Influence of natural organic matter on the adsorption of metal ions onto clay minerals

An eco-environmental assessment of harmful algal bloom mitigation using modified clay

The application of modified clay (MC) to mitigate harmful algal blooms (HABs) is becoming a widespread practice in China because of its low cost, high efficiency and environmental friendliness. Due to its success in China, this technology has also gained international recognition in recent years. Nevertheless, the eco-environmental impacts of this technology still remain to be a concern to many customers and government agencies, which has motivated scientists in both China and many other countries to evaluate its potential effects on nontarget aquatic organisms, water quality and the benthic environment. This paper reviews the results from these studies both in the laboratory and in the field, which showed that MC does not harm nontarget aquatic organisms and has no negative impact on water quality or the benthic environment. Additionally, findings show that MC can alter nutrient cycling and reduce algal toxins in water bodies. Furthermore, researchers also found that MC affects cyst formation and germination in sedimentary environments. This review is expected to provide scientific guidance for mitigating HABs in China and worldwide using clay or MC.

Synthesis of potential Ca-Mg-Al layered double hydroxides coated graphene oxide composites for simultaneous uptake of europium and fulvic acid from wastewater systems

High background electrolyte and natural organic matter are favorable to migration of hazardous radionuclides in geochemical repository. Herein, Ca-Mg-Al layered double hydroxide coated onto graphene oxide (Ca-Mg-Al LDH/GO) composites were successfully synthesized, characterized and adopted to decontaminate Eu(III) and fulvic acid (FA) under diverse experimental conditions. Diverse concentration gradients and different addition sequences on Eu(III) and FA were also obtained, which revealed different interaction mechanisms. The experimental results displayed that the coexistence of FA and Eu(III) respectively promoted adsorption performance of Eu(III) and FA under the ternary systems. The acquired Ca-Mg-Al LDH/GO composites were adopted to remove Eu(III) and FA, which further illustrated excellent chemo-physical stability and adsorption capacity of 1.12 × 10^-3 mol/g and 3.54 × 10^-4 mol/g, respectively. The remarkable adsorption performances of Ca-Mg-Al LDH/GO were confirmed through kinetic procedures and depending-temperature isotherms, illustrating that the kinetics processes were simulated using pseudo-second-order pattern, and the adsorption isotherms were splendidly simulated using Langmuir pattern. XPS spectrum analysis revealed that these containing oxygen groups took significant part in the restricting of Eu(III) and FA onto the surfaces of Ca-Mg-Al LDH/GO composites. In view of experimental results, the Ca-Mg-Al LDH/GO composites can be as potential adsorbents with availably recycled reusability for the decontamination of Eu(III) and FA from nuclear fuel partition or nuclear wastewater systems.

EDTA-Inspired Polydentate Hydrogels with Exceptionally High Heavy Metal Adsorption Capacity as Reusable Adsorbents for Wastewater Purification

Water pollution by heavy metal ions is a critical threat to public health. To remove the heavy metal pollutants from large waterbodies, we have synthesized a biocompatible, cost-effective, metal ion non-specific, and ethylenediaminetetraacetic acid (EDTA)-inspired polydentate hydrogel with exceptionally high adsorption capacity and reusability. The hydrogel was synthesized by the transamidation reaction between hydrolyzed polyacrylamide and branched polyethylenimine (BPEI). The mechanical strength of the synthesized hydrogel displayed an increasing trend with the wt % of the cross-linker (BPEI) and achieved a maximum storage modulus (G_max^') of 1093 Pa. Scanning electron microscopy revealed a porous network structure of the hydrogel (pore size: 30-70 μm), resulting in a very high swelling ratio of 5800%. The porous hydrogel manifested the maximum adsorption capacity of 482.2 mg/g when adsorbing from a mixture of metal ions (Cr³⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺, and Pb²⁺), higher than any EDTA-grafted material known to date. The high adsorption capacity of the hydrogel was attributed to the existence of numerous EDTA-mimicking coordinating functional groups, as confirmed by X-ray photoelectron spectroscopy. In addition, the hydrogel showed the self-healing property and preserved more than 85% adsorption efficiency even after five cycles of reuse. Furthermore, the hydrogels showed no or moderate toxicity toward mammalian cells.

