling of proton and Cd adsorption onto an algal cell wall

This study quantifies Cd adsorption onto the cell wall of the algal species Pseudokirchneriella subcapitata by applying a surface complexation approach to model the observed adsorption behavior. We use potentiometric titrations to determine deprotonation constants and site concentrations for the functional groups on the algal cell wall. Adsorption and desorption kinetics experiments illustrate that adsorption of Cd onto the cell wall is rapid and reversible, except under low pH conditions. Adsorption experiments conducted as a function of pH and total Cd concentration yield the stoichiometry and site-specific stability constants for the important Cd-algal surface complexes. We model the acid/base properties of the algal cell wall by invoking four discrete surface functional group types, with pKa values of 3.9 +/- 0.3, 5.4 +/- 0.1, 7.6 +/- 0.3, and 9.6 +/- 0.4. The results of the Cd adsorption experiments indicate that the first, third, and fourth sites contribute to Cd adsorption under the experimental conditions, with calculated log stability constant values of 4.1 +/- 0.5, 5.4 +/- 0.5, and 6.1 +/- 0.4, respectively. Our results suggest that the stabilities of the Cd-surface complexes are high enough for algal adsorption to affect the fate and transport of Cd under some conditions and that on a per gram basis, algae and bacteria exhibit broadly similar extents of Cd adsorption.

Detoxification of mercury in the environment

In this study, a "green chemistry" approach was developed as an option for remediation of toxic mercury in the environment. Twenty mercury compounds were treated with an environmentally friendly agent cyclodextrin to produce stable non-toxic mercury in soil and water. The binding efficiency was determined using high performance liquid chromatography with diode-array detection. The stability of the cyclodextrin mercury complexes toward environmental microorganisms in water was estimated under OECD guidelines using gas chromatography-mass spectrometry. The toxicity of the cyclodextrin mercury compounds to terrestrial organisms was investigated by use of internationally recognized toxicity methods using mercuric acetate as a model contaminant. Key process conditions, for example pH, temperature, and amount of detoxifying agent were investigated and found to have significant effects on the toxicity of mercury. It was found that organic and inorganic mercury pollutants could be mineralized in the environment with cyclodextrins. The bound mercury compounds resisted biodegradation and were found to be non-toxic to environmental microorganisms under laboratory conditions.

Influence of water chemistry and natural organic matter on active and passive uptake of inorganic mercury by gills of rainbow trout (Oncorhynchus mykiss)

To distinguish physiologically regulated uptake from passive uptake of inorganic Hg in fish, rainbow trout (Oncorhynchus mykiss) were exposed to inorganic Hg (0.5, 1, or 2 microM total Hg) in ion-poor water with various treatments. Addition of ions to the water (mM concentrations of Ca, K, Cl) did not consistently alter Hg accumulation by trout gills, although there was a trend to higher Hg accumulation at higher ion concentrations. The apical Ca channel blockers Verapamil and lanthanum also did not consistently affect Hg accumulation by trout gills. Pre-treatment of trout with the Na channel blocker Phenamil decreased Hg uptake by about half. These results suggest a combination of physiologically regulated and passive uptake of Hg by trout gills. Strong complexing agents of Hg (EDTA, NTA, ethylenediamine, cysteine) decreased Hg-binding by trout gills in a dose-dependent manner. From these data, a conditional equilibrium binding constant for Hg to the gills was estimated as logK(Hg-gill) = 18.0, representing very strong binding of Hg to the gills. This value is a first step in creating a biotic ligand model (BLM) for inorganic Hg and fish. Natural organic matter (2-10 mg C/L) also decreased Hg-binding by trout gills, although mM concentrations of Na, K, and Cl interfered with this effect. At low concentrations of these ions, natural organic matter samples isolated from various sources bound Hg to similar degrees, as judged by Hg accumulation by trout gills. A conditional binding constant to natural organic matter (NOM) was estimated as logK(Hg-NOM) = 18.0 with about 0.5 micromol binding sites per mg C, representing strong binding of Hg to NOM.

