Organic Matter-bound Fractions Research Articles

Lead in paint-contaminated residential soils with varying physicochemical properties from three large US cities: assessment of geochemical forms and recommendations on amendments for immobilization

Lead in yard soils has been recognized as the principal source of excess lead absorption among young children. The hazard imposed by soil lead is dependent on the geochemical forms of lead in soils. Soil properties such as pH, soil organic matter, clay, and carbonate content influence the geochemical forms of lead in soil. This study was conducted to investigate the correlation between soil properties and the geochemical speciation of lead in lead paint-contaminated residential soils from three major US cities. A comprehensive field survey was conducted, involving the collection of soils from ten houses in each of the cities: Baltimore, San Antonio, and Detroit. The influence of soil properties on geochemical speciation was analyzed to identify effective immobilization amendments for each soil type. Results showed that soils collected from San Antonio were slightly alkaline, whereas those from Baltimore were slightly acidic. Soils collected from Detroit were neutral to mildly alkaline in pH. San Antonio soils had relatively high soil salinity, high clay content, moderate to high soil organic matter (SOM), and high total carbon (TC). In contrast, soils collected from Baltimore had lower salinity and clay content, low SOM, and total carbon. Soils from Detroit exhibited relatively high salinity, clay, SOM, and TC contents. The average total soil lead concentrations were as follows; San Antonio 4,073 mg/kg, Baltimore 2,706 mg/kg, and Detroit 850 mg/kg. Geochemical speciation studies revealed significant differences in lead distribution among the studied soils. San Antonio soils exhibited high carbonate-bound and organic matter-bound fractions, while Baltimore soils had elevated soluble + exchangeable fractions. Detroit soils showed substantial lead in organic matter-bound fractions. Correlation analysis showed that the soil properties influencing exchangeable lead, were pH, total Al, and total Ca for San Antonio soils; pH and total P for Baltimore soils; and SOM and total Al for Detroit soils. Correlation analysis showed that there is a significant negative correlation (p < 0.05) between exchangeable lead and total Al (r = −0.653), and total Ca (r = −0.438) for San Antonio soils; pH (r = −0.286) and total p (r = −0.314) for Baltimore soils; and SOM (r = −0.628) and total Al (r = −0.408) for Detroit soils. Based on these results, the best potential immobilization amendments for each of these cities were predicted.

Geochemical Contamination, Speciation, and Bioaccessibility of Trace Metals in Road Dust of a Megacity (Guangzhou) in Southern China: Implications for Human Health

Road dust has been severely contaminated by trace metals and has become a major health risk to urban residents. However, there is a lack of information on bioaccessible trace metals in road dust, which is necessary for an accurate health risk assessment. In this study, we collected road dust samples from industrial areas, traffic intersections, and agricultural fields from a megacity (Guangzhou), China, and conducted a geochemical enrichment, speciation, and bioaccessibility-based health risk assessment of trace metals. In comparison with local soil background values, the results revealed a significant accumulation of trace metals, including Zn, Cd, Cu, and Pb in the road dust, which is considered moderate to heavy pollution. Sequential extraction indicated that most trace metals in the road dust were primarily composed of a Fe/Mn oxide-bound fraction, carbonate-bound fraction, and residual fraction, while the dominant fraction was the organic matter-bound fraction of Cu, and the residual fractions of As, Cr, and Ni. The in vitro gastrointestinal (IVG) method revealed that high percentages of Zn, Cd, Cu, and As were bioaccessible, suggesting the possible dissolution of trace metals from adsorbed and carbonate-associated fractions in road dust exposed to the biological fluid matrix. The IVG bioaccessibility-based concentration largely decreased the noncarcinogenic health risk to a negligible level. Nevertheless, the entire population is still exposed to the cumulative probability of a carcinogenic risk, which is primarily contributed to by As, Cd, Cr, and Pb. Future identification of the exact sources of these toxic metals would be helpful for the appropriate management of urban road dust contamination.

