Assessment of the metal pollution, potential toxicity and speciation of sediment from Algeciras Bay (South of Spain) using chemometric tools

A study to determine total and mobile heavy metals concentrations in sediments from Algeciras Bay was performed and pollution hotspots were identified. The effects on aquatic organisms were established using sediment quality guidelines (SQGs). Ni and Cr exceeded the effect range medium and low levels, respectively, around industrial area. Potential toxicity of metals was determined by diethylenetriaminepenta-acetic acid (DTPA) extractions and low alert levels of Co, Cu, Zn, Cd, Ni and Pb were exceeded at most sampling sites. Three pollution indicators were used showing significant values for As, Ni, Cr, Pb and Cd. Sediment speciation using the sequential extraction BCR procedure was carried out, being Cd, Zn, Pb and As the most available metals. Principal component, cluster and ANOVA analyses were performed in order to assess the sources of metals and the influence of seasonality and anthropogenic activities on the sediment quality. Two principal component analysis (PCA) factors were obtained identifying the sampling sites affected by anthropogenic activities; Ni-Cr and Zn-Cu-V clusters were also obtained associated with stainless steel and petrochemical industrial activities. ANOVA showed the outstanding sites because of total metal concentration and significant differences among sampling sites by the acid extractable and reducible fractions for all metals except for Ba and V.     

Speciation of heavy metals in untreated sewage sludge by using microwave assisted sequential extraction procedure

A fast microwave assisted extraction procedure was developed and optimized for their eventual exploitation in the three-stage sequential extraction procedure proposed by modified BCR protocol (the community Bureau of Reference now the European Union "Measurement and Testing Programme"). The effects of the microwave treatment on the extraction of Cd, Cr, Cu, Ni, Pb and Zn from untreated sewage sludge collected from Hyderabad city (Pakistan) were compared with those obtained from sequential BCR extraction procedure. In sequential BCR method, each extraction step takes 16 h, where as with the use of compromised microwave conditions, extraction steps could be completed in about 120 s, for each step, respectively. Extractable Cd, Cr, Pb and Ni obtained by both comparable methodologies were measured by electrothermal atomic absorption spectrometry (ETAAS), while for Cu and Zn flame atomic absorption spectrometry (FAAS) was used. The validations of both extraction techniques were compared by the analysis of certified reference material of soil amended with sewage sludge (BCR 483). The results of the partitioning study of untreated waste water sewage sludge, indicate that more easily mobilized forms (step 1) were predominant for Cd, Ni and Zn (28.3, 28.4 and 43.7%), in contrast, the largest amount of Cd and Pb (66.4 and 72.8%) was associated with the iron/manganese oxide while Cr and Ni (71.2 and 38.7%) in organic matter/sulphide fractions. The overall metal recoveries in steps 1-3 (excluding residual step) were 95.3-104% of those obtained with the sequential BCR protocol. The accuracy of the proposed microwave extraction method (expressed as %R.S.D.) was lower than 10% for all metals.     

Assessment of single extraction methods for the prediction of bioavailability of metals to Brassica juncea L. Czern. (var. Vaibhav) grown on tannery waste contaminated soil

Various single extractant (DTPA, EDTA, NH(4)NO(3), CaCl(2), and NaNO(3)) was used to evaluate the bioavailability of heavy metals from tannery wastewater contaminated soil and translocation of metals to the plant of Brassica juncea L. Czern. (var. Vaibhav). The extraction capacity of the metals was found in the order: EDTA>DTPA>NH(4)NO(3)>CaCl(2)>NaNO(3). Cluster analysis between different extractants showed close relationship between DTPA, CaCl(2), NH(4)NO(3) except EDTA and NaNO(3), which showed dispersed relationship. Principal components analysis (PCA) applied to metals extracted with EDTA showed different grouping of metals (i) Na, Co, Pb, Ni and (ii) K, Mn, Zn, Cr, in the loading plot which showed similar availability from contaminated soil. PCA applied on metals accumulation data in the plants also exhibited different grouping of variables (i) Cu, Co, Ni, Cd and (ii) Mn, Zn, Pb, Fe showed almost similar accumulation pattern in the plants. The data displayed positive loading for Mn and negative loading for Cr with PC(2). Cd and Zn have shown high loadings in PC(1) and PC(2), respectively. The translocation of most of the tested metals (Pb, Mn, Cd, Ni, Fe) in the shoot of the plant was found better except Cr, Cu, Co and K. The correlation analysis between different extractable metals and metal accumulation in the shoot of the plant showed significant positive correlation with Pb and Cr. Overall, extraction capacity and cluster analysis augmented that EDTA was found suitable extractant for tannery wastewater contaminated soil to B. juncea.     

