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Few studies have carried out soil washing experiments using pot experiments to simulate in situ soil washing operations, particularly for alkaline soils. This study explored the effects of multiple washing operations using pot experiments on the removal efficiencies of potentially toxic metals (PTM) from alkaline farmland soil and the reuse strategy of washed soil for safe agricultural production. The results showed that the removal efficiencies of Cd, Pb, Cu, and Zn after seven washings with a mixed chelator (EDTA, GLDA, and citric acid) were 41.1%, 47.1%, 14.7%, and 26.5%, respectively, which was close to the results of the EDTA treatment. For the alkaline soil studied, the second washing with the mixed chelators most effectively removed PTM owing to the activation of them after the first washing operation. The mixed chelator more effectively increased the proportion of stable fraction of PTM and maintained soil nutrients (e.g., nitrogen content) than EDTA, indicating little disturbance of alkaline soil quality after washing with the mixed chelator. After the amendment of the washed soil, there was no visible difference in the biomass weight of crops from the soils washed with different agents, indicating that the inhibitory effect of both washing agents on plant growth was effectively alleviated. The Cd and Pb contents in Z. mays were below the threshold of Hygienical Standard for Feeds of China (GB 13078–2017) (1 and 30 mg·kg−1). Moreover, after three cropping operations, the available concentrations of PTM in the soil washed with the mixed chelator were lower than those in the soil washed with EDTA, indicating the value and potential of agricultural reuse of alkaline farmland soil washed with the mixed chelator. [ABSTRACT FROM AUTHOR]

The mixed chelators (MC) of N, N-bis(carboxymethyl) glutamic acid (GLDA) and citric acid (CA), which were biodegradable chelating ligand, were employed to remove heavy metals from sewage sludge. The extraction of Cd, Cu, Zn, Mn, and Cr from sludge was studied under different experimental conditions. Results showed that the removal efficiencies of heavy metals from both sludges were the highest with MC of GLDA and CA at the molar ratio of 1:5. For the sludge of plant A, the extraction efficiencies increased with increasing contact time, but only slowly increase was observed after 24 h. However, for the sludge of plant B, the removal efficiencies were significantly decreased after 16 h, and sharply decreased after 24 h. The removal efficiencies of heavy metals decreased with the increase of the solution pH, while they increased with the increase of the solution concentration. For the sludge of plant A, single washing with 200 mmol·L−1 MC might be favourable to remove heavy metals. However, for the sludge of plant B, duplicate washing with 100 mmol·L−1 MC could remove much more heavy metals. The concentration of Cu in sludge of plant B decreased from 695.79–139.16 mg·kg−1, which was conformed to the standard (GB 4284-2018). These results suggested that MC may be useful and environmentally friendly chelators in the removal of heavy metals from sewage sludge. [ABSTRACT FROM AUTHOR]

Soil washing is an effective technology for the remediation of multi-metal contaminated soils. However, bioavailability of residual heavy metals in soils and soil properties could be changed during washing processes. This study investigated the effects of EDTA, FeCl 3 and mixed chelators (MC) on bioavailability of residual heavy metals in soils and soil biological properties after soil washing. The results showed that soil washing by chelators successfully decreased the total concentration of heavy metals in soils, while it did not effectively decrease the exchangeable fraction of heavy metals, especially for calcareous contaminated soil. The toxic effects of the washed soils seemed to exhibit higher correlations with the changes in the soil properties such as soil pH and nutrient concentrations. As compared with FeCl 3 and EDTA, MC tended to moderately change soil properties (e.g., pH, total N, available N, available P, and exchangeable K, Ca, and Mg). Additionally, MC-washed soil had the least influence on the soil enzymes activities, and had the highest germination and growth of Chinese cabbage. Accordingly, MC is a moderate washing solution in the removal of heavy metals from multi-metal contaminated soils, and had minimal negative effects on soil qualities. [ABSTRACT FROM AUTHOR]

A pot experiment was conducted to investigate the effects of biochars on the availability of heavy metals (Cd, Cu, Mn, Ni, Pb, and Zn) to ryegrass in an alkaline contaminated soil. Biochars only slightly decreased or even increased the availability of heavy metals assesses by chemical extractant (a mixture of 0.05 mol L −1 ethylenediaminetetraacetic acid disodium, 0.01 mol L −1 CaCl 2 , and 0.1 mol L −1 triethanolamine). The significantly positive correlation between most chemical-extractable heavy metals and the ash content in biochars indicated the positive role of ash in this extraction. Biochars significantly reduced the plant uptake of heavy metals, excluding Mn. The absence of a positive correlation between the chemical-extractable heavy metals and the plant uptake counterparts (except for Mn) indicates that chemical extractability is probably not a reliable indicator to predict the phytoavailability of most heavy metals in alkaline soils treated with biochars. The obviously negative correlation between the plant uptake of heavy metals (except for Mn) and the (O + N)/C and H/C indicates that biochars with more polar groups, which were produced at lower temperatures, had higher efficiency for reducing the phytoavailability of heavy metals. The significantly negative correlations between the plant uptake of Mn and ryegrass biomass indicated the “dilution effect” caused by the improvement of biomass. These observations will be helpful for designing biochars as soil amendments to reduce the availability of heavy metals to plants in soils, especially in alkaline soils. [ABSTRACT FROM AUTHOR]

