Exchangeable Fraction Of Cd Research Articles

Geochemical Fractionation and Source Identification of Pb and Cd in Riparian Soils and River Sediments from Three Lower Reaches Located in the Pearl River Delta.

Pb and Cd accumulation in riparian soils and river sediments in river basins is a challenging pollution issue due to the persistence and bioaccumulation of these two trace metals. Understanding the migration characteristics and input sources of these metals is the key to preventing metal pollution. This study was conducted to explore the contents, geochemical fractionation, and input sources of Pb and Cd in riparian soils and river sediments from three lower reaches of the Pearl River Delta located in the Guangdong-Hong Kong-Macao Greater Bay Area. The total concentration of all Pb and Cd values exceeded the background values to varying degrees, and the exchangeable fraction of Cd in riparian soils and river sediments accounted for the largest proportion, while that of Pb was dominated by the residual fraction. Geoaccumulation index calculations showed that in the riparian soils, the average accumulation degree of Pb (0.52) in the Beijiang River (BJR) was the highest, while that of Cd (2.04) in the Xijiang River (XJR) was the highest. Unlike that in riparian soils, the maximum accumulation of Pb (0.76) and Cd (3.01) in river sediments both occurred in the BJR. Furthermore, the enrichment factor results also showed that Pb and Cd in the riparian soils and river sediments along the BJR were higher than those in the XJR and Dongjiang River (DJR). The relationship between enrichment factors and nonresidual fractions further proved that the enrichment factors of Cd were significantly correlated with the nonresidual fractions of Cd, which may imply various anthropogenic sources of Cd in the three reaches. Moreover, source identification based on principal component analysis (PCA) and Pb isotope ratio analysis indicated that riparian soils and river sediments have inconsistent pollution source structures. The PCA results showed that Pb and Cd were homologous inputs in the DJR, and there were significant differences only in the riparian soils and river sediments. Pb isotope tracing results further showed that the bedrock of high geological background from upstream may be the main reason for Cd accumulation in the XJR. However, the ultrahigh accumulation of Cd in the BJR is mainly caused by the input of the upstream mining and metallurgy industry. The control of upstream input sources will be the key to the prevention of trace metal pollution in these regions.

Wheat straw- and maize straw-derived biochar effects on the soil cadmium fractions and bioaccumulation in the wheat–maize rotation system

Wheat–maize rotation is one of the most popular systems and successful intensification cropping systems in Northern China, while soils in some of this area are contaminated by cadmium (Cd). However, few studies have performed experiments on the reduction of Cd accumulation in the wheat–maize rotation system. In this study, wheat- and maize-derived biochars are applied to the Cd-contaminated soil to reduce the Cd accumulation in the wheat and maize plants. The results showed that soil biochar applications can significantly decrease DTPA-extracted Cd concentrations by 12.7–26.0% and 13.1–20.5% by wheat- and maize-derived biochars, respectively. Sequential extractions showed that biochar applications significantly reduced the Cd mobility and bioavailability in soils and changed the exchangeable and carbonate-bound fractions of Cd to organic material-bound and residual fractions. The biochar applications increased the plant growth, yield, and quality of both wheat and maize, especially a significant increase in high dosages. The biochar applications also improved the antioxidant enzyme activities and reduced the reduction of MDA in both wheat and maize roots. The Cd concentrations in wheat grains were reduced by 38.1–91.5% and 65.9–80%, and maize grain Cd concentrations were reduced by 20.9–54.2% and 30.8–44% by wheat- and maize-derived biochar applications, respectively, and the Cd concentrations in the root, stem, and leaf were also significantly reduced. The wheat-derived biochar was more effective on the Cd reduction in soil bioavailable fractions and Cd accumulation in crop plants.

Utilization of Straw Resources May Affect the Speciation of Cd and Its Solubility in Cd-Contaminated Paddy Soil