Total concentrations and fractions of Cd, Cr, Pb, Cu, Ni and Zn in sewage sludge from municipal and industrial wastewater treatment plants

Heavy metals are one of the important factors that affect the final disposal of sewage sludge. In this paper, the metal mobility and bioavailability of heavy metals in sewage sludge were studied by using Community Bureau of Reference (BCR) sequential extraction procedure to get more information for the reasonable disposal of sludge. Sewage sludge was collected from five municipal wastewater treatment plants and three industrial wastewater treatment plants. The sludge was examined for and the total concentrations and different chemical fractions of Cd, Cr, Pb, Cu, Ni and Zn. The total metal concentrations of heavy metals in sludge varied greatly. The contents of Zn and Cu were the highest, followed by then Cr, Ni and Pb and the content of Cd was the least. There was no significant difference in total metal concentration between municipal and industrial wastewater treatment plants. Fractions extracted by the BCR sequential procedure were acid soluble/exchangeable, reducible and oxidizable fraction. Sludge pH was found to have profound effect on the chemical fractions of heavy metals. Acidic sludges (Xiamen and Jinlin Petrochemical Group Co., wastewater treatment plant) had higher proportion of the acid soluble/exchangeable fractions than in neutral sludge. In neutral sludges, Pb and Cr were principally distributed in between the oxidizable fraction and the residual fraction; Cu was in the oxidizable fraction; Cd mainly in the residual fraction in municipal wastewater treatment plants and had high percentage of acid soluble/exchangeable and reducible fractions in industrial wastewater treatment plants; Ni and Zn had higher percentage in the acid soluble/exchangeable and the oxidizable fraction.     

Decontamination and/or revegetation of fly ash dykes through naturally growing plants

Present study is focused on the decontamination and/or revegetation of fly ash dykes through naturally growing plants, namely Calotropis procera, Cassia tora, Chenopodium album, Sida cardifolia, Blumea lacera. The results of sequential extraction study showed that maximum amount of metals (Na, K, Fe, Mn, Cr, Pb, Ni, Cd) were associated with residual and Fe-Mn fractions. Diethylenetriamine penta acetic acid (DTPA)-triethanolamine (TEA) extraction assessed the bioavailability of the metals. The total metal accumulation in tested plants was found in the order; C. album>S. cardifolia>C. tora>C. procera>B. lacera. The maximum bioconcentration factor (BCF) was recorded in S. cardifolia for the metals (Na, Fe, Zn, Cd), in C. procera for the metals (Mn, Cu, Ni, Cr) and in C. album for the metals (Co, Pb). However, the translocation factor (TF) of most of the metals was found more in S. cardifolia followed by C. album than other plants. Among all the plants, C. album have shown high BCF and low TF values for toxic metals (Pb, Cd) and suitable for phytostabilization of these metals. Principal component analysis was used to predict translocation behavior of the metals in different parts of the plants which was found similar for the metals (Cu, Zn, Mn, Cr). All examined plants are suitable for revegetation (naturally grows on fly ash dykes) and S. cardifolia and C. album may be used for decontamination purposes.     