Characterization of the pesticide chlordane in estuarine river sediments

Sediments are increasingly recognized as both carrier and potential source of contaminants in aquatic environments. This study investigated the characteristics and spatial distribution of total chlordane and its three most abundant compounds, including alpha-chlordane, gamma-chlordane, and trans-nonachlor, in sediments from the Cedar and Ortega rivers, Florida, USA, using geographic information system (GIS)-based kriging analyses and field measurements. Kriging analysis showed that two areas, one from the Cedar River area and the other from the northern end of the Ortega River area, were contaminated. The maximum concentrations of total chlordane, gamma-chlordane, alpha-chlordane, and trans-nonachlor in the sediments were, respectively, 101.8, 20.1, 26.3, and 19.2 microg/kg. A plot of total organic carbon (TOC)-normalized chlordane concentrations showed that effects of grain size on sediment chlordane contamination were negligible. A principal axis analysis further revealed that a linear correlation existed between alpha-chlordane and total chlordane as well as between gamma-chlordane and total chlordane, whereas no correlation existed between trans-nonachlor and total chlordane. Comparison of total chlordane concentration with Florida Sediment Assessment Guidelines showed that the Cedar River and the northern end of the Ortega River had total chlordane concentrations above the probable effect level (4.79 microg/kg), which could pose a potential risk to aquatic life.

Are methylmercury concentrations in the wetlands of Kejimkujik National Park, Nova Scotia, Canada, dependent on geology?

In the relatively pristine ecosystem in Kejimkujik Park, Nova Scotia, methylmercury (MeHg) concentrations in loons, Gavia immer, are among the highest recorded anywhere in the world. This study investigated the influence of bedrock lithology on MeHg concentrations in wetlands. Twenty-five different wetland field sites were sampled over four different bedrock lithologies; Kejimkujik monzogranite, black sulfidic slate, gray slate, and greywacke. Soil samples were analyzed for ethylmercury (EtHg), MeHg, total Hg, acid-volatile sulfides (AVS), organic matter, and water content as well as the biological parameters, mercury methyltransferase (HgMT) activity, sulfate reduction rates, fatty acid methyl ester (FAME) composition, and acidity. Methylmercury concentrations in the wetlands were highly dependent (P < 0.08) on lithology with no significant difference between bogs, fens, and swamps. Methylmercury concentrations in wetland soils developed on Kejimkujik monzogranite averaged 900 ng kg(-1) compared with only 300 ng kg(-1) in wetland soils developed on black sulfidic slate. Fatty acid methyl ester composition was also lithologically dependent (P < 0.001) with biomarkers for Desulfobulbus spp. discriminating between sites containing high and low MeHg concentrations. Levels of MeHg in wetlands were predicted mainly (41% of the sum of squares) by HgMT activity that differed (P < 0.009) between wetlands, with activity in bogs almost three times that present in swamps. Wetland MeHg concentrations are highly dependent on the lithology on which they have developed for largely biological reasons.

Influence of Bacillus subtilis cell walls and EDTA on calcite dissolution rates and crystal surface features

This study investigates the influence of EDTA and the Gram-positive cell walls of Bacillus subtilis on the dissolution rates and development of morphological features on the calcite [1014] surface. The calcite dissolution rates are compared at equivalent saturation indicies (SI) and relative to its dissolution behavior in distilled water (DW). Results indicate that the presence of metabolically inactive B. subtilis does not affect the dissolution rates significantly. Apparent increases in dissolution rates in the presence of the dead bacterial cells can be accounted for by a decrease of the saturation state of the solution with respect to calcite resulting from bonding of dissolved Ca2+ by functional groups on the cell walls. In contrast, the addition of EDTA to the experimental solutions results in a distinct increase in dissolution rates relative to those measured in DW and the bacterial cell suspensions. These results are partly explained by the 6.5-8 orders of magnitude greater stability of the Ca-EDTA complex relative to the Ca-B. subtilis complexes as well as its free diffusion to and direct attack of the calcite surface. Atomic force microscopy images of the [1014] surface of calcite crystals exposed to our experimental solutions reveal the development of dissolution pits with different morphologies according to the nature and concentration of the ligand. Highly anisotropic dissolution pits develop in the early stages of the dissolution reaction at low B. subtilis concentrations (0.004 mM functional group sites) and in DW. In contrast, at high functional group concentrations (4.0 mM EDTA or equivalent B. subtilis functional group sites), dissolution pits are more isotropic. These results suggest that the mechanism of calcite dissolution is modified by the presence of high concentrations of organic ligands. Since all the pits that developed on the calcite surfaces display some degree of anisotropy and dissolution rates are strongly SI dependent, the rate-limiting step is most likely a surface reaction for all systems investigated in this study. Results of this study emphasize the importance of solution chemistry and speciation in determining calcite reaction rates and give a more accurate and thermodynamically sound representation of dead bacterial cell wall-mineral interactions. In studies of natural aquatic systems, the presence of organic ligands is most often ignored in speciation calculations. This study clearly demonstrates that this oversight may lead to an overestimation of the saturation state of the solutions with respect to calcite and thermodynamic inconsistencies.