Rice-wheat rotation has long been the most typical cropping system along the mid-lower reaches of the Yangtse River in China, and the full amount returning of wheat straw before rice season is widely practiced in the rotation system. However, massive incorporation of fresh wheat straw may activate Cd in the soil solid phase and cause more labile Cd entering soil solution in Cd-contaminated soil during the flooded rice season. An incubation experiment was conducted under flooded conditions to investigate the solubility and speciation of Cd and its variation and driving factors with an ongoing decomposition phase in simulated Cd-contaminated paddy soil treated with wheat straw addition. The results indicated that wheat straw addition enhanced Cd solubility, and soluble Cd concentration increased with the ongoing decomposition phase. The observed significant positive correlation (r = 0.648, p < 0.01, n = 77) between soluble Cd concentration and DOC content in soil leachate was gradually strengthened with ongoing decomposition. Wheat straw addition decreased the exchangeable fraction of Cd and increased the residual fraction during the first 5 days of decomposition, while the exchangeable Cd increased and the residual Cd decreased after 10 days of decomposition. The exchangeable Cd increased and the residual Cd decreased gradually with ongoing decomposition. Correlation analysis showed that the exchangeable and residual fractions were sensitive to DOC content. Nitrogen fertilizer input further activated Cd by promoting wheat straw decomposition. In summary, wheat straw addition would activate Cd by increasing Cd solubility and promoting the translation from immobile fraction to labile fraction in Cd-contaminated paddy soil under flooded conditions.

The preparation of paddy soil amendment using granite and marble waste: Performance and mechanisms

The wastes generated from the mining and processing of granite and marble stone are generally regarded as useless. However, these waste materials were used as the soil amendments for the first time. The functional groups, crystalline structure and micro-morphology of granite and marble wastes amendments (GMWA) were different from the original wastes demonstrated by X-ray diffractometer (XRD), Fourier transform infrared spectrometer (FT-IR) and Scanning electron microscope-energy dispersive spectrometer (SEM-EDS) analyses. With the addition of the amendments, the cation exchange capacity, electrical conductivity and nutrient availability of the soil increased, and the extractable heavy metals of the soil reduced significantly. Under the condition of the addition of 3% amendments, 7.0%, 99.9%, 99.7% and 70.5% of Cu, Pb, Zn and Cd in exchangeable fractions in soil were transformed to the more stable Fe-Mn oxides- or carbonates-bounded fractions. Tessier method and correlation analysis showed that the reduction of extractable metals in the acidic paddy soil can be attributed to the adsorption of available SiO2, the co-precipitation induced by the elevated pH value, the complexation induced by Fe-Mn oxides and the cation exchange induced by mineral nutrients. This study provides a new strategy for resource recovery of waste stones and remediation of heavy metal-contaminated soil.

Responses of Labile Organic Carbon and Extractable Cadmium Fractions in an Agricultural Soil Following Long-Term Repeated Application of Pig Manure and Effective Microbes.

Long-term pig manure addition has been widely applied in red soil to improve soil fertility. However, the influence of combined utilization of pig manure and effective microbes (EM) on soil organic carbon (SOC) and Cd are not well understood. This study conducted a 23-year (1996-2019) long-term fertilization field trial to investigate the changes of different fractions of SOC and Cd under chemical fertilization (CF), pig manure (PM), and pig manure with effective microbes (PM + EM) treatments in an agricultural soil of Jiangxi Province, South China. The results showed that the pig manure addition significantly enhanced the contents of SOC and Cd in the soils compared with the CF treatments. Furthermore, with the increment of SOC, the PM + EM treatment significantly increased the contents of soil microbial biomass carbon, dissolved organic carbon and easily oxidizable carbon compared with the pig manure application alone. Meanwhile, compared with the CF treatments, the EM addition significantly enhanced the exchangeable and oxidizable fractions of Cd, thus the potential Cd environment risk due to pig manure application should be carefully assessed.

The efficiency of potato peel biochar for the adsorption and immobilization of heavy metals in contaminated soil

We investigated the potential application of potato peel biochar (PPB) for the adsorption and immobilization of heavy metals (Cd, Pb, and Ni) in contaminated acidic soil. The addition of PPB to the soil, especially at the application rate of 8%, increased soil pH, cation exchange capacity (CEC), and organic carbon (OC). The maximum adsorption capacity of Cd, Pb, and Ni in the soil amended with PPB at the application rate of 8% was 3215.9, 4418.67, and 3508.51 mg kg−1, respectively. Compared to the control, the addition of 8% PPB to the soil decreased the soluble and exchangeable fraction of Cd, Pb, and Ni to 84.3, 90.6, and 79.1 mg kg−1, respectively. In contrast, the addition of 8% PPB to the soil increased the organically-bound and residual fractions of metals in the following order: Pb > Cd > Ni, and Cd > Pb > Ni, respectively. The results of this study showed that potato peel biochar has the potential to stabilize and reduce the bioavailability of heavy metals in contaminated acidic soil. Therefore, potato peel biochar can serve as an eco-friendly, low-cost, and efficient adsorbent to immobilization of heavy metals in contaminated acidic soils. NOVELTY STATEMENT Effect of biochar produced from potato peel on the adsorption of the heavy metals in contaminated acidic soil. Immobilization of heavy metals in contaminated acidic soil amended with potato peel biochar. Improving the chemical properties of soil amended with potato peel biochar.

