|Other Abstract||Heavy metal contamination of farmland seriously threatens the sustainable use of soil resources and the safe production of agricultural products, while heavy metals entering the food chain can pose great risks to human health. Environmentally friendly permanent removal of heavy metals from soils is the key to the remediation and safe utilization of farmland soils. Washing is an efficient, economical and permanent technique for soil heavy metals removal, but traditional washing technologies may destroy soil structure and decrease fertility while waste eluent are difficult to deal with. Small-molecule organic acids such as citric acid can effectively remove heavy metals in soil, and have good biodegradability, which will not cause secondary pollution to the soil environment. However, at present, related research mostly uses analytically pure citric acid with higher price to carry out indoor shaking washing parameter optimization experiments, and its field application costs are high, while there are few reports on the effect of citric acid on soil heavy metal removal efficiency at low purity. In recent years, waste extracts produced by agricultural and forestry industries have been considered as potential substitutes for traditional synthetic washing agents due to their easy biodegradation, wide range of waste sources and certain removal efficiency of heavy metal. However, only a few waste extracts washing agents (wine residue, kitchen waste compost, straw waste liquid, etc.) with high liquid-soil ratio have been proved to have a certain removal efficiency of heavy metals in polluted soil. There are relatively few studies on agricultural and forestry peel waste materials, and lack of simple and effective soil heavy metal elution waste liquid treatment materials and methods. We proposed and verified a feasible and eco-friendly solubilization and eluent drainage system using industrial grade citric acid and/or extracts from agro-forestry waste to remove cadmium (Cd) and copper (Cu) from farmland soil. The removal efficiencies of Cd and Cu, washing mechanism, fraction distribution, potential ecological risk, soil fertility, soil enzyme activity and the feasibility of waste eluent treatment method using waste residue were discussed to provide a scientific reference for the construction of a green energy-saving and consumption-reducing remediation technologies for agricultural soil. The main findings of this paper include the following aspects:
(1) The agricultural land solubilization and drainage system can effectively remove heavy metals from soil, and can realize the recycling and reuse of waste eluent. The sequential citric acid-water washing method had higher removal rates of heavy metals in soil than single citric acid, and the removal efficiencies of Cd and Cu were 68.9% and 41.4%, respectively. The Cd and Cu at different soil depth were removed more effectively using the two-stage (citric acid-water) washing with wheat straw than crushed stone as underdrainfilling material. The potential risks of Cd in soils all decreased by approximately 75% from considerable to low risk. The citric acid-water washing with wheat strawfilling significantly mitigated the soil acidification and increased soil organic matter (SOM), available ammonium (N-NH4) and available phosphorus (AP) increased by 40.9%, 57.3%, 32.0%, respectively. The clam powder could efficiently removed 78.7% Cd and 57.5% Cu from the waste eluent, and the regenerated citric acid after shell powder treatment can achieve rapid systemization remediation of heavy metals in polluted soil. The combined solubilization and eluent drainage system is thus a low-cost, high-efficiency, environmentally friendly and potentially practical agricultural soil heavy metal remediation technology.
(2) The extracts of different agricultural and forestry peels and wine-making wastes can realize mild remediation of heavy metals in agricultural soil. Different mild peel and wine-making waste extracts have certain removal effects on soil heavy metals in agricultural land, and the mild pineapple peel material extract has the best heavy metals removal efficiency (juiced pineapple peel extract has an average removal of 15.9% Cd, while the crushed pineapple peel extract is 21.1% Cd from soil). The optimal parameters were selected through the soil washing batch test of crushed pineapple peel extract: the concentration of pineapple peel extract was 5.0% (50 g·L-1), the washing reaction time was 180 min, the pineapple peel extract solution was pH 3 and the solid-liquid ratio 1:5. The average removal rate of different pineapple peel mixed extracts was significantly increased than the single pineapple peel extract. Compared with one-stage washing, the two-stage washing method of pineapple peel citric acid mixed extract increased soil Cd and Cu removal rates by 2.2%-13.2% and 3.1%-9.0%, respectively, which could effectively reduce the potential ecological potential risk of heavy metals in farmland soil, alleviate soil acidification, improve most soil nutrient indicators, and promote the reuse of soil after remediation. The two-stage sequential washing method of pineapple peel mixed washing agents has potential application feasibility in removing heavy metals from polluted agricultural soils.