In a field experiment on multi-metal contaminated soil, we investigated the efficiency of Cd, Pb, Zn, and Cu removal by only mixture of chelators (MC) or combining with FeCl 3 . After washing treatment, a co-cropping system was performed for heavy metals to be extracted by Sedum alfredii and to produce safe food from Zea mays . We analyzed the concentration of heavy metals in groundwater to evaluate the leashing risk of soil washing with FeCl 3 and MC. Results showed that addition of FeCl 3 was favorable to the removal of heavy metals in the topsoil. Metal leaching occurred mainly in rain season during the first co-cropping. The removal rates of Cd, Zn, Pb, and Cu in topsoil were 28%, 53%, 41%, and 21% with washing by FeCl 3 +MC after first harvest. The application of FeCl 3 reduced the yield of S. alfredii and increased the metals concentration of Z. mays in first harvest. However, after amending soil, the metals concentration of Z. mays in FeCl 3 +MC treatment were similar to that only washing by MC. The grains and shoots of Z. mays were safe for use in feed production. Soil washing did not worsen groundwater contamination during the study period. But the concentration of Cd in groundwater was higher than the limit value of Standard concentrations for Groundwater IV. This study suggests that soil washing using FeCl 3 and MC for the remediation of multi-metal contaminated soil is potential feasibility. However, the subsequent measure to improve the washed soil environment for planting crop is considered. [ABSTRACT FROM AUTHOR]

According to the bulletin of the national soil pollution reported by Ministry of Environmental Protection and Ministry of Land and Resources of the Peoples Republic of China, the total above standard rate is 16.1% in national soil, and above standard soils of Cd, Zn, Pb and Cu are 7.0%, 0.9%, 1.5% and 2.1%, respectively. Soil washing with chelating agents and phytoextraction by chelator-enhanced is potentially useful technique for remediating the heavy metal-contaminated soils. EDTA is the most frequently cited chelating agent in these techniques because of its strong chelating ability for different heavy metals. However, the slow degradation rate and persistence of residual EDTA in soil potentially increases the metal leaching risk which may cause groundwater contamination. But the environmental risk of EDTA reported in literature is from pot and column leaching experiments in laboratory scale. In order to understand the environmental risk of residual EDTA in the remediation of metal-contaminated soil, the field investigation and the incubation experiments were conducted to investigate the residue and degradation of EDTA in soil. The results of Lechang field investigation revealed that EDTA residue was not detected in the topsoil and deep soil after EDTA applied for 6 years. In Fogang field, the concentration of EDTA in soil was 0.039 - 0.056 mmol/kg soil, which was 2% to5% of the applied amount in the 4th month after application. However, the EDTA was not detected in soil after 1 year. In Wengyuan field, the concentration of EDTA in topsoil was approximately 50% of added amount (3.3 mmol/kg soil) after 45 d of EDTA application, while it was only 2.6% of added amount after application in one year. EDTA residue was detected in the deep soil. However, the EDTA was not detected in groundwater. In addition, the concentration of heavy metals in groundwater was not increased after EDTA application. The deep soils have considerable fixation capacity for the heavy metal-chelator complexes, which help preventing the metal-chelator complexes from leaching down to groundwater. Incubation experiments were carried out to evaluate the degradation of EDTA in different soils. The air-dried soil (2 kg with < 5 mm particle size) was placed in plastic pots with the rate of 10 mmol/kg soil EDTA addition. Soils were incubated at room temperature and about 60%-70% soil water-holding capacity. Then soil samples were taken in 0, 3, 10, 20, 30, 50 and 72 d after incubation. The results indicated that degradation of EDTA in soils followed the first-order kinetic equation. Degradation rate constant of EDTA in latosolic red soil, cinnamon soil and metal-contaminated soil was 4.6×10-3, 1.4×10-2 and 5.8×10-3, and the half-life was 71, 25 and 53 d for each soil, respectively. The half-life of EDTA had a good correlation with the organic matter content and CEC in soil. Microorganisms had a marked influence on the degradation of EDTA in soils. The finding suggested that EDTA enhanced remediation technology can be used on remediating heavy metals contaminated soil, but the added amount should be controlled. As such, the environmental risk of EDTA is minimum. [ABSTRACT FROM AUTHOR]

Land application of sewage sludge usually leads to increased levels of heavy metals in soil, plants and groundwater. Pre-treatment using plants has been proposed to reduce the contents of heavy metals and water in sludge prior to land application. This study quantified the transfer of Zn, Cd, Pb and major nutrients in a sludge-soil-plant-leachate system during the treatment of sewage sludge. To accomplish this, a two year pot experiment was carried out to collect leachate, mono- and co-cropping ofSedum alfrediiand feed crops was conducted in sludge with an under-layer soil support. Sludge phyto-treatment increased Zn and Cd concentrations in the under-layer soil, but not Pb. Specifically, 70%, 70% and 80% of the original Zn, Cd and Pb, respectively, remained in the sludge, while about 40%, 70% and 60% of the original N, P and K remained. Only 3% to 5% of Cd and Zn and < 1% of Pb were transferred into the under-layer soils or leachates, while more than 12% of the N and P were transferred. Co-plantingS. alfrediiand feed crops led to a significant reduction of heavy metals in leachates when compared with sludge without planting. Overall, sludge leachate is more appropriate than whole sludge for recycling in agriculture since it reduces the chance of heavy metal contamination in the agro-ecosystem; therefore, co-cropping phyto-treatment of sludge can be coupled with sludge leachate recycling for crop production and re-collection of the sludge residue for landfilling. [ABSTRACT FROM PUBLISHER]