Comparative effectiveness of different biochars and conventional organic materials on growth, photosynthesis and cadmium accumulation in cereals

Although biochar and conventional organic materials have been widely studied for lowering cadmium (Cd) uptake by plants but information regarding their comparative effectiveness is lacking. In this study, biochars from different feedstocks viz. rice husk biochar (RHB), cotton sticks biochar (CSB) and wheat straw biochar (WSB) were compared with conventional organic materials viz. farm manure (FM), poultry manure (PM) and press mud (PrMd) for their effectiveness to promote plant growth and to reduce Cd uptake by wheat and rice plants grown rotationally in a Cd-spiked (50 mg kg^-1) soil. Each amendment was applied at the rate of 2% (w/w) in three replicates. Results showed that the application of amendments improved the soil properties and plant growth, by retaining Cd in the soil and restricting its uptake by plants. The amendments decreased the ammonium bicarbonate diethylene penta acetic acid extractable soil Cd, and improved soil organic carbon (SOC) and cation exchange capacity (CEC) as compared to only Cd-contaminated soil. The highest SOC content of 2.68 and 1.68% and CEC of 8.77 and 9.39 cmol_c kg^-1 were found in RHB treated post-wheat and post-rice soil, respectively. Amendments treated soil showed lower concentrations of bioavailable Cd and the maximum reduction was recorded in RHB and PrMd amended soil. Similarly, bioaccumulation of Cd was decreased with the application of all amendments; the maximum decrease was recorded in RHB and PrMd treated soil. Our results suggested that RHB and PrMd could be used for reducing the bioaccumulation of Cd in cereal grains in alkaline soils.

Agricultural use of Samarco's spilled mud assessed by rice cultivation: A promising residue use?

Mining activity is one of the main responsible for accumulation of potentially toxic elements in the environment. These contaminants are absorbed by plants served as food that could be a risk to human health, such rice. Rice is a staple food with known accumulation of toxic elements. The recent collapse of a mining dam operated by Samarco Mining Company spilled around 50 million m³ of Fe-mining waste in the environment, including rivers and farming areas. In the present study, concentrations of As, Cd, Hg, Pb, Co, Zn, Mn, Cu, Fe, Al, Se, and Sr were determined in soils, roots and grains of rice plants cultivated in soil containing Samarco's residual mud (0, 16, 34 and 50%). Further, rice plant agronomic parameters (chlorophyll, carotenoids, grain yield, mass, height) were assessed. Rice cultivated at Samarco's residual mud produced grains with low levels of As, Cd and Pb. However, the excess of mud (50%) during the rice cultivation reduced roots' growth and grains yield. Chlorophyll (a and b) and carotenoids contents were significantly lower in all mud cultivations, mainly mud-50%. Our findings suggest that plant alterations induced by the mud were associated to the deficiency of nutrients and the physical properties of the mud. Soil fertilization by organic matter and top soil provided conditions for plant development. Therefore, considering the experimental conditions here used, we showed that is possible the use of the affected land for agriculture and reforestation after soil amendment.

Reducement of cadmium adsorption on clay minerals by the presence of dissolved organic matter from animal manure

Clay minerals are the most popular adsorbents/amendments for immobilizing heavy metals in contaminated soils, but the dissolved organic matter (DOM) in soil environment would potentially affect the adsorption/immobilization capacity of clay minerals for heavy metals. In this study, the effects of DOM derived from chicken manure (CM) on the adsorption of cadmium (Cd²⁺) on two clay minerals, bentonite and zeolite, were investigated. The equilibrium data for Cd²⁺ sorption in the absence or presence of CM-DOM could be well-fitted to the Langmuir equation (R² > 0.97). The presence of CM-DOM in the aqueous solution was found to greatly reduce the adsorption capacity of both minerals for Cd²⁺, in particular zeolite, and the percentage decreases for Cd²⁺ sorption increased with increasing concentrations of Cd²⁺ as well as CM-DOM in aqueous solutions. The adsorption of CM-DOM on zeolite was greater than that on bentonite in the absence of Cd²⁺, however, a sharp increase was observed for CM-DOM sorption on bentonite with increasing Cd²⁺ concentrations but little change for that on zeolite, which can be attributed to the different ternary structures on mineral surface. The CM-DOM modified clay minerals were utilized to investigate the effect of mineral-adsorbed CM-DOM on Cd²⁺ sorption. The adsorbed form was found to inhibit Cd²⁺ sorption, and further calculation suggested it primarily responsible for the overall decrease in Cd²⁺ sorption on clay minerals in the presence of CM-DOM in aqueous solutions. An investigation for the mineral surface morphology suggested that the mineral-adsorbed CM-DOM decreased Cd²⁺ sorption on bentonite mainly through barrier effect, while in the case of zeolite, it was the combination of active sites occupation and barrier effect. These results can serve as a guide for evaluating the performance of clay minerals in immobilizing heavy metals when animal manure is present in contaminated soils.

