Residual Fraction Of Cd Research Articles

Cadmium isotope fractionation during leaching with nitrilotriacetic acid

This study aimed to determine the leaching mechanism of cadmium (Cd) from contaminated soil by nitrilotriacetic acid (NTA). 114Cd/110Cd isotope technology was adopted as the main characterization method, and experiments were conducted with a typical high-Cd soil to determine the speciation distribution of Cd, isotopic composition, and changes in bonding environment before and after leaching. The speciation distribution of Cd in the soil before leaching in order from most to least was exchangeable Cd, iron‑manganese oxide Cd, residual Cd, carbonate-bound Cd, and organic fraction Cd. The removal efficiency of exchangeable Cd was the highest, followed by iron‑manganese oxide Cd and carbonate-bound Cd. Residual Cd and organic fraction Cd content did not significantly change. Cd(NTA)− and Cd(NTA)24− were the major species of Cd in the leaching reagent, which was enriched in the heavy isotope 114Cd and had a strong bonding environment. The isotope fractionation coefficient α decreased as the proportion of Cd remaining in the soil decreased. Cd speciation distribution had an important influence on the 114Cd/110Cd fractionation degree in the leaching process. The Cd isotope composition in soil (from −0.12‰ ~ 0.04‰ to −0.35‰ ~ −0.18‰ without buffer and − 0.28‰ ~ −0.09‰ with buffer) can reflect the change of Cd speciation distribution, which determines the characteristics of the bonding environment and, in turn, determines the efficiency of NTA leaching to remove soil Cd. As such, isotope fractionation characteristics can be a significant theoretical support related to NTA leaching Cd contaminated soil.

Effects of sheep bone biochar on soil quality, maize growth, and fractionation and phytoavailability of Cd and Zn in a mining-contaminated soil

Biochar prepared from various feedstock materials has been utilized in recent years as a potential stabilizing agent for heavy metals in smelter-contaminated soils. However, the effectiveness of animal bone-derived biochar and its potential for the stabilization of contaminants remains unclear. In the present study, sheep bone-derived biochar (SB) was prepared at low (500 °C; SBL) and high temperatures (800 °C; SBH) and amended a smelter-contaminated soil at 2, 5, and 10% (w/w). The effects of SB on soil properties, bioavailable Zn and Cd and their geochemical fractions, bacterial community composition and activity, and the response of plant attributes (pigments and antioxidant activity) were assessed. Results showed that the SBH added at 10% (SBH10) increased soil organic carbon, total nitrogen, and phosphorus, and also increased the oxidizable and residual Zn and Cd fractions at the expense of the bioavailable fractions. The SBH10 lowered the Zn and Cd contents in maize roots (by 57 and 60%) and shoot (by 42 and 61%), respectively, compared to unamended control. Additionally, SBH10 enhanced urease (98%) and phosphates (107%) activities, but reduced dehydrogenase (58%) and β-glucosidase (30%) activities. Regarding the effect of the pyrolysis temperature, SBH enhanced the activity of Acidobacteria, Bacteroidetes, Firmicutes, Nitrospirae, Verrucomicrobia, Chlorobi, and Microgenomates, but reduced Actinobacteria and Parcubacteria in comparison to SBL. However, only the SBL10 reduced the Proteobacteria community (by 9%). In conclusion, SB immobilized Zn and Cd in smelter-affected soils, enhanced the bacterial abundance and microbial function (urease, phosphates), and improved plant growth. However, validation of the results, obtained from the pot experiment, under field conditions is suggested.

Immobilization of Cd and Pb in a contaminated acidic soil amended with hydroxyapatite, bentonite, and biochar