Remediation of Cd-contaminated soil through different layered double hydroxides: The weakness of delamination and mechanism

The great variability of the reaction of layered double hydroxides (LDHs) with anions and cations, especially the ultra-thin thickness, large specific surface area and fully-exposed active sites of single LDHs, may be crucial factors in the remediation of Cd-contaminated soil. In this study, the environmental behavior of Cd was investigated through sequential extraction method and plant potting tests, so as to verify the immobilization efficiency of LDHs and single LDHs in Cd-polluted soil. The results revealed that S-LDHs differed significantly from the original LDHs, and Ca-LDH were the best material for immobilizing Cd. Through sequential extraction procedures, the exchangeable fraction of Cd decreased and its residual fraction increased by forming CdCO3(s) and Cd5(OH)8(NO3)2.2 H2O. After treating the Cd-contaminated soil, the LDH layers were restacked through the XRD-pattern-confirmed S-Mg-LDH, while S-Ca-LDH were more converted to CaCO3 and Cd5(OH)8(NO3)2.2 H2O. Through investigating the structural changes of stabilizers in soil remediation via a series of characterization, the difference between Mg-LDH and S-Mg-LDH was in the generation of CdCO3(s), while that between Ca-LDH and S-Ca-LDH was in the formation of CdAl-LDH-CO3. Single LDHs and LDHs showed the opposite effect when reacting with different pollutants (Cd and Cr), and single LDHs were not as effective for Cd in soil as was previously reported. These results helped with a better understanding of the use of LDHs as a solidified agent as well as the mechanisms involved.

Effects of soil aging conditions on distributions of cadmium distribution and phosphatase activity in different soil aggregates

Aging behaviors of metals in the field differ from those in a controlled laboratory environment. Whether aging conditions influence the fates of metals in soil remains unclear. In this study, distributions of cadmium (Cd) and phosphatase activity were compared in soil aggregates (i.e., >2, 1–2, 0.25–1, and <0.25 mm) along a profile (0–5, 5–10, and 10–15 cm) at the end of 500-day aging experiments under both controlled laboratory and field conditions. Cd concentration in the 0–5 cm layer was lower and Cd concentration in the 5–10 cm layer was higher in field-aged soil compared to laboratory-aged soil. 25.26–35.62% of soil Cd was loaded in >2 mm aggregates of field-aged soils, and 58.41–66.95% was in laboratory-aged soils. Higher loadings of Cd in 0.25–1 and <0.25 mm aggregates were found in field-aged soil. A higher proportion of exchangeable Cd fraction (20.93% of total soil Cd) was found in the 0–5 cm layer of field-aged soil than in laboratory-aged soil (17.63%), while the opposite tendency was found in deeper soil layers. Soil phosphatase activities in field-aged soils were 1.13–1.26 times higher than in laboratory-aged soils. Phosphatase loadings in the >2 mm aggregates were lower and loadings in both the 1–2 and 0.25–1 mm aggregates were higher in field-aged soils than in laboratory-aged soils. Furthermore, correlation analysis and principal component analysis indicated that available Cd fractions accounted for most of the variations in phosphatase activities. In summary, the fates of the exogenous metal Cd differed between field and controlled laboratory conditions. To better understand the behaviors of heavy metals in soil, especially in a seasonal freeze-thaw area, further field studies are needed.

Effects of biowaste-derived biochar on the dynamic behavior of cadmium fractions in soils.