Assessment of the hazard posed by metal forms in water and sediments

This study aimed to describe the prevalence heavy metals (Zn, Cu, Pb, and Cd) forms in the ecosystem of the Utrata river in order to determine the mobile forms and bioavailability of metals. To extract the dissolved forms of metals in the water of the Utrata PHREEQC2 geochemical speciation model was used. The river waters show a high percentage of mobile and eco-toxic forms of Zn, Cu and Pb. The percentage of carbonate forms for all the studied metals was low (<1%). The content of carbonates in the water and the prevailing physical and chemical conditions (pH, hardness, alkalinity) reduce the share of toxic metal forms, which precipitate as hardly soluble carbonate salts of Zn, Cu, Cd and Pb. Cu in the water in 90% of cases appeared in the form of hydroxyl compounds. To identify the forms of metal occurrence in the sediments Tessier's sequential extraction was used, allowing to assay bound metals in five fractions (ion exchange, carbonate, adsorption, organic, residual), whose nature and bioavailability varies in aquatic environments. The study has shown a large share of metals in labile and bioavailable forms. The speciation analysis revealed an absolute dominance of the organic fraction in the binding of Cu and Pb. Potent affinity for this fraction was also exhibited by Cd. The rations of exchangeable Zn and Cu forms in the sediments were similar. Both these metals had the lowest share in the most mobile ion exchange fraction.

Heavy metal contamination and ecological risk in Futian mangrove forest sediment in Shenzhen Bay, South China

Surface sediments in the Futian mangrove forest (South China) were analyzed for heavy metals including cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb) and zinc (Zn). The heavy metal distributions varied greatly in surface sediments, reflecting some sediment heterogeneity. The heavy metal concentrations decreased in the order of Zn>Cr>Pb>Cu>Cd. According to the mean probable effects level quotient, the combination of studied metals had a 21% probability of being toxic. The potential ecological risk index and geo-accumulation index also revealed high metal contamination. Cd was of primary concern due to its higher assessment values and potential for adverse biological effects. Multivariate analysis implied that clay and silt played a significant role in raising the levels of Cr, Cu and Zn. The percentage of mobile heavy metals was relatively higher, without considerable ecological risk to the biota based on the risk assessment code.

Ecotoxicological risk assessment of trace metals in humid subtropical soil

In this work, several physicochemical properties of sub-tropical soil (up to 20 cm depth) like water holding capacity, organic carbon content, cation exchange capacity, texture, pH, and electrical conductivity were determined along with the trace metals, Co, Cr, Cu, Mn, Ni, Pb and Zn, in order to evaluate inter-relations among the trace metals and the soil properties. The contribution of the trace metals to ecotoxicological risk was assessed using various tools. Cr, Cu, Mn and Zn contents were found to be lower than the world average, but Co, Ni, and Pb had higher contents. The trace metal concentrations were utilized to obtain the pollution index and the potential ecotoxicological aspects. The trace metals were shown to have come from similar origin and their retention in the soil was contributed by properties like organic carbon, cation exchange capacity, clay content and water holding capacity of the soil. The pollution index showed that the trace metals had the sequence of Pb (considerably polluted) > Co, Ni (moderately polluted) > Cr, Cu, Mn and Zn (unpolluted). The composite ecological risk index was the highest in agricultural land with irrigation and fertilizer use, and was the lowest in the forest land.