Geochemical speciation and risk assessment of heavy metals in the river estuarine sediments--a case study: Mahanadi basin, India

Sequential extraction technique was used to study the mobility and dynamics of operationally determined chemical forms of heavy metals in the sediments and their ecological risk on the biotic species. The results reveal that high environmental risk of Cd, Ni, Co and Pb, are due to their higher availability in the exchangeable fraction. Substantial amount of Cd, Co, Mn, Cu, Zn, Ni and Pb, is observed as carbonate bound, which may result due to their special affinity towards carbonate and their co-precipitation with its minerals. Colloids of Fe-Mn oxides act as efficient scavengers for the heavy metals like Zn, Pb, Cu, Cr, Co, and Ni. Toxic metals like Ni, Pb and Cd are of concern, which occasionally may be associated with adverse biological effects based on the comparison with sediment quality guidelines (SQGs). The risk assessment code (RAC) suggests that the highest mobility of Cd poses a higher environmental risk and also threat to the aquatic biota. Factor analysis reveals that the enrichment of heavy metals in bioavailable fraction is mostly contributed from anthropogenic sources. These contributing sources are highlighted by cluster analysis.     

Speciation of heavy metals by modified BCR sequential extraction procedure in different depths of sediments from Sungai Buloh, Selangor, Malaysia

The sequential extraction procedure proposed by the European Standard, Measurements and Testing (SM&T) program, formerly the Community Bureau of Reference (BCR), was applied for partitioning of heavy metals (HMs) in river sediments collected along the course of Sungai Buloh and the Straits of Malacca in Selangor, Malaysia. Eight elements (V, Pb, Cd, Cr, Co, Ni, Cu and Zn) from seven stations (S1-S7) and at different depths were analyzed using the modified BCR Sequential Extraction Procedure (SEP) in combination with ICP-MS to obtain the metal distribution patterns in this region. The results showed that heavy metal contaminations at S2 and S3 was more severe than at other sampling sites, especially for Zn, Cu, Ni and Pb. Nevertheless, the element concentrations from top to bottom layers decreased predominantly. The samples from the Straits of Malacca (S4-S7) the highest contamination factors obtained were for Co, Zn and Pb while the lowest were found for V and Cr, similar to Sungai Buloh sediments. The sediments showed a low risk for V, Cr, Cu and Pb with RAC values of less than 10%, but medium risk for Co, Zn (except S3), Cd at S1 and S2 and Ni at S1, S3 and S5. Zn at S3 and Cd at S3-S7 showed high risk to our sediment samples. There is not any element of very high risk conditions in the selected samples.     

Assessment of heavy metal contamination and its mobilization from municipal solid waste open dumping site

Influence of heavy metals was investigated by conducting various tests on the samples collected from Nonthaburi dumpsite in Thailand. The heavy metal concentration in the solid waste and its mobility potential based on its binding forms was studied. The sequential extraction method was used to determine the binding forms of metals. From the analysis, Zn was found to be highest concentrated heavy metal compared to Mn, Cu, Cr, Cd, Pb, Ni and Hg in the solid waste. From the sequential extraction, Mn, Zn and Cd mostly found in reducible form, showed its susceptibility to be leached easily. Cu and Cr were found predominantly in oxidizable form and stable under anaerobic condition. Pb and Ni were present in residual form, which is inert. The estimated individual contamination factor (C(f)(i)), showed Zn with highest affinity to leach. The concentration level of all the heavy metals in the leachate except for Cr was noticed to be below the National effluent standards. Though, indicated to be safe for disposal, its effect in any concentration proved toxic to the plant life from the seed germination toxicity test using synthetic chelate ethylene diamine tetraacetic acid (EDTA).     