Cr(VI) immobilization in soil using lignin hydrogel supported nZVI: Immobilization mechanisms and long-term simulation

A novel nanocomposite, named as nZVI@LH, was prepared by nanoscale zero-valent iron (nZVI) supported on lignin hydrogel and was used in the remediation of Cr(VI)-contaminated soil collected from an industrial site. Meanwhile, scanning electron microscopy with energy dispersive X-ray (SEM-EDX) and X-ray diffractometry (XRD) results determined that nZVI nanoparticles disperse uniformly on hydrogel. After the 14 days remediation, the immobilization efficiency of Cr(VI) could reach over 87% in the treatment of 3% (w/w%) nZVI@LH and 26% in the treatment of bare-nZVI. Leaching experiment results showed that the treatment group with 3% (w/w%) nZVI@LH was up to the national leaching toxicity identification standard, and there was no threat in simulation of acid rain over the long term. The water-soluble (WS) fraction in 3# nZVI@LH treatment decreased 31.1%, while the Fe–Mn oxide bound (OX) fraction and organic matter-bound (OM) fraction increased 10.9% and 13.4%, respectively. Moreover, nZVI@LH had limited impact on soil properties and the capability to immobilize Cr over a long period exposure to acid rain. This work prove that nZVI@LH has the potential to remediate Cr contaminated soil. Furthermore, details of possible mechanistic insight into the Cr remediation were carefully discussed.

Transformation of Chromium Speciation during High Hexavalent Chromium-Contaminated Soil Remediation by CPS and Biostimulation

To address the secondary pollution problem of chemical reduction and the defects of a long bioremediation period for the soil around a chromium (Cr) salt plant in China, calcium polysulfide (CPS) combined with biostimulation (adding nutrient solution with glucose and urea) was used to reduce and stabilize hexavalent chromium [Cr(VI)] in the soil. The results showed that the remediation effect of adding CPS and nutrient solution was better than that of CPS alone for Cr(VI)-contaminated soil. An amount of 2.15% CPS with the Cr(VI) concentration reduced to approximately 330.0 mg/kg was selected as the optimal reducing agent dosage. Following the addition of 11 g/kg glucose and 13 g/kg urea, the degradation rate of Cr(VI) reached over 92% after 3 days, and the water-soluble fraction decreased by 95.6% after 40 days. The stability of the Cr was significantly enhanced. The correlation and regression analyses of Cr speciation indicated that the water-soluble fraction had significant negative correlations with the iron-manganese (Fe-Mn) oxide-bound fraction and the organic matter-bound fraction. There were significant negative correlations between the exchangeable fraction and the carbonate-bound fraction as well as extremely significant positive correlations between the Fe-Mn oxide-bound fraction and the organic matter-bound fraction. It was speculated that both the Fe-Mn oxide-bound fraction and the organic matter-bound fraction were transformed from the water-soluble fraction, and the exchangeable fraction and the carbonate-bound fraction were mutually transformed. This study enables us to acquire more knowledge about the speciation transformation of Cr in soil and provides an efficient, low-cost, and low-risk technology for the remediation of high Cr(VI)-contaminated soil.

Immobilization of cadmium in contaminated soils using sulfidated nanoscale zero-valent iron: Effectiveness and remediation mechanism

Sulfidated nanoscale zero-valent iron (S-nZVI) has shown excellent removal capacity for the removal of cadmium (Cd) in aqueous phase. Herein, the effectiveness and the mechanism of S-nZVI for the remediation of Cd contaminated soil were investigated for the first time. The results of sequential extraction procedures (SEP) showed that the exchangeable (EX) Cd was decreased by over 97.6% at the optimal dosage of 5 g kg-1 S-nZVI during 30 d incubation and converted to less available Cd such as iron-manganese oxides-bound (OX) and organic matter-bound (OM) fractions. pH has negligible effect on the immobilization of Cd in soil, since OX fraction was stabilized in the range of 72–92% at initial soil pH range from 5.3 to 7.5. SEM-EDS analysis of the separated magnetic particles implied that Cd was successfully enriched on S-nZVI and the distribution of Cd was closely related to Fe, S, and O. CdO and CdS was confirmed as the key products for Cd immobilization in soil. Meanwhile, the S-nZVI was oxided to α-FeOOH, γ-FeOOH, and γ-Fe2O3. The existence of CdO was visibly related to the iron oxides, suggesting the synergetic immobilization effect by iron oxides. Overall, S-nZVI was promising for the remediation of Cd-contaminated soil.