As a commonly used amendment to soil contaminated by heavy metals, biochar has attracted great attention and has been applied for decades due to the benefits to the soil. However, the effects of biochar on the dynamic behavior of soil properties and metal fractions are still unclear. Here, we used two biochars, derived from biowastes (reed and bamboo willow), to treat two cadmium (Cd)-contaminated soils, S1 (loamy sand) and S2 (sandy loam), and determined the dynamic effects. The incubation experiments were designed to investigate the effects of biochar on the dynamic behavior of soil pH, dissolved organic matter (DOM), bioavailable Cd, and the transformation of Cd fractions for 270days. The results showed that the soil pH, DOM, and bioavailable Cd initially increased and then decreased with incubation time, and the soil pH and DOM were higher, but bioavailable Cd content was lower than the original value. The transformation of the metal fractions changed dynamically, and the exchangeable fraction of Cd decreased with incubation time. Furthermore, the correlation results showed that the DOM can directly control the redistribution of Cd fractions, while soil pH can control it indirectly by regulating the DOM. This study highlighted that biochar can affect soil pH and DOM, redistribute Cd fractions, decrease bioavailable Cd content, and lower the potential risk of heavy metals. This study suggests ways to immobilize heavy metals in contaminated soils using biochar.

Cadmium isotope systematics for source apportionment in an urban–rural region

Cadmium (Cd) is a highly carcinogenic metal that can cause toxic reactions, even at low concentrations. In the urban–rural region of Chengdu (China), the Cd concentration of soil ranged from 0.02 mg kg−1 to 5.03 mg kg−1. To decipher Cd source apportionment, samples were collected from topsoil, weathered material, and bedrocks (profiles reaching 200 m in depth) and from local anthropogenic endmembers (pigment, fertilizer, and urban dust). Geogenic Cd weathering processes were determined using the alteration index and exchangeable fraction of Cd in soil. Simultaneously, the Cd isotope composition was measured to determine the geogenic isotopic trend of Cd during weathering and the impacts of anthropogenic inputs on the soil surface. Overall, we observed a high variability in the mass transfer of Cd, indicating the presence of multiple Cd sources. Chemical weathering led to the preferential depletion of heavy Cd isotopes through mineral dissolution and the preferential enrichment of light Cd isotopes during Cd sorption onto newly formed soil phases (−0.35‰ < Δ114/110Cdsoil-bedrock < 0.01‰). Anthropogenic inputs were associated with a wide range of δ114/110Cd values (−0.50 ± 0.02‰ to 0.08 ± 0.02‰). It can be inferred that anthropogenic inputs alter the typical Rayleigh fractionation observed during chemical weathering and modify the Cd isotopic trend in the soil surface of urban–agricultural areas.

Biochar combined with phosphate fertilizer application reduces soil cadmium availability and cadmium uptake of maize in Cd-contaminated soils.

Cadmium (Cd) has become the primary pollution factor in farmland, which seriously threatens crop growth and food safety. A pot experiment was conducted to investigate the effect of combined application with biochar and P fertilizer on soil Cd availability and translocation, in which biochar was 0 (C0) and 20gkg-1 (C20), P fertilizer was 0 (P0), 20 (P20), and 40mg P kg-1 (P40). Results showed that, compared with C0 level, the content of DTPA-Cd in soil was significantly decreased with biochar addition after 60days of cultivation, under C20 level, soil DTPA-Cd in C20P40 treatment were significantly increased. Under both C levels, the percentage of exchangeable Cd fraction at P40 rate was significantly lower than that at P20 rate, because the excess P in soil could precipitate Cd. The percentage of residual-Cd fraction was significantly increased with the combined addition of biochar and P fertilizer, particularly in C20P40 treatment, which was 75.95%, while it was only 61.65% in C0P0 treatment. The Cd translocation factor (TF) and bioconcentration factor (BCF) were also significantly reduced in C20P20 and C20P40 treatments compared with C0P0 treatment. Therefore, the combined high P and biochar application was a good choice in inhibiting soil Cd availability and plant Cd uptake, which benefited to the safe utility of the Cd contaminated soil.

Inter-annual reduction in rice Cd and its eco-environmental controls in 6-year biannual mineral amendment in subtropical double-rice cropping ecosystems