Spatial variation, environmental risk and biological hazard assessment of heavy metals in surface sediments of the Yangtze River estuary

30 samples of eight heavy metals were collected in February 2011 within Yangtze River estuary (YRE). The mean concentrations met the primary standard criteria based on Marine Sediments Quality of China. The spatial distribution showed that a gradient concentration decreased gradually from inner-estuary to river mouth. Anthropogenic inputs might be the main contributor, and fine grained sediments might also aggravate the heavy metal contamination. The assessment results indicated that the YRE was in low risk of contamination caused by every single heavy metal. However, it was in considerable degree of contamination considering combination of all the heavy metals. The toxicities of heavy metals might be elevated when heavy metals were in combination. Arsenic should be of primary concern due to its higher assessment values and the potential of adverse biological effects. And the concentration of As in the YRE had a trend to increase because of anthropogenic activities nearby.

Effect of fulvic acid surface coatings on plutonium sorption and desorption kinetics on goethite

The rates and extent of plutonium (Pu) sorption and desorption onto mineral surfaces are important parameters for predicting Pu mobility in subsurface environments. The presence of natural organic matter, such as fulvic acid (FA), may influence these parameters. We investigated the effects of FA on Pu(IV) sorption/desorption onto goethite in two scenarios: when FA was (1) initially present in solution or (2) found as organic coatings on the mineral surface. A low pH was used to maximize FA coatings on goethite. Experiments were combined with kinetic modeling and speciation calculations to interpret variations in Pu sorption rates in the presence of FA. Our results indicate that FA can change the rates and extent of Pu sorption onto goethite at pH 4. Differences in the kinetics of Pu sorption were observed as a function of the concentration and initial form of FA. The fraction of desorbed Pu decreased in the presence of FA, indicating that organic matter can stabilize sorbed Pu on goethite. These results suggest that ternary Pu-FA-mineral complexes could enhance colloid-facilitated Pu transport. However, more representative natural conditions need to be investigated to quantify the relevance of these findings.

Coating of AFM probes with aquatic humic and non-humic NOM to study their adhesion properties

Atomic force microscopy (AFM) was used to study interaction forces between four Natural Organic Matter (NOM) samples of different physicochemical characteristics and origins and mica surface at a wide range of ionic strength. All NOM samples were strongly adsorbed on positively charged iron oxide-coated silica colloidal probe. Cross-sectioning by focused ion beam milling technique and elemental mapping by energy-filtered transmission electron microscopy indicated coating completeness of the NOM-coated colloidal probes. AFM-generated force-distance curves were analyzed to elucidate the nature and mechanisms of these interacting forces. Electrostatics and steric interactions were important contributors to repulsive forces during approach, although the latter became more influential with increasing ionic strength. Retracting force profiles showed a NOM adhesion behavior on mica consistent with its physicochemical characteristics. Humic-like substances, referred as the least hydrophilic NOM fraction, i.e., so called hydrophobic NOM, poorly adsorbed on hydrophilic mica due to their high content of ionized carboxyl groups and aromatic/hydrophobic character. However, adhesion force increased with increasing ionic strength, suggesting double layer compression. Conversely, polysaccharide-like substances showed high adhesion to mica. Hydrogen-bonding between hydroxyl groups on polysaccharide-like substances and highly electronegative elements on mica was suggested as the main adsorption mechanism, where the adhesion force decreased with increasing ionic strength. Results from this investigation indicated that all NOM samples retained their characteristics after the coating procedure. The experimental approach followed in this study can potentially be extended to investigate interactions between NOM and clean or fouled membranes as a function of NOM physicochemical characteristics and solution chemistry.