Effects of nearly four decades of long-term fertilization on the availability, fraction and environmental risk of cadmium and arsenic in red soils

Fertilizers are important for agricultural production because they can effectively promote crop productivity. However, long-term fertilization can cause heavy metal accumulation in soils and crops. This study utilized sequential extraction, the diffusive gradient in the thin films (DGT) technique and risk assessment models to estimate the effects of the longest long-term fertilization (38 years) in China on cadmium (Cd) and arsenic (As) accumulation in soils. The treatments included no fertilization (CK); inorganic nitrogen, phosphorus, and potassium fertilization (NPK); manure fertilization (M); and NPK plus M cofertilization (NPKM). The results indicated that the soils treated with NPKM, M and NPK had significantly increased total and available concentrations of Cd and As after 38 years of long-term fertilization. Cd mainly originates from cattle manure, while As originates from phosphate fertilizer. Sequential extraction results indicated that the application of manure increased the acid/exchangeable fraction (F1) and organic matter-bound fraction (F3) of Cd and As. The risk assessment results showed that the environmental risks of both Cd and As increased during long-term fertilization, and Cd contamination in the soil was at a moderate-high level, while As remained at a relatively low level. According to the calculations of the maximum numbers of years of soil productivity and rice production, Cd was labile and accumulated in the soils, and As was more labile than Cd in terms of accumulating in rice, indicating that the true risk from As in rice is higher than that from Cd. Controlling the heavy metals in fertilizers, mitigating effective amendments, and identifying plant types that accumulate low amounts of contaminants may be good choices for cleaner crop production.

Occurrence and mobility of trace elements during oxidant stimulation of shales in Yichang, Hubei province of China

Oxidant stimulation is a promising technology for shale permeability enhancement, but it is still faced with the problem of produced water with high total dissolved solids. For the consideration of environmental protection and water reuse, the mobility of trace elements (TEs) should be evaluated before the in-situ application of this technology. In this study, carbonate-rich shale and silicate-rich shale, collected in Yichang, Hubei province of China, were used to react with sodium persulfate (Na2S2O8) at different experimental conditions. The sequential chemical extraction was used to analyze the occurrence of TEs in shales and their mobilization mechanism. Influence factors including oxidant concentration, solid:liquid ratio, initial pH and temperature were systematically investigated to explore their effect on the mobility of TEs. Results showed that a similar affinity between TEs and fractionation phases was observed in both two shales. Tough the TEs associated with residual fraction was most obvious in shales, various associations between TEs and other extractable fractions were also apparent, such as Co (61.7% on average) and Cu (59.2% on average) in organic matter-bound fraction, and Sr (53.2% on average) in carbonates-bound fraction. The occurrence of TEs determined their extent of mobilization during oxidant stimulation, and the complex interactions including acidization, oxidation, adsorption and precipitation influence the mobility of TEs as well. The carbonate minerals and pyrite were both critical minerals for decreasing the mobility of TEs. Carbonate minerals in shale could effectively buffer the pH of the system to mitigate the acidization reaction. The near-neutral environment was also beneficial for the Fe(OH)3 precipitation then resulted in the fixation of TEs through adsorption. Besides, the generated gypsum from carbonate dissolution also contributed to the incorporation of TEs. Meanwhile, sulfate generated from pyrite oxidation and persulfate hydrolysis also directly precipitated with TEs such as Ba and Sr to reduce their dissolved content in the reaction system. Influence factors analysis showed that the mobility of TEs was strongly dependent on the oxidant concentration and solid: liquid ratio and to a lesser extent on the temperature and initial pH. Considering the environmental risk and water management cost, systematic investigation on shale composition and associated TEs, and optimization of engineering parameters should be carried out before the in-situ application of this technology.