This study aims to investigate the dynamic changes over time in the immobilization efficiency of hydroxyapatite, bentonite, and biochar for Cd and Pb co-contaminated soil given the potential remobilization of immobilized heavy metals for long-term stabilized soils. Hydroxyapatite, bentonite, and biochar were applied only once at 2.5% (w/w) of soil during a 5-year pot experiment with pepper-cabbage (Capsicum annuum L.-Brassica pekinensis) rotation. The dynamic changes of soil pH, soil organic carbon (SOC), TCLP (toxicity characteristic leaching procedure)-, DTPA-, and PBET (simulated human gastric solution extraction)-Cd and Pb in soil, fractionation of Cd and Pb in soil, as well as the dynamic variations of bioaccessible Cd and Pb in vegetables were determined. The results indicated the application of hydroxyapatite, bentonite, and biochar significantly increased the proportion of residual fraction of Cd and Pb, and reduced the mobility, bioavailability, and bioaccessibility of Cd and Pb analyzed by TCLP, DTPA, and PBET, respectively. Moreover, the concentrations of TCLP-, DTPA-, and PBET-Cd and Pb gradually increased over time. The proportions of acid-extractable Cd and Pb in amended treatments in 2019 were higher than those in 2016. The increment for acid-extractable Cd fraction was 4.69%, 5.53%, and 4.19% in hydroxyapatite, bentonite, and biochar, respectively. While the increment for acid-extractable Pb fraction was 1.48%, 2.08%, and 1.91%, respectively. The bioaccessible Cd and Pb in pepper and cabbage significantly decreased after remediation while no remarkable changes were observed for Pb from 2017 to 2019. The three amendments were effective ameliorants for Cd and Pb co-contaminated acidic soil, but the immobilization efficiency decreased over time. Moreover, hydroxyapatite showed the highest efficiency on metal stabilization and vegetable safety.

Immobilization of Cd, Pb and Zn through Organic Amendments in Wastewater Irrigated Soils

Due to the scarcity of water, raw sewage effluents are often used to irrigate arable suburban soils in developing countries, which causes soil contamination with toxic metals. Soil microorganisms involved in biochemical transformations are sensitive to heavy metals contamination. The study was designed to investigate the effect of organic amendments on the microbial activity of cadmium (Cd), lead (Pb) and zinc (Zn) fractions and their bioavailability in soils contaminated with wastewater irrigation. Three metal contaminated soils under wastewater irrigation were collected, ground, sieved and added to incubation jars. Two organic amendments: wheat straw and chickpea straw, were applied (1% w/w) to the soil before incubation for 84 days at 25 °C. The CO2-C evolution after 1, 2, 3, 5, 7, 10 and 14 days was measured and thereafter was also measured weekly. Soil samples collected at 0, 14, 28, 42, 56, 70 and 84 days after incubation were analyzed for microbial biomass carbon (MBC). Sequential extraction for metal fractionation of samples was carried out collected at 0, 28, 56 and 84 days. Three soils differed significantly in evolved MBC and ∑CO2-C. Chickpea straw addition significantly increased soil MBC as compared to the wheat straw. Organic amendments significantly increased ∑CO2-C evolution from the soils, which was higher from chickpea straw. The addition of crop residues did not affect total Pb, Cd and Zn contents in soils. The concentration of exchangeable, carbonate bound and residual fractions of Pb, Cd and Zn decreased (6–27%), while the organic matter bound fraction increased (4–75%) with straw addition. Overall, the organic amendments improved microbial activity and reduce the bioavailability of toxic metals in wastewater irrigated soils. Furthermore, organic amendments not only reduce economic losses as they are cheap to produce but also minimize human health risks from heavy metals by hindering their entry into the food chain.

Emerging contaminants: Evaluation of degradable chelators towards enhancing cadmium phytoextraction efficiency of bioenergy crop grown on polluted soil

Exogenous application of chelators towards enhancing heavy metals extraction efficiency of bioenergy crop has received considerable attention in recent time. However, little is known about optimal application rate, comparative evaluation of degradable versus non-degradable chelators and their impact on cadmium (Cd) speciation and uptake, physiological and biochemical activity of sweet sorghum grown under Cd stress. Four chelators namely, Nitrilotriacetic acid (NTA), tetrasodium N,N-diacetate (TDA), Ethylenediamine tetraacetic acid (EDTA), iminodisuccinic acid (IDA) and control (CK) were applied and compared at rates of 2, 5, and 10 mmol kg−1 towards augmenting phytoextraction efficiency of sweet sorghum for Cd remediation in a screen house study. Results showed that sweet sorghum augmented with TDA significantly (P < 0.05) increased biomass, enhanced Cd uptake and accumulation when compared to EDTA at application rate of 2–5 mmol kg−1. TDA influenced Cd speciation via increasing acid-soluble, reducible, oxidizable, and residual fractions of Cd, thus increasing Cd bioavailability. NTA and TDA increased proline concentrations, antioxidant enzymes and net photosynthetic activity rate (Pn) comparably to EDTA in sweet sorghum, thus enhancing stress tolerance and stabilizing photosynthetic activities. Overall, exogenous application of degradable TDA at 2–5 mmol kg−1 could be recommended as replacement for non-degradable EDTA in a chelate assisted approach towards augmenting bioenergy crop for phytoremediation of Cd polluted soil.