The alkaline mineral amendment is a practical means of alleviating Cd concentration in rice grain (CdR) in the short-term; however, the long-term remediation effect of mineral amendment on the CdR and the eco-environmental controls remains unknown. Here a mineral (Si–Ca–Mg) amendment, calcined primarily from molybdenum tailings and dolomite, was applied biannually over 6 years (12 seasons) to acidic and moderately Cd-contaminated double-rice cropping ecosystems. This study investigated the inter-annual variation of Cd in the rice-soil ecosystem and the eco-environmental controls in subtropical rice ecosystems. CdR was reduced by 50%–86% following mineral amendment. The within-year reduction in CdR was similar between early rice (50%–86%, mean of 68%) and late rice (68%–85%, mean of 74%), leading to CdR in all early rice and in 83% of late rice samples below the upper limit (0.2 mg kg−1) of the China National Food Safety Standards. In contrast, the inter-annual reduction in CdR was moderately variable, showing a greater CdR reduction in the later 3 years (73%–86%) than in the former 3 years (54%–79%). Three years continuous mineral amendment was required to guarantee the safety rice production. The concentrations of DTPA-extractable and exchangeable Cd fractions in soil were reduced, while the concentration of oxides-bound Cd was increased. In addition, the soil pH, concentrations of Olsen-P and exchangeable Ca and Mg were elevated. These imply a lower apparent phytoavailability of Cd in the soil following mineral amendment. An empirical model of the 3-variable using soil DTPA-Cd, soil Olsen-P, and a climatic factor (precipitation) effectively predicted temporal changes in CdR. Our study demonstrates that Cd phytoavailability in soil (indexed by DTPA-extractable Cd) and climatic factors (e.g., temperature and precipitation) may directly/indirectly control the inter-annual reduction in CdR following mineral amendment in slightly and moderately Cd-contaminated paddy ecosystems.

Comparative study on the potential risk of contaminated-rice straw, its derived biochar and phosphorus modified biochar as an amendment and their implication for environment

Direct application of contaminated-rice straw (CRS) to soil can cause the secondary pollution in agricultural land because of high content of Cd in rice straw. This study employed biochar or modified biochar technique to reduce the potential pollution risk of Cd in CRS. In the pot experiment, the CRS, straw biochar prepared at 300 °C (B300) and 500 °C (B500), and phosphorus modified biochar pyrolyzed at 300 °C (PB300) and 500 °C (PB500) were added at dosage of 5% into three typical paddy soils. The results showed that CRS and its derived biochar could enhance soil pH, EC, Eh, organic carbon, exchangeable base cations (K+, Na+, Ca2+ and Mg2+), and available phosphate. The application of CRS, biochar and phosphorus modified biochar significantly increased the contents of total Cd in soils relative to control soil. Compared to CRS, the biochar application (especially the PB500) decreased the contents of 0.01M CaCl2-extractable Cd. The application of CRS significantly increased the content of exchangeable Cd fraction (F1), whereas biochar increased residual Cd content (F4). The biochar and phosphorous modified biochar significantly decreased the contents of bioavailable Cd in soils compared to CRS application. The increased soil pH and dissolve organic matter were found to be the main factors in reducing the release of Cd in biochar. The possible mechanisms of biochar in reducing bioavailability of Cd were to significantly increase soil pH, enhance the complexation of Cd ions, and promote the transformation of Cd from easily available to stable (residual) forms. It could conclude that conversion of contaminated rice straw into biochar was an efficient way to minimize Cd availability in soil and reduce the pollution risk of Cd in rice straw.

Cd diminution through microbial mediated degraded lignocellulose maize straw: Batch adsorption and bioavailability trails

Lignocellulose degraded maize straw (LMS) was prepared with the interaction of soil-indigenous microorganisms and further deployed to attenuate the Cd contamination in polluted soil. The Lignocellulose degrading ratio was determined and results revealed the significant degradation of cellulose, hemicellulose and lignin by 33.03, 26.7 and 15.97% respectively as compared to pristine maize straw (PS). Moreover, LMS was also categorized through FE-SEM, FTIR, BET analysis, elemental analysis and XPS technique and the analytical results indicated that lignocellulose structure in maize straw was successfully degraded and was involved in metal-ion complexation. Batch sorption trials revealed that Cd2+ sorption onto LMS was explained well by Langmuir isotherm and pseudo-second-order kinetic model. The LMS showed maximum adsorption capacities (9.84 mg g−1) for Cd2+ as compared to PS (3.30 mg g−1). Moreover, the soil incubation trials (60 days) depicted the availability of Cd decreased by 11.03 and 34.7% with PS and LMS application respectively. The addition of LMS significantly decreased the exchangeable fractions of Cd and ensued an increase in organic matter and Fe–Mn oxides bound fractions. This work clarified the LMS as a promising amendment for effective remediation of Cd-contaminated matrices.