PGPR inoculation of a contaminated soil affects plant growth and phytoavailability of Cd and Pb

ABSTRACT To evaluate the effects of cabbage varieties and PGPR inoculation on the mobility and phytoavailability of Pb and Cd in a contaminated soil, a factorial (3 × 6) pot experiment with completely randomized design and three replications was performed. The cabbage varieties were ornamental cabbage, broccoli and cabbage and the PGPR strains included Pseudomonas putida PTCC1694, Bacillus megaterium PTCC1656, Azotobacter chroococcum, Proteus vulgaris PTCC1079 and Bacillus subtilis PTCC1715. The results indicate that the PGPR inoculation increased the amounts of plant biomass, the Pb and Cd concentrations in the root and shoot, the Pb and Cd uptake performances, the mobility, phytoavailability and translocation factor. The Pb bound to Fe-Mn oxides (42.7%) and the Cd bound to carbonate (38.1%) were the predominant heavy metal fractions. The percentages of the Pb and Cd in the soluble, exchangeable and organic matter-bound fractions increased while those of the Pb and Cd in the carbonate and Fe-Mn oxide-bound fractions decreased by lowering the soil pH and increasing the concentration of dissolved organic carbon (DOC). It is concluded that inoculation of heavy metal contaminated soils by the PGPR strains, especially Bacillus megaterium PTCC1656, enhances the phytoremediation process by increasing the metal phytoavailability and biomass production of the plant. Abbreviations HMe: Heavy metals; PGPR: Plant growth-promoting rhizobacteria; DOC: Dissolved organic carbon; HCN: Hydrogen cyanide; IAA: Indole-3-acetic acid; MIC: Minimum Inhibitory Concentrations; LB: Liquid broth; EC: Electrical conductivity; OC: Organic carbon; FC: Field capacity; WS: Water soluble; EX: Exchangeable; CAR: Bound to carbonates; OX: Bound to Fe-Mn oxides; OM: Bound to organic matter; RES: Residual; BOA: Ornamental cabbage; BOC: Cabbag; BOI: Broccoli; Pp: Pseudomonas putida PTCC1694; Bm: Bacillus megaterium 107 PTCC1656; Ac: Azotobacter chroococcum; Pv: Proteus vulgaris PTCC1079; Bs: Bacillus subtilis 108 PTCC1715; C-: Uninoculated non-rhizosphere soil; C+: Uninoculated rhizosphere soil; TF: Translocation factor; MF: Mobility factor; ANOVA: Analysis of variance

Influence of Soil Characteristics and Land Use Type on Existing Fractions of Radioactive 137Cs in Fukushima Soils

Radioactive cesium (137Cs) in distinct soil fractions provides key information to its bioavailability, and therefore determining the effect of soil characteristics and land use types on existing fractions of 137Cs in soils is important for predicting future 137Cs mobility in Fukushima. Thus, the objective of this study was to investigate the influence of soil characteristics and land use types on sequentially extracted fractions of 137Cs in Fukushima. In this study, five coniferous forest soils, eight arable soils, and eight paddy rice soils were sampled in 2012 and 2013. The 137Cs in the soils were separated into four fractions; water-soluble, exchangeable, organic matter-bound and residual fractions. More than 90% of the soil 137Cs fraction for arable and paddy rice soils was found in the residual fraction, implying significantly reduced bioavailable 137Cs with higher fixation. In contrast, forest soils measured higher exchangeable and organic matter-bound fractions of 5%–33% and 9%–44%, respectively, implying future 137Cs mobility in the forest ecosystem. Correlation analysis showed a significant negative correlation (p < 0.05) between the organic matter fraction and residual fraction in both arable and paddy rice soils. There was a significant positive correlation (p < 0.05) for both exchangeable and residual fractions with cation exchange capacity (CEC), total carbon (TC) and total nitrogen (TN) values in arable soils. Organic matter content influenced both exchangeable and residual fractions. It was not clear whether organic matter played a direct role in 137Cs fixation or mobility in the agricultural soils. In paddy rice soils, the organic matter fraction showed a significant negative correlation with TC and TN values. Soil pH was significantly negatively correlated (p < 0.05) with both water-soluble and residual fractions in forest soils but positively (p < 0.1) with the organically bound 137Cs fraction.