Attapulgite and processed oyster shell powder effectively reduce cadmium accumulation in grains of rice growing in a contaminated acidic paddy field

Heavy-metal contamination is widespread in agricultural soils worldwide, especially paddy soils contaminated by Cd. Amendment-induced immobilization of heavy metals is an attractive and effective technique, provided that cost-effective materials are used. This field experiment compared three alkaline passivators (attapulgite, processed oyster shell powder, and mixed soil conditioner) at a rate of 2.25 t ha−1 for their effectiveness in decreasing Cd bioavailability in soils and accumulation in rice plants in a paddy field contaminated by Cd (0.38 Cd mg kg−1). The utilization of attapulgite and processed oyster shell powder decreased labile fractions but increased stable fractions of Cd in soils through ion exchange, precipitation and complexation. The addition of attapulgite decreased the concentration of bioavailable Cd in both bulk and rhizosphere soils, whereas the amendment of processed oyster shell powder decreased it only in bulk soil. The Cd accumulation in rice plants correlated significantly with acid-soluble and residual Cd fractions in the rhizosphere soil but not in the bulk soil. The addition of attapulgite and processed oyster shell powder decreased Cd accumulation in rice grains from 0.26 mg kg−1 to 0.14 and 0.19 mg kg−1, respectively, meeting the National Food Safety Standard (< 0.20 mg kg−1). However, the mixed soil conditioner did not decrease the Cd accumulation in rice shoots or grains. This study demonstrated that attapulgite and processed oyster shell powder were economic agents in reducing Cd accumulation in rice grains.

Biochar derived from cadmium-contaminated rice straw at various pyrolysis temperatures: Cadmium immobilization mechanisms and environmental implication

The total concentration, chemical speciation and availability of Cd in biochar derived from Cd-contaminated rice straw were determined to evaluate the potential environmental risk of Cd in biochar and the possibility of biochar as effective way to dispose Cd-contaminated straw. The Cd was concentrated with the increased pyrolysis temperature, while the bioavailability of Cd in biochar was reduced. The sequence extraction indicated that residual fraction of Cd increased and acid exchangeable fraction decreased as pyrolysis temperature increased. The biochar modified by phosphate could significantly reduce the concentration of total and DTPA-extractable Cd in biochar. The pot experiment demonstrated that pyrolysis reduced the potential environmental risk of Cd in biochar. The precipitation and co-precipitation, physical sorption, surface electrostatic interaction, and functional group complexation could be the potential mechanisms of Cd immobilization in biochar. These findings suggested that pyrolysis would be an acceptable and feasible way to dispose contaminated rice straw.

Effects of Mesophilic Anaerobic Digestion and Thermophilic Anaerobic Digestion on the Risk and Stability of Heavy Metals in Sludge

Anaerobic digestion is widely used for sewage sludge stabilization. Different anaerobic digestion methods have different impacts on the speciation and stability of heavy metals in sludge. This study investigated the response of the speciation and stability of heavy metals in sludge to the treatment of mesophilic anaerobic digestion-mechanical dewatering and thermal hydrolysis pre-treatment-thermophilic anaerobic digestion. The results showed that the process of mesophilic anaerobic digestion increased the contents of Cd, Cr, Cu, Ni, and Zn in the sludge, increasing its contamination level and potential ecological risks. In contrast, the thermophilic anaerobic digestion process increased the contents of Cd, Cr, and Pb in the sludge, reducing the level of heavy metal contamination and potential ecological risks. It was found that the main contaminated metals were Cd and Zn in Plant N and Cd in Plant S. Cd has the highest risk factor and is the highest risk contributor to the potential ecological risk of all the types of sludge. The sum of reducible and the exchangeable fractions of Cd, Ni, Pb, and Zn reduced, and the residual fraction of Cd, Cr, Cu, and Ni decreased after mesophilic anaerobic digestion. This indicated that the process of mesophilic anaerobic digestion changed the proportion of potential toxicity and steady state of the heavy metals into direct toxicity. After thermophilic anaerobic digestion, the exchangeable proportion of Cd, Cr, Cu, Pb, and Zn decreased, and the steady state of Cd, Cr, Cu, Ni, Pb, and Zn decreased. After the thermophilic anaerobic digestion process, the proportions of bio-availability and mobility of heavy metals therefore decreased, and the proportions of steady state also reduced. The potential toxicity increased.