Adsorption of Cd to natural biofilms in the presence of EDTA: effect of pH, concentration, and component addition sequence

Both dissolved organic matters (DOM) and natural biofilms are important substances in controlling the behavior of trace metals in natural aquatic environments. In this study, ethylenediaminetetraacetic acid (EDTA) was selected as a typical DOM to investigate the effect of DOM on the adsorption of trace metals to the biofilms in natural waters. The adsorption of Cd to biofilms, including adsorption isotherm at a fixed pH (pH = 6.0) and pH-edge adsorption (pH ranging from 4.3 to 9.0) with different adsorption sequences, was determined without EDTA and in the presence of EDTA ([EDTA] = 0.5 μmol/L for isotherms measurement and [EDTA] = 0.5 and 2.0 μmol/L for pH-dependent adsorption). The presence of EDTA generally decreased the adsorption of Cd, and the effect was determined by solution pH, concentration of EDTA, and adsorption sequence. Higher concentration or higher pH usually resulted in a more significant decrease. The influence of adsorption sequence on the effect of EDTA was insignificant in lower pH range, while the adsorption usually decreased in the order of Cd only adsorption > Cd first adsorption > EDTA first adsorption ≈ simultaneous adsorption in higher pH range. The effect of EDTA could be attributed to the conversion of Cd speciation, the competition with the biofilms for Cd, and the dissolution of Mn oxides from the biofilms. EDTA affected the adsorption of Cd to natural biofilms, and the effect could be fairly significant. The role of Mn oxides in determining the behavior of trace metals might be underestimated.

Concentration and fractionation of trace metals in surface sediments of intertidal Bohai Bay, China

Surface sediments from intertidal Bohai Bay were sampled for the geochemical and environmental assessment of six trace metals (Cd, Cr, Cu, Ni, Pb and Zn). Results indicate that sediment grain size plays an important role in controlling the distribution and fractionation of them. Metal concentrations in clayey silt sediments are all clearly higher than in sand and silty sand ones. Cd and Pb in clayey silt sediments are more mobile than in sand and silty sand ones. Two sediment quality guidelines and two geochemical normalization methods (index of geoaccumulation and enrichment factor) were used to judge the potential risk and accumulation of metals. According to the mean probable effects level quotient, the combination of studied metals may have a 21% probability of being toxic. The sediments with high fraction of clay and silt have been contaminated by trace metals to various degrees, among which Cr contributes the most to contamination.

Heavy metals: their pathway from the ground, groundwater and springs to Lake Góreckie (Poland)

The migration pathways of heavy metals derived from an area previously in agricultural use was investigated in the Wielkopolski National Park (mid-western Poland). The heavy metals involved (Cd, Cu, Cr, Pb, Ni and Zn) were determined in groundwater, the springs that feed Lake Góreckie and the lake itself. In order to show how the heavy metals may be set free and what is their biological availability, soil and sediment samples were subjected to single-stage extraction, using 0.01 M CaCl(2), 0.02 M EDTA, 0.005 M DTPA, 0.1 M HCl, 1 M HCl and de-ionised water. Varying metal concentrations were recorded in the water samples during the study period (from November 2009 to July 2010), usually with higher values in winter and lower ones in summer. The seasonal changes may be ascribed to natural processes taking place in the ground- and surface waters of Lake Góreckie. On the other hand, the concentration levels (mostly of Cd, Pb and Cr) are indicative of anthropogenic activity. It should be mentioned in this context that the highest metal concentrations were found in the soil layer. The concentrations were also found to exceed both the Polish and the World Health Organization water-quality standards. It appears that the soils are highly contaminated, mostly with cadmium. The long-lasting effect of acid precipitation in the area makes it possible for immobile forms to become mobile, thus facilitating further migration into the environment.

Comparison of advanced oxidation processes for the removal of natural organic matter

This study examined the impact of UV, ozone (O(3)), advanced oxidation processes (AOPs) including O(3)/UV, H(2)O(2)/UV H(2)O(2)/O(3) in the change of molecular weight distribution (MWD) and disinfection by-product formation potential (DBPFP). Bench-scale experiments were conducted with surface river water and changes in the UV absorbance at 254 nm (UV(254)), total organic carbon (TOC), trihalomethane and haloacetic acid formation potential (THMFP, HAAFP) and MWD of the raw and oxidized water were analyzed to evaluate treatment performance. Combination of O(3) and UV with H(2)O(2) was found to result in more TOC and UV(254) reduction than the individual processes. The O(3)/UV process was found to be the most effective AOP for NOM reduction, with TOC and UV(254) reduced by 31 and 88%, respectively. Application of O(3)/UV and H(2)O(2)/UV treatments to the source waters organics with 190-1500 Da molecular weight resulted in the near complete alteration of the molecular weight of NOM from >900 Da to <300 Da H(2)O(2)/UV was found to be the most effective treatment for the reduction of THM and HAA formation under uniform formation conditions. These results could hold particular significance for drinking water utilities with low alkalinity source waters that are investigating AOPs, as there are limited published studies that have evaluated the treatment efficacy of five different oxidation processes in parallel.