The effects of chicken manure on the immobilization and bioavailability of cadmium in the soil-rice system

ABSTRACT Soil cadmium (Cd) contamination has become a serious environmental concern worldwide. We conducted a two-year field experiment to investigate the effect of chicken manure as a soil amendment for the in situ immobilization of Cd, by assessing Cd bioavailability, fractions, and its uptake by rice plants. Results showed that soil pH and DTPA-extractable Cd were significantly elevated and reduced, respectively, with increasing doses of chicken. Sequential soil extractions indicated that chicken manure converted Cd to more immobilized fractions by significantly decreasing the exchangeable Cd fraction and increasing the carbonate-, oxide-, and organic matter-bound fractions. Accordingly, chicken manure significantly reduced the concentrations of Cd in rice tissues, including the brown rice (by up to 77.8%; the Cd content was below the recommended tolerable limits of 0.2 mg kg−1); such effects did not vary much in the 2 years. In this study, chicken manure could effectively reduce the bio-availability of Cd as it increased the soil pH, decrease the content of DTPA-extractable Cd, and transformed Cd into more stable fractions; as a result, chicken manure can be used as an amendment to significantly reduce Cd concentrations in rice if the heavy metal input through its application is considered in a long run.

Mobility and potential risk of sediment-associated heavy metal fractions under continuous drought-rewetting cycles

Ecological decline in the water level fluctuating (WLF) zone of the Three Gorges Reservoir (TGR) has been well established over the past decades. However, the effect of heavy metal fractions present in the sediment and their potential ecological risk under the anti-seasonal hydrological regime are still unclear. The Pengxi River is a tributary of the Yangtze River and it has a typical annual water level fluctuation ranging from 145 to 175m above sea level. The current study examined heavy metal fractions in sediments containing Cr, Cd, Cu, Pb and Mn collected along the WLF zone using the Tessier sequential extraction scheme. The total organic carbon (TC), total nitrogen (TN), carbon/nitrogen ratio (C/N), pH, particle size composition and content of nitrate (NO3−-N), and ammonium (NH4+-N) differed dramatically among the sampled altitudes and depths and was significantly correlated with the flooding time of the WLF zone. At lower altitudes of the WLF zone, the amounts of the exchangeable (EXC), carbonate-bound (CA) and total heavy metal contents of the surface sediment were much higher compared to those of higher altitudes of the WLF zone. The risk assessment code (RAC) for Cd and Mn showed an opposite trend to that of Cr, Cu, and Pb and mainly depended on the organic matter-bound fraction (OM). The modified RAC (mRAC) indicated a very high potential adverse effect for the whole WLF zone, although the risk value was much lower at the lower altitudes and upper depths of the WLF zone. Our results showed that the positive response of the loosely bound fractions (LOS) of heavy metals to the drought-rewetting (DRW) process minimizes the risk of heavy metals in the WLF zone sediment.

Remediation of Cr(VI)-contaminated soil with co-composting of three different biomass solid wastes

This study was focused on co-composting of Cr(VI)-contaminated soil with three different biomass solid wastes for immobilizing Cr(VI). Sequential chemical extraction of the soil was performed to determine Cr fractions changing over time during the compost process. Fluorescence spectroscopy was applied to study the composition and evolution characteristics of organic matter from the municipal sludge and Cr(VI)-contaminated co-composting. The influence of organic matter evolution on the bioavailability of Cr by multivariate statistical analysis was further investigated. Results proved that the Cr(VI)-contaminated soil can be co-composted successfully and this practice results in significant shifts in Cr availability. The concentrations of non-available fractions increased as well as available fractions decreased during the composting stage. The results from the Pearson correlation analysis showed positive relation between non-available fractions and TOC and negative relation between organic matter-bound fraction and TOC. Furthermore, principal factor analysis among Cr and organic matter indicated that the available fractions Cr were bound with humic-like substances and their bioavailability was low. It can be concluded that co-composting provides a method for remediating Cr(VI)-contaminated soil through the decrease of bioavailability of Cr and the binding between available fractions Cr and humic-like substances.