Fe-Modified Common Reed Biochar Reduced Cadmium (Cd) Mobility and Enhanced Microbial Activity in a Contaminated Calcareous Soil

The immobilization of soil cadmium (Cd) by biochar and modified biochar is an eco-friendly and cost-effective strategy. In the current study, the effect of raw biochar (BC) and iron-modified biochar (Fe-BC) derived from common reed on the fractionation and mobility of Cd was evaluated, as was its effect on soil microbial activity in contaminated calcareous soil. Treatments involved a combination of two factors: type of biochar (CK: Control, BC, and Fe-BC) and soil Cd concentration (0, 15, and 30 mg kg−1). Treatments were applied to the soil and incubated for 90 days. The application of both biochars increased soil pH and soil organic carbon content (16.6–48.0%), microbial biomass carbon (40.5–75.1%), basal respiration (16.6–48.0%), substrate-induced respiration (12.4–41.9), and dehydrogenase activity (25.5–102.1%), while it reduced diethylene-triamine pentaacetic acid (DTPA)-extractable Cd (22.1–39.5%). The addition biochars, particularly Fe-BC, prominently decreased the concentration of exchangeable and carbonate fractions and increased the concentration of Fe-MnOx, as well as the organic and residual fractions of Cd in the soil. Moreover, relative to the control treatment, the incorporation of raw and Fe-modified biochar into 30 mg kg−1 Cd-spiked soil significantly decreased the Cd mobility factor (MF) value by 14.5 and 21.8%, respectively. Fe-modified biochar had a more significant impact than raw biochar on the immobilization of Cd in the soil, and its improved soil microbial activity to a greater extent. Overall, the findings indicate that Fe-modified biochar derived from common reed can immobilize Cd and improve soil microbial attributes in contaminated calcareous soil. Therefore, it can be used as an eco-friendly amendment for restoring Cd-contaminated calcareous soil.

Study on immobilization of diatomite, Ca(H2PO4)2, CaCO3, HAP and nano-HAP for heavy metal contaminated sediment

Abstract The immobilization performances of Diatomite, Ca(H2PO4)2, CaCO3, Hydroxyapatite (HAP) and Nano-HAP (n-HAP) for Zn, Mn, Pb, and Cd contaminated sediments were investigated by immobilization experiments and evaluated by the European Community Bureau of Reference (BCR) extraction test, toxicity characteristic leaching procedure (TCLP). The result of BCR indicated that HAP and Nano-HAP (n-HAP) had a better immobilization effect on metal contaminated sediments, and the residual fractions of Zn, Mn, Pb, and Cd increased from 30.4, 31.9, 55.49 and 54.27% to 36, 39, 72, and 57%, respectively. The order for immobilized effects of additive was: n-HAP &amp;gt; HAP &amp;gt; CaCO3 &amp;gt; Ca(H2PO4)2 &amp;gt; diatomite. However, the cost-effectiveness of HAP was slightly higher than that of n-HAP, so HAP was more suitable for immobilization of heavy metals in sediment. The TCLP test showed that with HAP as immobilization the leaching amount was reduced by approximately 76, 28, 78, and 85% for Zn, Mn, Pb, and Cd, respectively, compared to the blank group. The results also proved that HAP would be an effective and economical agent for immobilizing heavy metals in sediment, with the optimum mass dosage of 10% (the mass ratios of HAP/sediment (dry weight)) of the sediment.

Remediation of Soil Polluted with Cd in a Postmining Area Using Thiourea-Modified Biochar.

Cadmium presence in soil is considered a significant threat to human health. Biochar is recognized as an effective method to immobilize Cd ions in different soils. However, obtaining effective and viable biochar to remove elevated Cd from postmining soil remains a challenge. More modifiers need to be explored to improve biochar remediation capacity. In this investigation, pot experiments were conducted to study the effects of poplar-bark biochar (PBC600) and thiourea-modified poplar-bark biochar (TPBC600) on Cd speciation and availability, as well as on soil properties. Our results showed that the addition of biochar had a significant influence on soil properties. In the presence of TPBC600, the acid-soluble and reducible Cd fractions were transformed into oxidizable and residual Cd fractions. This process effectively reduced Cd bioavailability in the soil system. Compared to PBC600, TPBC600 was more effective in improving soil pH, electrical conductivity (EC), organic matter (SOM), total nitrogen (TN), ammonium nitrogen (), nitrate nitrogen (), available potassium (AK), available phosphorus (AP), and available sulfur (AS). However, this improvement diminished as incubation time increased. Results of Pearson correlation analysis, multivariate linear regression analysis, and principal component analysis showed that soil pH and available phosphorus played key roles in reducing the available cadmium in soil. Therefore, TPBC600 was shown to be an effective modifier that could be used in the remediation of soil polluted with Cd.

