Preparation And Modification Of Rice Husk Biochar And Its Remediation Of Cd And Se In Water And Soil

Rice（Oraza sativa）is one of the important food crops for human beings,and the total annual global production of rice grain exceeds 740 million tons,producing about140 million tons of rice husk.As an agricultural waste,rice husk has a high calorific value and is commonly used for biomass power generation.In recent years,the application of rice husk biochar,which are produced in large quantities,in the field of adsorption and separation of environmental pollution has attracted much attention.Since rice husk biochar has a porous silicon structure and also has a large number of negatively charged organic functional groups on its surface,it can strongly adsorb heavy metal cations and reduce the biological activity of heavy metals,making it particularly suitable for immobilizing or removing environmental heavy metals.In addition,rice husk biochar is rich in beneficial elements and biodegradable organic matter,and has been shown to improve soil fertility and increase crop yield and quality.In this study,the main directions of heavy metal adsorption in water and heavy metal passivation in soil were aimed to clarify the effect of rice husk biochar on the solidification and removal of pollutants,and to enhance its performance by mixing and modification,and finally to reveal its basic mechanism for the application of rice husk biochar in environmental remediation.The main research results obtained are as follows.1.Different temperatures were used to prepare rice husk biochar,and the effect of preparation temperature on the adsorption of cadmium and selenium by rice husk biochar was investigated.The results showed that as the production temperature increased from 300 to 800°C,the yield of rice husk biochar decreased from 41.6%to28.2%,p H increased from 7.5 to 9.8,surface area increased from 64.8 m²/g to 478.6m²/g,the number of functional groups decreased,and aromaticity decreased.Based on our experiment,the higher the preparation temperature of rice husk biochar,the better the adsorption performance on Cd and Se,and the physical adsorption was enhanced while the chemisorption was weakened.The maximum adsorption amounts of cadmium and selenium were 67.7 mg/g and 0.024 mg/g by Langmuir model fitting.2.Rice husk biochar was modified with iron and manganese oxides under hydrothermal conditions to investigate the adsorption performance of the modified rice husk biochar for cadmium and selenium.The results showed that the p H of the modified rice husk biochar increased and the loading efficiency of Fe and Mn was24.5%and 31.2%,respectively.Compared with the original rice husk biochar,the Mn-modified rice husk biochar improved the adsorption of Cd（Ⅱ）,while the Fe modified rice husk biochar improved the adsorption of Se（Ⅳ）.The adsorption data were consistent with the Langmiur model,and the maximum adsorption capacities of Mn-modified rice husk biochar for Cd（Ⅱ）and Fe modified rice husk biochar for Se were 123.7 and 0.758 mg/g,respectively.the pseudo-second-order kinetic model could better describe the adsorption kinetic processes.The adsorption mechanism is mainly ion exchange and coordination.3.The adsorption,oxidation and desorption processes of Se（Ⅳ）on manganese oxide（δ-Mn O₂）were studied using a stirred flow reactor to simulate the soil flow environment.The results showed that about 9.5%to 25.3%of Se（Ⅳ）was continuously and slowly oxidized to Se（Ⅵ）in the stirred flow system with a kinetic rate constant k of 0.032 h^-1,which was significantly higher than the quasi-level kinetic rate constant of 0.0014 h^-1 of the batch method.The oxidation reaction was driven by the proton concentration and its rate also depended on the Se（Ⅳ）fluid concentration,flow rate,andδ-Mn O₂ dosage.The spectroscopic technique analysis showed that Mn（Ⅱ）was produced and strongly adsorbed on the Mn oxide surface and the reaction of Se（Ⅵ）onδ-Mn O₂ produced Mn（III）as the main product.4.Rice Cd contamination control experiments were carried out in farmlands with different levels of Cd contamination to study the effects of adding different ratios of steel slag and rice husk ash on Cd accumulation and growth of rice.The results showed that both steel slag and rice husk biochar could reduce Cd accumulation in all parts of rice in Cd contaminated areas to different degrees,enhance rice growth parameters and yield,promote the transfer of Cd from the soil exchange state to the reduced and residue states,and increase soil p H.compared with steel slag,rice husk biochar performed better in areas with low soil Cd content（Cd<3 mg/kg）,with19.4%～32.5%increase in rice yield and 33.3%～58.5%decrease in Cd accumulation in brown rice.The amount of Cd accumulated in brown rice decreased by33.3%～58.4%.Moreover,increasing the amount of rice husk charcoal could reduce the exchange state Cd and raise the soil p H.5.The growth and heavy metal accumulation of wheat（Triticum aestivum L.）in two complex heavy metal contaminated（Cd,Zn,Pb and As）soils were investigated using silica-rich soil conditioners such as silicate,rice husk charcoal and rice husk charcoal in combination with bentonite in a wheat pot experiment.The results showed that the addition of silica-rich soil conditioners decreased the heavy metal content in wheat stems and leaves at seedling stage and rice grains at maturity,and the concentrations of Cd,Zn,Pb and As in grains were reduced by up to 20.5%,13.1%,22.2%and 17.8%,respectively,relative to the untreated soil.Photosynthetic parameters in leaf tissues were improved.Heavy metal mobility was reduced by the transfer of heavy metals from intracellular soluble fractions to the cell wall.Soil heavy metal sequential extraction analysis showed that silica-rich soil conditioners could convert bioavailable heavy metals to less bioavailable forms to varying degrees.In addition,with the application of the conditioners,the enzymatic activity of plant antioxidant systems was decreased and plant oxidative damage was reduced.