(3) The residue of waste pineapple peel extracts can realize the simple and friendly removal of heavy metals in waste eluent.The removal efficiency of Cd from the waste eluent by the adsorption materials varied greatly under different conditions. The optimal conditions were as follows: citric acid extraction modified residue of pineapple peel as the optimal material, the modified residue of pineapple peel dosage 10 g L-1, the waste eluent pH> 3.5, and the adsorption reaction time 60 min. The modified residue of pineapple peel has a high removal efficiency of heavy metals from the waste eluent through the combined effect of physical adsorption and chemical complexation. The average removal efficiencies of Cd and Cu in Changsha dryland waste eluent by the modified residue of pineapple peel were 60.1% and 57.4%, Baiyin dryland waste eluent were 64.1% and 53.8%, and Yantai orchard brown soil waste eluent were 78.8% and 75.6%, respectively, which indicated that the waste pineapple peel residue could be recycled and reused by modifying the residue.
(4) The mild pineapple peel waste extract has the potential for field soil remediation of heavy metals. The mild pineapple peel mixed washing agents could effectively remove the heavy metals in the field farmland soil under the combined solubilization and eluent drainage system. The two-stage (PP+CA-H2O) sequential washing method of pineapple peel (PP) and citric acid (CA) mixed washing agents to Yantai orchard brown soil and Baiyin dryland sierozem soil Cd removal efficiencies were 38.21% and 29.58%, respectively, and the residual concentration of Cd in two field soil could be reduced below below the risk screening value of heavy metals in agricultural land in China. Compared with citric acid (CA-H2O), the two-stage (H2O+PP-H2O and CA+PP-H2O) sequential washing method increased the urease activity in the Yantai orchard brown soil and Baiyin dryland sierozem soil by 22.8% and 29.7%, and the soil β-glucosidase activity averaged increased by 14.8% and 12.2%, respectively. The two-stage (CA+PP-H2O) sequential washing of the two soils increased soil microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) by an average of 151.65 mg kg and 9.14 m-1g kg-1 than the original soil, respectively. The germination rate, root length, shoot length and dry weight of wheat grown in the soil after two-stage sequential washing of the different pineapple peel mixed extracts were increased than the two original soils. The average removal efficiencies of Cd from the waste eluent of Yantai orchard brown soil and Baiyin dryland sierozem soil were 65.0% and 53.5% by the modified residue of pineapple peel, which achieved the comprehensive utilization of agricultural and forestry waste resources and heavy metal remediation of energy-saving and consumption-reducing for agricultural soils.
(5) The dissolution mechanism of waste pineapple peel extracts to heavy metals in agricultural soils involves many factors.The removal process of Cd and Cu in agricultural soil from pineapple peel extract can be divided into a fast reaction stage (0-3 h) and a slow reaction stage (3-24 h). The Elovich equation is more suitable to describe the kinetics of the dissolution of soil Cd and Cu from pineapple peel extract. The dissolution and removal process is a heterogeneous diffusion process, which is mainly controlled by the diffusion factor. The differences in the removal efficiency of soil heavy metals from different waste extracts were related to the different concentrations and compositions of organic acids, and the concentration of total organic acids is 4715.5 mg L-1-15237.9 mg L-1. Among them, the removal efficiency of soil heavy metals was the best when citric acid was used as the main organic acid component. The carboxyl, hydroxyl, amine and carbonyl groups were the main functional groups complexed with heavy metal ions in pineapple peel extracts. The washing agents of pineapple peel after different extraction treatments have similar physical structures to those of un-extracted pineapple peels, and their broken and irregular distribution is conducive to full reaction and removal of heavy metals in soil.|