Distribution of heavy metals in Lakes Doirani and Kerkini, Northern Greece

The distribution of heavy metals in two lakes of high ecological significance, Doirani and Kerkini, located in Northern Greece was studied. Eight metals (Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn) were determined in water, total suspended solids, fine and coarse sediments. Moreover, the modified BCR fractionation scheme was employed in sediments and suspended solids to determine soluble, oxidisable, reducible and residual fractions of metals. The Lake Doirani presents higher metal concentrations in aqueous phase than Lake Kerkini; Cd, Cu, Ni, Pb and Zn are above the chronic freshwater quality criteria for aquatic life. In both lakes, Fe and Mn are the most abundant elements in total suspended solids whereas Cd the less abundant. The Lake Kerkini exhibits higher concentrations of all the examined metals in sediments comparing to the Lake Doirani, however the concentrations are lower than the sediment quality guidelines. Cd in sediments is mainly in soluble fraction, Pb and Cu exhibit significant oxidisable fractions whereas, Cr and Fe associated mainly with residual fraction.     

Speciation of heavy metals in sediment by conventional, ultrasound and microwave assisted single extraction methods: a comparison with modified sequential extraction procedure

Sequential extraction procedure proposed by the European Standards, Measurements and Testing (SM&T) Program, formerly the Community Bureau of Reference (BCR), has been applied for the heavy metals (HMs) partitioning in fresh water lake sediment samples. The results obtained by conventional sequential extraction BCR method (SEB) for Cd, Cu, Cr, Ni, Pb and Zn were compared with those estimated from three alternative single extractions, conventional (CSE), ultrasonic (USE) and microwave assisted (MSE), using identical operating conditions applied in each individual BCR fraction and validated by the CRM BCR 701. Extractable HMs contents obtained by all compared methodologies were measured by atomic absorption spectrometery. With the use of compromised sonication and microwave conditions, steps 1-3 of the sequential extraction (excluding the hydrogen peroxide digestion in step 3) could be completed between 15-30 min using ultrasonic bath, while 60-120 s were required for MSE. The total extractable metal contents obtained by three single extractions ranged from 75.1% to 114.0% except Cr in first step, which was extracted (125.3%) by MSE method as compared to those obtained by SEB procedure. The precision of the proposed BCR single extraction methods (expressed as RSD%) was found in the range of (3.99-9.6%) for all metals.     

Time saving modified BCR sequential extraction procedure for the fraction of Cd, Cr, Cu, Ni, Pb and Zn in sediment samples of polluted lake

The mobility, availability and persistence of heavy metals (Cd, Cr, Cu, Ni, Pb and Zn) in contaminated lake sediment samples were evaluated by means of sequential extraction scheme, proposed by the Community Bureau of Reference protocol (BCR). The metal content in the extracts was measured by atomic absorption spectrometry. The precision and accuracy of the proposed procedure were evaluated by using a certified reference material BCR 701. The maximum recoveries for heavy metals (HMs) were observed for all three steps of BCR protocol at 32h total shaking period instead of previously reported 51h, with p>0.05. The lixiviation tests (DIN 38414-S4) were used to evaluate the leaching of HMs from sediment samples and it was observed that levels of water extractable HMs were low as compared to those values obtained by acid-soluble fraction of the BCR protocol.     

Use of sequential leaching, mineralogy, morphology and multivariate statistical technique for quantifying metal pollution in highly polluted aquatic sediments--a case study: Brahmani and Nandira Rivers, India

The particle size distribution, geochemical composition and sequential leaching of metals (Fe, Mn, Ni, Cu, Co, Cr, Pb, Zn and Cd) are carried out in core sediments (<88 microm) from the Brahmani and Nandira Rivers, India. To confirm the contamination of downstream sediments by fly ash, mineralogical and morphological characterizations were carried out. High environmental risk of Co, Pb and Ni is due to their higher availability in exchangeable fraction. The metals like Zn, Cu and Mn represent an appreciable portion in the carbonate phase. Metals such as Zn, Pb, Cd, Co and Ni are associated with reducible phase may be due to adsorption. The organic bound Cu, Zn, and Pb seem to be second dominant fraction among non-lithogenous in Nandira sediments. Factor analysis data reveals that textural parameters, Fe-Mn oxy/hydroxides, organic precipitation and coal fly ash disposals, are individually responsible for the enrichment of heavy metals. The relationships among the stations are highlighted by cluster analysis to identify the contamination levels.     