Immobilization Mechanism of Four Types of Amendments on Cu and Cd in Polluted Paddy Soil

Cu and Cd are common pollutants in the soil surrounding copper smelting enterprises. The regional characteristics of southern China results in a high risk of Cu and Cd reactivation in soil after immobilization with soil amendment. To clarify the degree of risk of secondary activation of Cu and Cd, four types of amendments, namely limestone (LS), maifanite (MF), biochar (BC), and iron modified biochar (Fe-BC), were used to study Cu and Cd fraction distribution in soil and soil colloids and the type and fractional changes of in-situ iron oxides. The results showed that the soil amendments were ranked by their immobilizing effect on soil Cu and Cd in the order LS, MF, Fe-BC, and BC; the exchangeable and carbonate-bound fractions of Cu in the soil reduced by 8.19% and 2.33%, and the readily reducible iron- and manganese-bound fractions of Cu increased by 8.00% and 2.69%, respectively, when treated with LS and MF. The risk of secondary activation of heavy metals was higher in soil treated with LS and MF than in soil treated with other amendments. The readily reducible iron- and manganese-bound fractions of Cu reduced by 2.21% and 5.90% and the organic-bound fractions of Cu increased by 4.75% and 3.48% when treated with BC and Fe-BC, respectively. This indicated that the immobilization effect tends to be stable. The exchangeable and carbonate-bound fractions of Cd in the soil decreased by 7.64%, 8.34%, 2.37%, and 6.73%, and the residual fractions increased by 8.27%, 9.18%, 5.73%, and 9.60% respectively, indicating that the amendment treatments resulted in better stability of Cd than Cu. The Cu and Cd contents of soil colloids were 489.92 mg ·kg-1 and 2.57 mg ·kg-1, respectively, which were considerably higher than those in soil (239.98 mg ·kg-1 and 1.93 mg ·kg-1, respectively). The amorphous iron oxide-bound Cu and Cd contents of soil colloids increased significantly with the application of each of the four amendment, which was the main reason and mechanism for the decrease in heavy metal bioavailability. With the extension of aging time, long-term immobilization can be achieved if the heavy metals are further transformed into crystalline iron oxide-bound and residual fractions.

The Combination of Lime and Plant Species Effects on Trace Metals (Copper and Cadmium) in Soil Exchangeable Fractions and Runoff in the Red Soil Region of China

The water-soluble heavy metal ions in contaminated soil may enter aquatic ecosystem through runoff, thus causing negative impact on the water environment. In this study, a two-year in situ experiment was carried out to explore an effective way to reduce the runoff erosion and water-soluble copper (Cu) and cadmium (Cd) in a contaminated soil (Cu: 1,148 mg kg−1, Cd: 1.31 mg kg−1) near a large Cu smelter. We evaluated the ability to influence soil properties by four Cu-tolerance plant species (Pennisetum sp., Elsholtzia splendens, Vetiveria zizanioides, Setaria pumila) grown in a contaminated acidic soil amended with lime. The results show that the addition of lime can significantly reduce the exchangeable fraction (EXC) of Cu and Cd in soil (81.1–85.6% and 46.3–55.9%, respectively). Plant species cannot change the fraction distributions of Cu and Cd in the lime-amended soils, but they can reduce the runoff generation by 8.39–77.0%. Although water-soluble Cu concentrations in the runoff were not significantly differed and water-soluble Cd cannot be detected among the four plant species, the combined remediation can significantly reduce 35.9–63.4% of Cu erosion to aquatic ecosystem, following the order: Pennisetum sp. &amp;gt; Elsholtzia splendens &amp;gt; Vetiveria zizanioides &amp;gt; Setaria pumila. The implication of this study would provide valuable insights for contaminated soil management and risk reduction in the Cu and Cd contaminated regions.

Crop straw-derived biochar alleviated cadmium and copper phytotoxicity by reducing bioavailability and accumulation in a field experiment of rice-rape-corn rotation system

Biochar has the potential to control the bioavailability and transformation of heavy metals in soil, thereby ensuring the safe crop production. A three seasons field experiment was conducted to investigate the effect of crop straw-derived biochar on the bioavailability and crop accumulation of Cd and Cu in contaminated soil. Wheat straw biochar (WSB), corn stalk biochar (CSB), and rice husk biochar (RHB) were applied at the rate of 0, 1.125, and 2.25 × 104 kg ha−1, respectively. The results showed that all types of biochar significantly increased soil pH, organic carbon and cation exchangeable capacity (CEC), compared to the control. The reduction in DTPA extractable Cd and Cu contents was much greater under high dosage biochar application, with a prominence at RHB treatment throughout the three cropping seasons, compared to the control. Moreover, the biological accumulation of Cd and Cu in the grains of rapeseed and corn significantly decreased after biochar application. Linear regression also confirmed the effective role of biochar in controlling the translocation and accumulation of Cd and Cu due to their inactive bioavailability. In addition, the sequential extraction indicated that exchangeable fraction (EXF) of Cu and Cd had decreased, while residual fraction (RSF) had increased under all biochar amendments. Contrarily, the oxidizable fraction (OXF) of Cd decreased while OXF of Cu increased under biochar treatments. Biochar application, especially RHB, could be an effective measure to enhance Cd and Cu adsorption and immobilization in polluted soils and thereby reducing its uptake and translocation to crops.