Nano-TiO2 affects Cu speciation, extracellular enzyme activity, and bacterial communities in sediments

In aquatic ecosystems, titanium dioxide nanoparticles (nano-TiO2) coexist with heavy metals and influence the existing forms and toxicities of the metal in water. However, limited information is available regarding the ecological risk of this coexistence in sediments. In this study, the effect of nano-TiO2 on Cu speciation in sediments was investigated using sequential extraction. The microcosm approach was also employed to analyze the effects of the coexistence of nano-TiO2 and Cu on extracellular enzyme activity and bacterial communities in sediments. Results showed that nano-TiO2 decreased the organic matter-bound fraction of Cu and increased the corresponding residual fraction Cu. As a result, speciation of exogenous Cu in sediments changed. During the course of the 30-day experiment, the presence of nano-TiO2 did not affect Cu-induced changes in bacterial community structure. However, the coexistence of nano-TiO2 and Cu restrained the activity of bacterial extracellular enzymes, such as alkaline phosphatase and β-glucosidase. The degree of inhibition also varied because of the different properties of extracellular enzymes. This research highlighted the importance of understanding and predicting the effects of the coexistence of nanomaterials and other pollutants in sediments.

Investigating speciation and toxicity of heavy metals in anoxic marine sediments—a case study from a mariculture bay in Southern China

The pollution of marine sediments by heavy metals is still a major concern, especially in zones affected by industry or mariculture. Toxicity of sediment heavy metal contents may be assessed using sequential extraction (SE) procedures, minding inherent constraints of such approaches. In this study, we investigated heavy metal speciation and toxicity in anoxic marine sediments in Zhelin Bay, a mariculture bay in Southern China, using an SE and acid volatile sulfur-simultaneously extracted metals (AVS-SEM) approach. Speciation of Cd, Cu, Ni, Pb, and Zn were studied by a modified SE of five fractions, adapted to separate organic and sulfidic metal fractions in anoxic sediments: F1 weak acid soluble (readily available), F2 reducible fraction, F3 organic matter-bound fraction, F4 sulfide-bound fraction, and F5 residually bound fraction. Toxicity predictions based on the sum of non-residual (NR) metal fractions from sequential extraction were compared to predictions based on AVS-SEM. Results showed that Cd, Ni, and Pb predominantly occurred in the weak acid soluble fraction (F1), residual fraction (F5), and sulfide-bound fraction (F4), respectively; Cu and Zn were mainly obtained in F4 and F5. Based on the distribution of indicator elements for metal fractions, the SEM from AVS extraction included different yields of non-residual and residual fractions besides the sulfidic fraction. Estimates for potential heavy metal toxicity based on NR metals of the SE procedure were thus based on a better-defined speciation compared to the simplistic approach of the AVS-SEM method. Based on the contents of NR metals and normalizing them by organic matter content, toxic effects are not expected for any of the sampling sites, irrespective of the presence or absence of mariculture. Using Pearson correlation analysis to identify predominant fractions influencing toxicity, we conclude that toxicity of heavy metals in anoxic sediments can be well predicted by their non-residual heavy metal contents.

Transformation of heavy metal fractions on soil urease and nitrate reductase activities in copper and selenium co-contaminated soil