Sorption and desorption of imidazolium based ionic liquids in different soil types

This study investigates the influence of the two different clay minerals kaolinite and smectite as well as of organic matter on the cation sorption and desorption behaviour of three imidazolium based ionic liquids -1-butyl-3-methyl-imidazolium tetrafluoroborate (IM14 BF(4)), 1-methyl-3-octyl-imidazolium tetrafluoroborate (IM18 BF(4)) and 1-butyl-3-methyl-imidazolium bis[(trifluoromethyl)sulfonyl]imide (IM14 (CF(3)SO(2))(2)N) - in soil. The German standard soil Lufa 2.2 - a natural soil classified as a loamy sand - was the basis substrate for the different soil compositions and also served as a reference soil. The addition of organic matter and clays increases the sorption of the substances and in particular smectite had striking effects on the sorption capacity for all three ionic liquids indicating that ionic interactions play an important role for sorption and desorption processes of ionic liquids in soil. One exception was for kaolinite-containing soils and the IM14 cation: with (CF(3)SO(2))(2)N(-) as an anion the sorption was identical at either 10 wt% or 15 wt% clay content, and with BF(4)(-) sorption was even lower at 15 wt% kaolinite than at 10 wt%. Desorption was weak for IM18 BF(4), presumably owing to the longer alkyl side chain. With regard to the influence of kaolinite on desorption, the same pattern was observed as it was found for the sorption of IM14 BF(4) and IM14 (CF(3)SO(2))(2)N.

Solid phase extraction of chromium(VI) from aqueous solutions by adsorption of its diphenylcarbazide complex on a mixed bed adsorbent (acid activated montmorillonite-silica gel) column

A novel approach has been developed for the solid phase extraction of chromium(VI) based on the adsorption of its diphenylcarbazide complex on a mixture of acid activated montmorillonite (AAM)-silica gel column. The effect of various parameters such as acidity, stability of the column, sample volume, interfering ions, etc., were studied in detail. The adsorbed complex could be easily eluted using polyethylene glycol-sulfuric acid mixture and the concentration of chromium has been determined using visible spectrophotometry. The calibration graph was linear in the range 0-1microgmL(-1) chromium(VI) with a detection limit of 6microgL(-1). A highest preconcentration factor of 25 could be obtained for 250mL sample volume using glass wool as support for the mixed bed adsorbent. Chromium(VI) could be effectively separated from other ions such as nickel, copper, zinc, chloride, sulfate, nitrate, etc., and the method has been successfully applied to study the recovery of chromium in electroplating waste water and spiked water samples.

Effects of organic matter on the distribution of uranium in soil and plant matrices

This work studied interactions of uranium with pure organic compounds, such as glutathione, and more complex mixtures, such as humic acid and aqueous plant extracts. High performance liquid chromatography with UV absorption interfaced to inductively coupled plasma mass spectrometry sequential detection was used to detect organouranium complexes in a variety of soils and plant materials, indicating that nearly 100% of the uranium extracted from certain plant tissues was bound to organic ligands. In addition, soil sorption experiments indicated that humic acid generally decreased uranium sorption to soils and promoted subsequent desorption of uranium because of uranium partitioning to the organic phase. These experiments demonstrate that organic compounds influence the mobility and chemistry of uranium in the environment.