Source apportionment and health risks of the bioavailable and residual fractions of heavy metals in the park soils in a coastal city of China using a receptor model combined with Pb isotopes

The differences in the characteristics and sources of heavy metals in the park soils between bioavailable and residual fractions remain insufficiently demonstrated. In this study, 52 park soil samples were collected in Xiamen, southeast China. A modified sequential extraction method recommended by the European Community Bureau of Reference (BCR) was used to determine the speciations of V, Cr, Co, Mn, Ni, Cu, Zn, As, Cd, Ba, and Pb. Differences between different fractions in the sources and risks of heavy metals were determined using the Pb isotopes, multivariate statistical analysis, and a human health risk model. Based on enrichment factors (EFs) and comparison with a previous record, the contents of Cu, Zn, As, Cd, and Pb in the park soils were significantly influenced by anthropogenic sources, while those of Mn, Ba, V, Cr, Ni and Co were partially impacted by human activities. The bioavailable fraction constitutes important proportions of Cd (82.0%), Cu (58.5%), Mn (58.4%), Zn (57.6%), Co (55.4%), and Pb (50.3%), but 68.6–85.4% of V, Cr, Ni, As, and Ba are mainly present in the residual fraction. Principal component analysis and multiple linear regression (PCA-MLR) revealed that anthropogenic sources are the dominant contributors to the bioavailable fractions of most heavy metals, excluding Cr and Ni. However, 55.1–100% of the anthropogenic sources still contribute to the residual fractions of Cd, Ba, Pb, As, and Mn, which was confirmed by Pb speciation isotopes. These results imply that anthropogenic sources influence both the bioavailable and residual fractions, but more significantly impact the bioavailable fraction. The major contributing elements of Cr, Pb, and As as health risks mainly originate from soil parent materials, coal combustion, pesticides, and fertilizers. This study provides an example of source appointment between bioavailable and residual fractions to identify actual source-specific health risks of heavy metals in the park soil.

Distribution and chemical speciation of heavy metals in various size fractions of aggregates from zonal soils

ABSTRACT Aggregates are the basic structural units of soil; therefore, understanding the distribution and speciation of heavy metals (HMs) in soil aggregates is crucial for the clarification of the environmental behaviour of HMs. In this paper, seven soil profiles from different climatic type regions were sampled and grouped into six size aggregates (>2, 2–1, 1–0.6, 0.6–0.25, 0.25–0.053, <0.053 mm) to determine the distribution and chemical speciation of Cd, Cu, Mn and Pb. The results showed that the vertical distribution of Cd, Cu, Mn and Pb was differed, and the content of total Cd and Pb was ranked in the same order at both subsoil and parent material layer. Although the proportion of <0.053 mm size aggregate was lowest in bulk soil, fine particles showed higher content of total HMs and higher per cent of labile form distribution. However, the contents of total HMs were not always positively correlated with the content of acid soluble or potentially labile fractions of HMs. Total contents of Cu and Mn in aggregates from acidic soils were high, but these two HMs only posed light ecological risk according to the risk assessment code values. Soil organic matter (SOM) and soil pH were involved in the speciation of HMs. Acid exchangeable fraction of Mn in aggregates showed a decreasing trend from alkaline to acidic soils, and SOM positively correlated with the content of oxidisable fraction. For Cd, Cu and Pb, the content of potentially labile fraction of Cu positively associated with the content of Pb potentially labile fraction, and the content of residual fraction of Cd were negatively correlated with the content of Cu residual fraction.