This study aims to explore the effects of the distribution, transformation and bioavailability of different fractions of copper (Cu) and selenium (Se) in co-contaminated soils on soil enzymes, providing references for the phytoremediation of contaminated areas and agriculture environmental protection. Pot experiments and laboratory analysis were used to investigate the transformation and bioavailability of additional Cu and Se for pakchoi (Brassica chinensis) in co-contaminated soil. In the uncontaminated soil, Cu mainly existed in residual form, whereas Se was present in residual form and in elemental and organic-sulfide matter-bound form. In the contaminated soil, Cu mainly bound to Fe–Mn oxidates, whereas Se was in exchangeable and carbonates forms. After a month of pakchoi growth, Cu tended to transfer into organic matter-bound fractions, whereas Se tended to bound to Fe–Mn oxidates. The IR (reduced partition index) value of Cu decreased as the concentrations of Cu and Se gradually increased, whereas the IR value of Se decreased as the concentration of Se increased. The IR value before pakchoi planting and after it was harvested was not affected by the concentration of exogenous Cu. Soil urease and nitrate reductase activities were inhibited by Cu and Se pollution either individually or combined in different degrees, following the order nitrate reductase>urease. The significant correlation between the IR value and soil enzyme activities suggests that this value could be used to evaluate the bioavailability of heavy metals in soil. Path analysis showed that the variations in exchangeable Cu and organic-sulfide matter-bound and elemental Se had direct effects on the activities of the two enzymes, suggesting their high bioavailability. Therefore, the IR value and the transformation of metals in soil could be used as indicators in evaluating the bioavailability of heavy metals.

Green Manure and Overlapped Rhizosphere Effects on Pb Chemical Forms in Soil and Plant Uptake in Maize/Canola Intercrop Systems: A Rhizobox Study

To investigate the effects of single and mixed cultures of maize and canola, green manure addition, and distance from the rhizosphere on chemical forms of soil lead (Pb) and its uptake by plants, a greenhouse experiment was conducted using rhizobox systems. After plant harvest, the soils from different parts of the rhizoboxes were sampled and analyzed for dissolved organic carbon (DOC) concentrations and pH values. A sequential extraction procedure was also carried out to differentiate the chemical forms of Pb in the soil samples. Results showed that root activity significantly affected soil DOC concentration, pH and Pb chemical forms, while the effects generally ceased from the rhizosphere through the bulk soil. The pH value increased and DOC concentration decreased with distance from the roots. Green manure addition enhanced DOC concentration and exchangeable, carbonate-associated, and organic matter-bound Pb fractions. Green manure addition reduced the Pb concentration and uptake of maize but increased those of canola. The concentration of Pb in the roots of maize and canola was increased in intercropping compared with monocropping systems. The conclusion is that chemistry and bioavailability of soil Pb are interactively influenced by overlapped rhizospheres as compared to individual rhizospheres, use of green manure, and distance from the root surfaces.

Heavy-Metal Fractionation in Surface Sediments of the Cauvery River Estuarine Region, Southeastern Coast of India

Geochemical fractionation of iron (Fe), manganese (Mn), copper (Cu), chromium (Cr), lead (Pb), zinc (Zn), and nickel (Ni) were determined using five-stage sequential extraction in sediments collected from estuarine stretches of Cauvery River delta on the eastern coast of India with emphasis on seasonal variation. Abundance of metals in terms of sum of total fractionations varied in the following order: Fe>Mn>Cr>Zn>Cu>Ni>Pb. Exchangeable fraction, believed to be bioavailable, showed differential abundances during the dry and wet seasons in the following order: Pb>Zn>Cu>Mn>Ni>Cr>Fe and Zn>Cu>Cr>Ni>Mn>Pb>Fe, respectively, indicating the possibility of anthropogenic influence. Among nonlithogenic fraction, organic matter-bound fraction is the second largest and a key scavenger for all of the heavy metals studied except Fe and Mn. A significant portion of Cu-associated organic matter fraction shows strong association of Cu with organic matter at most of the sampling sites. Environmental risk of metals evaluated using risk-assessment code and mobility factor showed low to high risk for Pb, Zn, and Cu. The results of the present study also hint at notable enrichment of heavy metals in the certain pockets of the Cauvery Estuary.