Characterization of sewage plant hydrocolloids using asymmetrical flow field-flow fractionation and ICP-mass spectrometry

Asymmetrical flow field-flow fractionation (AF4) was applied to characterize aquatic colloids from biological sewage plants and to infer information of colloidal loads, sources, and sinks within the plants, resp. the colloidal interaction with the aqueous phase and the sewage sludge. To characterize the colloids further, especially the distributions of colloid associated heavy metals, the AF4 system was coupled to an inductively coupled plasma mass spectrometer (ICP-MS). The size distribution is determined by AF4 with UV absorbance and fluorescence detection after a calibration by monodisperse polystyrene sulfonate standards (PSS). Samples from different sewage plants and from different depths and locations within a plant were compared. The fulvic/humic acid fraction with a particle diameter d(p) < 10 nm appeared to be comparable in all samples and decreases only slightly along the plants, whereas larger colloids with d(p) > 10 nm almost completely passed into the sewage sludge. The concentrations of the initial colloidal heavy metals decreased along the plants.

Field-flow fractionation-inductively coupled plasma mass spectrometry: an alternative approach to investigate metal-humic substances interaction

Interaction between metal ions and humic matter was investigated using a hyphenated technique, field-flow fractionation-inductively coupled plasma mass spectrometry (FFF-ICP-MS). Aggregation of a metal-spiked commercial Aldrich humic acid in an aqueous solution of calcium ion or in seawater was examined over time intervals of 0-4320 min. The aggregation was demonstrated by shifts in peak maximum of humic matter from smaller size (2.9 nm) to larger size (5.1 or 5.8 nm in Ca2+ solution or in seawater, respectively) and also by the broadening of size distribution profiles. With FFF, size distribution of humic aggregate was characterized. Further, dominant particle size (2.9 nm), mean particle size (3.8 nm), and diffusion coefficient (1.51 x 10(-6) cm2/s) of humic acid solution were determined. With FFF-ICP-MS, associations of Cd, Cu, and Pb with humic aggregates were examined. The mean diameters of Cd-, Cu-, and Pb-bound humic aggregates in the metal-spiked humic acid were 4.1, 4.5, and 5.8 nm, respectively. These diameters were shifted to 6.0, 6.0, and 6.9 nm, respectively, in the humic acid incubated with calcium solution, whereas they were shifted to 6.5, 5.7, and 7.4 nm, respectively, in the humic acid incubated with seawater for three days. Humic aggregate of small size showed more affinity for Cu than Cd and Pb, whereas the large aggregate showed more affinity for Pb than Cd and Cu, respectively.

Characterzation of colloidal and humic-bound Ni and U in the "dissolved" fraction of contaminated sediment extracts

The dissolved phase of environmental aqueous samples is generally defined by filtration at 0.2 microm or even 0.45 microm. However, it is also acknowledged that colloids <0.2 microm suspended in the aqueous phase can be important for determining contaminant availability and mobility. We have used flow field-flow fractionation (FI FFF) and size exclusion chromatography (SEC) coupled to UV-absorbance (UVA) and inductively coupled plasma mass spectrometry (ICP-MS) to study the dissolved organic matter (DOM) and colloidal binding of U and Ni in water extracts of sediments collected from a contaminated area of the Savannah River Site, a U.S. Department of Energy former nuclear materials production and processing facility, near Aiken, SC. High-performance SEC-UVA-ICP-MS was well-suited to the separation of DOM overthe molecular weight (MW) range of approximately 200-7000 Da. The ICP-MS element specific data indicated that a significant fraction of U was associated with DOM. Uranium exhibited a bimodal distribution and the other fraction was greater than the exclusion limit for the column and coeluted with Al. Flow FFF was used to size this fraction as colloidal with an approximate effective spherical diameter of 0.09-0.12 microm. Element specific ICP-MS data confirmed that U and Al were associated with the colloidal phase. High-field FI FFF was also applicable to sizing DOM but resolution was poorer than SEC. The results of this study suggest that "dissolved" U at this site is predominantly either complexed by DOM or bound to a colloidal fraction while Ni is predominately present as labile complexes or the free cation and, therefore, potentially bioavailable.