Application of EDTA decontamination on soils affected by mining activities and impact of treatment on the geochemical partition of metal contaminants

Two soil samples were collected at mining areas located in southern Hunan Province, China. EDTA extraction of Pb, Zn, Cu and Cd from these two tailing soils was studied using column leaching experiments. The redistributions of heavy metals (HMs) were determined using the modified BCR (Community Bureau of Reference) sequential extraction procedure, before and after EDTA extraction. The results indicated that EDTA was an effective extractant because of its strong chelating ability for various HMs. The proportions of Pb, Zn, Cu and Cd in the four fractions varied largely after EDTA extraction. The extraction efficiency of EDTA of the acid-extractable fraction (AEX) was significant in shallow soil column, while in deeper soil column, decrease of the extraction efficiency of reduced (RED), oxidizable (OX) and residual fractions (RES) was obtained, which was mainly due to the decrease of EDTA concentration.     

Analysis of heavy metals during composting of the tannery sludge using physicochemical and spectroscopic techniques

The major limitation of direct application of tannery sludge compost in agriculture is the total heavy metal contents and their bioavailability to the soil-plant system. This study focused on the heavy metal characterization and the influence of changing the physicochemical properties of the medium throughout the composting on the concentrations, bioavailability or chemical forms of Cr, Cu, Zn, Pb and Cd in tannery sludge. The study shows that throughout the 60 days of composting, physicochemical analysis and Fourier-transformed infrared (FTIR) spectroscopic characterization show that all parameters elaborated and reached relatively stable levels reflecting the stability and maturity of the final product, and revealed the biodegradation of components that can be easily assimilated by microorganism. The C/N ratio reaches the optimal range of stable compost; inorganic nitrogen is transformed into stable organic forms. The total concentration of Cr, Zn, Cu, Pb and Cd is very low rendering final compost acceptable for agricultural use. The germination index for both Chinese cabbage and lettuce was 97% after 60 days of composting, showing that the final compost was not phytotoxic. Furthermore, in using a sequential extraction method in sludge compost at different phases of treatment, a less than 2% of metals bound to bioavailable fractions X-(KNO(3)+H(2)O). A large proportion of the heavy metals were associated to the residual fraction (75-85%) and more resistant fractions to extraction X-NaOH, X-EDTA, X-HNO(3) (15-25%). Mobile fractions of metals are poorly predictable from the total content. Bioavailability of all fractions of elements tends to decrease.     

Characteristics and accumulation of heavy metals in sediments originated from an electroplating plant

Heavy metals in river water and sediments originated from an electroplating plant in Jiangsu Province of China were studied and analyzed for their environmental impact. The results indicated that the wastewater from the plant degraded the quality of the aquatic environment downstream from the plant. In surface water, considerable concentrations of Cu, Ni, Zn, Mn and Cr were present at the sites near the plant. Unsafe levels of Cu were observed at all sites, and unsafe levels of Ni, Zn, and Cr were present at some sites. Significant accumulation of Ni, Cu, Zn and Cr was identified, and heavy metal longitudinal distribution in sediments was similar to that in water. The contents of Ni, Cu and Cr at all sites and Zn at some sites were likely to result in harmful effects on the environment. The risks posed by Ni, Cu, Zn and Cr in water and sediments decreased with increasing downstream distance. Moreover, a modified sequential extraction procedure was employed to determine exchangeable, carbonate-bound, iron-manganese oxide bound, organic matter bound and residual fractions of metals in sediments. The results showed that Ni was distributed in every fraction except for iron-manganese oxide bound, significant Mn exhibited in exchangeable fractions, and high percentage of Cu was in the organic matter and residual fractions. Residual fraction was the dominant fractions for Pb and Zn. According to RACs, Ni and Mn posed a high risk to the environment, Zn exhibited medium to high risk, Cu had low to high risk, and Pb possessed a low to medium risk.     

Chelant extraction of heavy metals from contaminated soils.