Efficiency of CH4N2S−modified biochar derived from potato peel on the adsorption and fractionation of cadmium, zinc and copper in contaminated acidic soil

The objective of this study was to investigate the characteristics of potato peel biochar (PPB) and modified potato peel biochar (MPPB), and their effects on the adsorption and fractionation of some heavy metals in contaminated acidic soil. The results showed that the modified potato peel biochar has a higher pH, cation exchange capacity (CEC), surface functional groups, oxygen/carbon (O/C), and hydrogen/carbon (H/C) ratios than the PPB. The addition of modified biochar to soil, especially at the application rate of 8 %, increased soil pH, CEC, and organic carbon (OC). The maximum adsorption capacity values of Cd, Zn, and Cu in the soil amended with MPPB at the application rate of 8 % were 5142.63, 3508.44, and 4993.12 mg kg−1, respectively. The addition of MPPB to the soil transformed the unstable fraction (soluble and exchangeable) of heavy metals to stable fractions (Fe/Mn oxides–bound, organically–bound, and residual). Compared to the control treatment, the soluble and exchangeable fraction of Cd, Zn, and Cu in soil amended with 8 % MPPB decreased to 102.97, 94.4, and 76.18 mg kg−1, respectively. The order of increment of organically–bound and residual fractions of heavy metals in soil amended with 8 % MPPB were Cu > Cd > Zn and Cd > Cu > Zn, respectively. The results of this study showed that MPPB can be used as a low-cost and eco-friendly adsorbent for the immobilization of heavy metals in contaminated acidic soils.

Feasibility and risk assessment of heavy metals from low-temperature magnetic pyrolysis of municipal solid waste on a pilot scale

Low-temperature magnetic pyrolysis (LMP) of municipal solid waste (MSW) was conducted in a pilot scale continuous reactor to investigate the distribution and transformation of heavy metals (HMs) in biochar. Environmental safety was evaluated by the risk assessment code (RAC) and the modified potential ecological risk index (MRI). Statistical analyses of HMs revealed that the total concentrations of HMs in biochar was higher than that in MSW and the exchangeable fraction of Cd in biochar under 200 °C and 250 °C were at high risk levels. Temperature increment indicates an increase in regular steps not only migrated more HMs into biochar, but also broke the immobilization of HMs, so resulted in higher environmental risks. The lowest direct toxicity to the environment was obtained by LMP at 200 °C. In light of the residual fraction and the high concentration of HMs in biochar produced in this work, it should be mixed with other uncontaminated plant waste for further application in agriculture. The results of economic assessment reveal that the value of net present value (NPV) and the internal rate of return (IRR) can be positive if high quality bio-products are produced with low operating costs. Optimized design of operation, feedstock and the investment are the key factors to improve the economic feasibility of LMP.

Fractionation and accumulation of selected metals in a tropical estuary, south-west coast of India.

Estimating the fractional distribution of sediment-bound heavy metals is highly significant for its ecological risk assessment in contaminated aquatic systems, since environmental factors enhance the mobility of heavy metals and its accumulation in different ecological matrices. In this study, the fractional distribution of Zn, Cd, Pb and Cu in the sediments of the Cochin estuary, along the south-west coast of India, was estimated along with its accumulation in four edible crustaceans. The high mobility of heavy metals in the Cochin estuary was evident from the distribution in fractions other than residual fraction. The exchangeable fractions of Zn and Cd were high in the Cochin estuary, indicating its high bioavailability. Even though the exchangeable fraction is negligible, Pb poses the risk of bioaccumulation due to the presence of oxidisable and reducible fractions. The level of heavy metals varies in different species of edible prawns, and high accumulation of all metals was observed in Metapenaeus dobsoni. Various risk assessment indices show that Cd and Pb pose significant ecological and human health risks in the Cochin estuary.