The effect of cow manure application on the Distribution Fractions of Fe, Mn and Zn in agricultural soils

The speciation of metals in environmental samples is a vital factor in assessing the potential environmental impacts. Fractionation of heavy metals in soils is needed to predict elemental mobility in soil and phytoavailability to plants. A study was conducted to determine the effect of cow manure on Fe, Mn and Zn availability and their redistribution among soil fractions. Chemical properties such as pH, EC, CEC and Lime of concerned soils are also analyzed. A soil was selected from agricultural soil in sistan va baluchestan, southeast Iran and amended with 0, 1 and 5% of Cow Manure (CM) with three triplications and incubated at 60% of field moisture. The amounts of Fe, Mn and Zn were determined from the soil after 4 months of incubation time using sequential extraction procedures. Tessier method was exerted to decompose the metal content into exchangeable (EXC), bond to carbonates (CAR), bound to Fe-Mn oxides (Oxids), bound to organic matter (OM) fractions and Residual (RES) fraction was determined in aqua regia digest. Results of these analysis showed that the concentrations of Fe and Mn decreased mainly in the carbonate-bound and residual fractions but increased in the exchangeable, Fe-Mn oxide and organic matter-bound fractions with cow manure application. It was concluded that cow manure was the most effective in immobilizing Zn. The application of Cow Manure (CM) levels significantly decreases the exchangeable, carbonate-bound and residual fractions (P < 0.05) of Zn and increased in the Fe-Mn oxide and organic matter-bound fractions.

Concentrations and chemical forms of heavy metals in agricultural soil near the world’s largest and oldest tungsten mine located in China

Tungsten (W) mining has taken place in Ganzhou in China for about 100 years. Such long-term W mining may release large amount of metals to soils and waters around these mines. Twenty soil samples were taken from the area around the W mines and 10 soil samples from an area much farther away. These soil samples were analysed for physicochemical properties, heavy metal content and their chemical forms. Results show that long-term W mining significantly increased both total and labile contents of Cd, Cu, Pb and Zn, but did not, or only slightly, increased the total content of Co, Cr and Ni in the soil near the mine. Average enrichment factor (EF) in the agricultural soils was 4.0, 2.4, 2.2, and 2.0 for Cd, Cu, Pb and Zn, respectively. The labile fraction was dominated by the carbonate-bound fraction for Cd (54.5%) and organic matter-bound fraction for Cu (37.9%), while the major labile fractions for Pb and Zn were associated with carbonates (30.2% and 6.4%), oxides (17.9% and 10.6%) and organic matter (9.2% and 18.8%). Consequently, there is a need to be cautious about Cd in the soils contaminated by W mining.

Effect of Wetting-Drying Cycles on Redistribution of Lead in Some Semi-Arid Zone Soils Spiked with a Lead Salt

Mobility and bioavailability of lead (Pb) could be affected considerably by soil physicochemical properties; however, less is known about the effect of Pb levels and aging time. This study was conducted to evaluate the effects of Pb levels and wetting-drying (WD) cycles on distribution and bioavailability of Pb in three semi-arid zone soils treated with different levels of Pb(NO3)2. Wetting-drying cycles simulated the actual field irrigation in the semi-arid soils. A soil with a long history of Pb contamination was also taken as a reference soil. The soils were spiked with various levels of Pb and incubated under WD cycles for 160 d. Sequential extractions and batch sorption experiments were performed to assess the fractionation of Pb in the spiked soils. Redistribution index (Uts) and reduced partitioning parameter (IR) were applied to semi-quantify the distribution of Pb in the spiked soils. A small amount of Pb sorbed was desorbed by the soils, indicating a strong and irreversible binding of Pb in the studied soils. Contribution of carbonate-bound (Car) and residual (Res) Pb fractions to the total Pb of the soils was more than 97%. The Car, soluble plus exchangeable (SE), and organic matter-bound (OMB) fractions of Pb were transferred to the Res fraction under the WD cycles. The IR and Uts values were influenced by Pb loading levels and WD; therefore, the Pb lability and/or redistribution pattern could semi-quantitatively be assessed via these parameters. At the end of the experiment, the Ir and Uts values for the Pb salt-spiked soils did not show the quasi-equilibrium state. The lability of Pb in the soils decreased with increasing incubation time and showed a strong dependence on Pb levels and soil chemical composition. WD cycles significantly affected the overall lability of Pb in soils through influencing the redistribution of Pb among solid-phase components.