The evaluation of in-site remediation feasibility of Cd-contaminated soils with the addition of typical silicate wastes

In-site remediation is a relatively promising and socially acceptable technique for heavy metal contaminated soils. But the key task is to select cost-effective and environment-friendly amendents for the consideration of practical application. Based on the property of four typical silicate wastes such as straw ash (SA), coal fly ash (CFA), ferronickel slag (FNS) and blast-furnace slag (BFS), effects of four wastes on available Cd content and Cd chemical speciation in amended soils, and physicochemical properties of the amended soils were carried out in the study. The results showed that four wastes were dominately composed of the amorphous phases with OH− ions readily released. When the weight ratio of silicate wastes to artificial Cd-contaminated soils reached 10%, the available Cd contents decreased from 4.12 mg/kg in untreated soils to 1.94, 1.92, 1.45 and 1.53 mg/kg in amended soils by adding SA, CFA, FNS and BFS respectively, after the soils were amended for 30 days. The residual fraction of Cd (R) was 2.54, 2.48, 2.77 and 2.58 times higher in amended soil than that in untreated soil when SA, CFA, FNS and BFS was added, respentively. The soil pH and CEC were improved. The amended soils by adding SA and FNS were looser than those by adding CFA and BFS, and air permeability of the amended soils by SA was better than that by FNS.

Apricot shell- and apple tree-derived biochar affect the fractionation and bioavailability of Zn and Cd as well as the microbial activity in smelter contaminated soil

The aim of this study was to elucidate the effects of apricot shell-derived biochar (ASB) and apple tree-derived biochar (ATB) on soil properties, plant growth, microbial communities, enzymatic activities, and Zn and Cd fractionation and phytoavailability in mining soils. Smelter soil contaminated by Zn (1860.0 mg kg−1) and Cd (39.9 mg kg−1) was collected from Fengxian, China, treated with different doses (0 (control), 1, 2.5, 5, and 10% w/w) of both biochars and cultivated by Brassica juncea in a greenhouse pot experiment. The acid-soluble, reducible, oxidizable, and residual fraction and plant tissue concentrations of Zn and Cd were determined. Biochar addition improved plant growth (22.6–29.4%), soil pH (up to 0.94 units), and soil organic matter (up to 4-fold) compared to the control. The ASB and ATB, particularly ATB, reduced the acid-soluble (21–26% for Zn and 15–35% for Cd) and the reducible (9–36% for Zn and 11–19% for Cd) fractions of Zn and Cd and altered these fractions in the organic and residual fractions. Therefore, the biochars decreased the metal concentrations in the roots (36–41% for Zn and 33–37% for Cd) and shoots (25–31% for Zn and 20–29% for Cd), which might be due to the increase in pH, biochar liming effects, and metal sorption by the biochar. The biochars impact on the bacterial community composition was selective. The ASB and ATB decreased the activities of soil β-glucosidase, dehydrogenase, and alkaline phosphatase while increasing the urease activity. The biochars, particularly ATB, can be considered as effective soil amendments for reducing the phytotoxicity of Zn and Cd in contaminated soils, improving plant growth, enhancing the abundance of specific bacterial groups and increasing urease activity; however, more attention should be paid to their negative effects on the activities of β-glucosidase, dehydrogenase, and alkaline phosphatase.

Biochar and bacteria inoculated biochar enhanced Cd and Cu immobilization and enzymatic activity in a polluted soil

The application of biochar in the remediation of heavy metal contaminated soil has received increasing global attention during the past decade. Although there has been some review work on the mechanism of heavy metals stabilization by biochar, the effects and mechanisms of interaction between biochar and functional microbes such as heavy metal tolerant, adsorption and transformation microbial strains remains unclear. In this paper, maize biochar and a heavy metal-tolerant strain Pseudomonas sp. NT-2 were selected to investigate the dynamic effects and potential mechanisms of biochar and bacteria loaded biochar on the stabilization of Cd and Cu mixed contaminated soil by a 75-day pot experiment. The results showed that, compared to the single biochar amendment, the application of biochar inoculated with NT-2 strain at the rate of 5% significantly increased the soil pH at the initial stage of incubation, and followed by a slight decline to a neutral-alkaline range during the reaction. The addition of NT-2 loaded biochar could also significantly increase the proportion of residual fraction of Cd and Cu, thus reduce the proportion of exchangeable and carbonate bound species in the soil, which lead to the decreasing of plant and human bioavailability of the metal in the soil indicated by DTPA and simulated human gastric solution extraction (UBM), respectively. Finally, the application of bacterial loaded biochar also markedly enhanced soil urease and catalase activities during the later stage of the incubation, and improved soil microbial community at the end of incubation, which indicates a recovery of soil function after the metal stabilization. The research results may provide some new insights into the development of functional materials and technologies for the green and sustainable remediation of heavy metal contaminated soil by the combination of biochar and functional microorganisms.