Assessment of the geochemical role of colloids and their impact on contaminant toxicity in freshwaters: an example from the Lambro-Po system (Italy)

The role of colloids in regulating element transport, behavior, and bioavailability in aquatic systems is now well-established. It appears that further progress in this research field is being slowed by (i) a limited integration between the geochemical and the biological aspects of the research on colloids and (ii) a persistent gap between well-controlled laboratory studies and real field situations. This paper presents a simultaneous evaluation of the role of colloids in controlling element environmental fate and bioavailability at the confluence between a major river and a polluted tributary. Fractionation of trace elements among suspended particulate matter, colloids, and true solution suggests that colloids may play a role in the removal of trace elements from the water column to bed sediments during the mixing of the two rivers. Toxicity testing of water samples indicates that, in this specific system, contaminants associated with colloids can contribute to water toxicity for the rotifer Brachionus calyciflorus but not for the green alga Pseudokirchneriella subcapitata. To the best of our knowledge, the results for B. calyciflorus are the first ones pointing to the possible contribution of colloid-bound contaminants to water toxicity in environmental samples. Despite the uncertainties associated with field variability, the results of chemical analysis and toxicitytesting show several points of convergence. Following these observations, a few innovative research approaches are suggested to improve the understanding of trace element biogeochemistry in real field situations.

Hydrolysis of carbaryl by carbonate impurities in reference clay SWy-2

The influence of clay preparation methods on the sorption and hydrolysis of carbaryl (1-naphthyl, N-methyl carbamate) by K+-saturated reference smectite SWy-2 was studied. Four methods were utilized: (1) The reference (or specimen) clay used as received was K+-saturated (hereafter referred to as whole clay). (2) High-speed centrifugation (3295g) of whole clay resulted in a pellet with three discrete bands. The upper, light-colored, low-density band was obtained by manual separation (light fraction). The high-density, dark-colored material comprising the lower band (heavy fraction) was also obtained manually. (3) SWy-2 was subjected to overnight gravity sedimentation to obtain the <2 microm particles (clay-sed.) and then K+-saturated. (4) SWy-2 was subjected to low-speed centrifugation (58-60g) to separate the <2 microm particle size (clay-cent.) and then K+-saturated. Each preparation of mineral fractions manifested significantly different abilities to hydrolyze carbaryl to 1-naphthol, decreasing in the order whole clay > heavy fraction >> clay-sed. > light clay > clay-cent. The extent of 1-naphthol disappearance from solution, accompanied by a progressive darkening of the clay, followed the order whole clay > heavy fraction >>> light clay > clay-sed. > clay-cent. Using ring labeled [14C]carbaryl, approximately 61 and 15% of the total 14C activity added to the whole clay and light fraction, respectively, remained unextractable. X-ray diffraction of the heavy fraction revealed several peaks corresponding to minor impurities, including calcite and dolomite. Aqueous slurries of whole clay, light fraction, clay-sed., and heavy fraction were alkaline, whereas the pH of slurried clay-cent. was neutral. It was concluded that dissolution of inorganic carbonate impurities in SWy-2 caused alkaline conditions in the slurries leading to the hydrolysis of carbaryl. Dissolution of carbonates with sodium acetate buffer eliminated hydrolytic activity associated with SWy-2. None of the four preparation methods reliably removed inorganic carbonates. The use of commercial or reference smectites in surface chemistry studies should be accompanied by a treatment with acetate buffer to remove carbonate impurities.

Predicting the rate and extent of cadmium and copper desorption from soils in the presence of bacterial extracellular polymer

The movement of cationic transition metals through the subsurface is strongly retarded by sorption to the porous media. However, dissolved organic ligands can compete with soil surfaces by providing binding sites for metals in solution. An extracellular polymer produced by a bacterium isolated from soil was used in this study to observe and model the influence of a naturally occurring ligand on the release of adsorbed metals from two test soils. Experimental results show that the presence of dissolved extracellular polymer enhanced the rate and extent of desorptive release of soil-bound cadmium and copper. A kinetic model that uses a gamma distribution of rate constants to account for the physical and chemical heterogeneity of the soil matrix was employed to describe the release of cadmium and copper in batch experiments. Model parameters describing soil, metal and extracellular polymer interactions were obtained through separate experiments. With these parameters the model successfully predicted the influence of dissolved polymer on the rate and extent of release of cadmium and copper from soil in independent batch experiments. These results suggest that the presence of natural metal-binding ligands such as bacterial extracellular polymers can act to increase the driving force for desorption by lowering the aqueous concentration of free unbound metals in solution.