The current state of the art regarding the use of chelating agents to extract heavy metal contaminants has been addressed. Results are presented for treatability studies conducted as worst-case and representative soils from Aberdeen Proving Ground's J-Field for extraction of copper (Cu), lead (Pb), and zinc (Zn). The particle size distribution characteristics of the soils determined from hydrometer tests are approximately 60% sand, 30% silt, and 10% clay. Sequential extractions were performed on the 'as-received' soils (worst case and representative) to determine the speciation of the metal forms. The technique speciates the heavy metal distribution into an easily extractable (exchangeable) form, carbonates, reducible oxides, organically-bound, and residual forms. The results indicated that most of the metals are in forms that are amenable to soil washing (i.e. exchangeable+carbonate+reducible oxides). The metals Cu, Pb, Zn, and Cr have greater than 70% of their distribution in forms amenable to soil washing techniques, while Cd, Mn, and Fe are somewhat less amenable to soil washing using chelant extraction. However, the concentrations of Cd and Mn are low in the contaminated soil. From the batch chelant extraction studies, ethylenediaminetetraacetic acid (EDTA), citric acid, and nitrilotriacetic acid (NTA) were all effective in removing copper, lead, and zinc from the J-Field soils. Due to NTA being a Class II carcinogen, it is not recommended for use in remediating contaminated soils. EDTA and citric acid appear to offer the greatest potential as chelating agents to use in soil washing the Aberdeen Proving Ground soils. The other chelating agents studied (gluconate, oxalate, Citranox, ammonium acetate, and phosphoric acid, along with pH-adjusted water) were generally ineffective in mobilizing the heavy metals from the soils. The chelant solution removes the heavy metals (Cd, Cu, Pb, Zn, Fe, Cr, As, and Hg) simultaneously. Using a multiple-stage batch extraction, the soil was successfully treated passing both the Toxicity Characteristics Leaching Procedure (TCLP) and EPA Total Extractable Metal Limit. The final residual Pb concentration was about 300 mg/kg, with a corresponding TCLP of 1.5 mg/l. Removal of the exchangeable and carbonate fractions for Cu and Zn was achieved during the first extraction stage, whereas it required two extraction stages for the same fractions for Pb. Removal of Pb, Cu, and Zn present as exchangeable, carbonates, and reducible oxides occurred between the fourth- and fifth-stage extractions. The overall removal of copper, lead, and zinc from the multiple-stage washing were 98.9%, 98.9%, and 97.2%, respectively. The concentration and operating conditions for the soil washing extractions were not necessarily optimized. If the conditions had been optimized and using a more representative Pb concentration (approximately 12000 mg/kg), it is likely that the TCLP and residual heavy metal soil concentrations could be achieved within two to three extractions. The results indicate that the J-Field contaminated soils can be successfully treated using a soil washing technique.     

Trace element exposure in the environment from MSW landfill leachate sediments measured by a sequential extraction technique

Dredged leachate sediments from eight MSW landfills were dried and homogenized, and metals sequentially extracted. The concentrations of cadmium (Cd), copper (Cu), chromium (Cr), iron (Fe), lead (Pb), manganese (Mn), nickel (Ni) and zinc (Zn) were found to be similar to those reported for sewage sludge, and generally below the EU limits for use of sludge on agricultural land. Sequential extraction of the samples showed that the largest fractions of the heavy metals were associated with Fe- or Mn-oxides. Cr and Cu were the only metals investigated that were associated with the organic matter in the sediments to any significant extent (2-10% of Cr and 10-28% of Cu). Since the largest fractions of the metals studied were bound to Fe-oxides, and thus had a low mobility, these metals will generally have a low bioavailability under aerobic conditions when present in leachate sediments. This is most likely also valid for particulate matter suspended in MSW landfill leachate and released to the environment, since this is the source of the sediments.     