Performance of the emerging biochar on the stabilization of potentially toxic metals in smelter- and mining-contaminated soils.

Soil potentially toxic metals (PTMs) pollution caused by anthropogenic activities has become serious concern with respect to the crop safety production. In this study, an emerging biochar derived from kiwi pruning branches waste was employed as amendment aiming to evaluate its remediation potential on smelter- and mining-contaminated soils. The effect of biochar on the soil physicochemical properties, leachability, and chemical fractions acted on stabilization practice of PTMs in soil was investigated. The results showed that the addition of biochar increased the soil pH, cation exchange capacity, organic matter, and enzymatic activities (dehydrogenase, urease, and sucrase) but reduced the extraction toxicity of PTMs in both smelter (Fengxian, FX) and mining (Tongguan, TG) soils. The fraction analysis showed that the maximum reduction of exchangeable fraction of Cd, Zn, and Pb in the 4% biochar amended soils decreased by 11.1, 13.3, and 24.7% in FX soil and 7.67, 22.8, and 7.89% in TG soil, respectively, in comparison with to control (no biochar added). Additionally, the residual fraction of Cd, Zn, and Pb increased by 55.9, 7.14, and 11.0% in FX soil and 23.7, 5.86, and 10.0% in TG soil, respectively. The further greenhouse experiment showed that the Indian mustard (Brassica juncea) production increased with the increasing application dosages of biochar, while the PTMs uptakes in plant notably decreased after amendments. Conversion of kiwi pruning branches waste into emerging biochar benefits the agricultural waste recycling utilization and enhances PTMs-contaminated soil remediation in practice. Graphical abstract.

Ecological risk assessment of heavy metals in road dust based on improved potential ecological risk index: A case study in Zhengzhou, China

In order to analyze the ecological risk of the heavy metals in road dust. The improved potential ecological risk index was established. In the improved method, the toxicity of different chemical speciations of heavy metals were considered. Since residual fraction of Cd are non-toxic in the improved method, the results of the improved method were much lower than the traditional method.The results showed that the total risks of 3# and 15# were medium risk. The highest risk appeared on14# with the value 337.49, which belonged to high risk, and the total risk of the other sampling sites were low risk. Therefore, 14# should be the primary control site. The improved method established in this study is more scientific.

Pyrolysis of various phytoremediation residues for biochars: Chemical forms and environmental risk of Cd in biochar

Various phytoremediation residues (PMRs), including Brassica napus L. (BN), Pennisetum sinese (PS) and Lolium perenne L.(LP), were pyrolyzed at 400, 500, 600 and 700 °C, respectively. A series of sequential and single extractions were employed to analyze the chemical speciation and potential environmental risk of Cadmium (Cd) in different phytoremediation residues-derived biochars (PMBs). The results showed that the exchangeable Cd fraction decreased but the residual Cd fraction increased, indicating the inhibition of bioavailability of Cd and low potential ecological risk index of PMBs. When the temperature was over 600 °C, the Cd in biochar was acceptable to the environment and the leaching concentration of Cd extracted by the three extraction methods (distilled water, SPLP and TCLP) were all under the standard limit. Findings from this study illustrated that the treatment of pyrolysis was feasible for the three kinds of PMRs at 600 °C with acceptable environment risk.

In-Situ Immobilization of Cd-Contaminated Soils Using Ferronickel Slag as Potential Soil Amendment.

The current study investigated the efficiency and mechanisms of in situ immobilization of artificially Cd-contaminated soils with ferronickel slag (FNS). The available Cd content of soil was measured and the modified European Community Bureau of Reference (BCR) sequential extraction procedure (SEP) was adopted to quantify the evolutions of Cd chemical speciation after the immobilization by the FNS. The results showed that the addition of FNS (5%‒15%) remarkably reduced the available Cd content and increased the pH and cation exchange capacity of soils. The passivation rate of Cd increased from 58.13% to 73.25% as the spiked Cd content rose from 10 to 120mgkg‒1. The BCR SEP test revealed that the FNS addition substantially reduced the acid soluble fraction and increased the residual fraction of Cd, indicating the reduction of mobility and bioavailability of Cd in soils. The chemical precipitation, ion exchange and surface complexation might be involved in in situ immobilization of Cd-contaminated soils by the FNS.