Removal of metals from landfill leachate by sorption to activated carbon, bone meal and iron fines

Sorption filters based on granular activated carbon, bone meal and iron fines were tested for their efficiency of removing metals from landfill leachate. Removal of Al, As, Ca, Cd, Co, Cr, Cu, Fe, Hg, Mg, Mn, Mo, Ni, Pb, Sr and Zn were studied in a laboratory scale setup. Activated carbon removed more than 90% of Co, Cr, Cu, Fe, Mn and Ni. Ca, Pb, Sr and Zn were removed but less efficiently. Bone meal removed over 80% of Cr, Fe, Hg, Mn and Sr and 20-80% of Al, Ca, Cu, Mo, Ni, Pb and Zn. Iron fines removed most metals (As, Ca, Co, Cr, Cu, Fe, Mg, Mn, Pb, Sr and Zn) to some extent but less efficiently. All materials released unwanted substances (metals, TOC or nutrients), highlighting the need to study the uptake and release of a large number of compounds, not only the target metals. To remove a wide range of metals using these materials two or more filter materials may need to be combined. Sorption mechanisms for all materials include ion exchange, sorption and precipitation. For iron fines oxidation of Fe(0) seems to be important for metal immobilisation.     

Monitoring of electrokinetic removal of heavy metals in tailing-soils using sequential extraction analysis

This research focused on the monitoring of the electrokinetic removal of heavy metals from tailing-soils, and emphasizes the dependency of removal efficiencies upon their physico-chemical states, as demonstrated by the different extraction methods adopted, which included aqua regia and sequential extraction. The tailing-soils examined contained high concentrations of target metal contaminants (Cd=179mgkg(-1), Cu=207mgkg(-1), Pb=5175mgkg(-1), and Zn=7600mgkg(-1)). The removal efficiencies of the different metals were significantly influenced by their speciations, mobilities and affinities (adsorption capacities) in the soil matrix. The removal efficiencies of mobile and weakly bound fractions, such as the exchangeable fraction were more than 90% by electrokinetic treatment, but strongly bound fractions, such as the organically bound species and residual fraction were not significantly removed (less than 30% removal efficiencies). In accordance with the general sequence of mobilities of heavy metals in soils, the removal efficiencies of more mobile heavy metals (Cd, Cu, and Zn) were higher than that of less mobile heavy metal (Pb).     

Investigation of the transport and fate of Pb, Cd, Cr(VI) and As(V) in soil zones derived from moderately contaminated farmland in Northeast, China

Some farmland in Shenyang had been irrigated with industrial wastewater since 1962. Although wastewater irrigation was ceased in 1992, soil had been heavily polluted by heavy metals, especially by Cd. For better understanding processes of soil-heavy metal interactions, in particular, the mobility and retention mechanism of heavy metal in soil, a study on the transport and fate of heavy metals in soil zones from Shenyang suburb was carried out by column leaching tests in laboratory. Breakthrough curves of Pb, Cd, Cr(VI) and As(V) fitted by Thomas model and Yoon–Nelson model. The results of fitted breakthrough curves showed that transport rates of the four heavy metals in the soil zones followed the order: Cr(VI) > As(V) > Cd > Pb, which indicated that Cr(VI) was much more mobile, and Pb was comparatively unmovable. Cr in effluents and As were almost entirely Cr(VI) and As(V), respectively, and no Cr(III) and As(III) was ever detected during the leaching tests. The contents of Pb, Cd, Cr and As in leached soils decreased in the order of Pb > Cd > Cr > As, which suggested that adsorption ability of soil to Pb was greatest and to As was least. The methods of selective sequential extraction and solvent extraction were used to determine the fractions of Pb, Cd, Cr, As and the valent states of Cr, As [Cr(VI) or Cr(III), As(V) or As(III)] in original soils and in leached soils. After leaching tests, the relative and absolute concentrations of exchangeable, carbonate, Fe–Mn oxide and organic fraction of each element were all increased, which enhanced the potential mobility and risk of Pb, Cd, Cr and As to soil/groundwater system. The relative concentrations of Cr(III) and As(III) in different depth of the soil zones after leaching tests were increased by about 6.0% and 5.6%, respectively. Cr(III) and As(III) tended to be adsorbed by soils, which reduced the mobility of them into groundwater.