Immobilization Of Arsenic And Cadmium In Contaminated Paddy Soil By Using Schwertmannite Combined With Biochar

Arsenic（As）and cadmium（Cd）are the major contaminants in agricultural soils in China.High contents of arsenic and cadmium in soil may lead to the accumulation of arsenic and cadmium in rice,thus threatening human health.However,the behaviors of arsenic and cadmium in paddy soils are different during their biogeochemical process,making it difficult to develop common stragies to simulataneously control their contamination.Schwertmannite（Sch）is a kind of secondary hydroxyl ferric sulfate mineral with poor crystallinity and metastable state.Schwertmannite has a high specific surface area and a large numbers of hydroxyl groups,sulfate groups and other groups on its surface,showing a strong affinity for arsenic.Previous studies have found that it can effectively immobilize arsenic in contaminated paddy soil.However,the application of schwertmannite into soils would decrease soil p H and thus increase the pollution risk of heavy metals（ie.,Cd）in contaminated soils.Biochar（BC）derived from pyrolysis of different types of biomassed under relatively high temperatures has shown potential in immobilizing heavy metals.However,whether the chemical loading or physical mixing of these two materials can realize the synchronous immobilization of As and Cd in the contaminated paddy soil is still worth further worth study.In this paper,we prepared the composite materials loaded with schwertmannite by biochar（Sch@BC）,and physical mixing of schwertmannite and biochar（Sch+BC）.Their adsorption and removal ability on As and Cd in water,and their immobilization ability for As and Cd in contaminated paddy soil have been compared.The obtained results would provide a novel method for the remediation of As and Cd contaminated soil.The main findings are summarized as follows:1.Based on the results of the influence of schwertmannite and biochar on the p H value of water,the appropriate composite ratio of schwertmannite and biochar was determined to be 1:10.Schwertmannite was successfully loaded on the biochar to form the composite material loaded with schwertmannite by biochar（Sch@BC）.The specific surface area(2.37m²·g^-1)of the Sch@BC was much higher than that of the schwertmannite(1.03 m²·g^-1)or biochar(0.58 m²·g^-1),potenianlly increasing the adsorption sites for As and Cd.2.Adsorption experiments were carried out to explore the sorbing capacities of schwertmannite,biochar,schwertmannite composite biochar（Sch+BC）,and biochar supported schwertmannite（Sch@BC）for As and Cd in water.The results showed that Sch,Sch+BC,Sch@BC exhibited much higher adsorption capacities for As（Ⅲ）,and BC,Sch+BC,Sch@BC exhibited much higher adsorption capacities for Cd（Ⅱ）.When the solution p H was 6.0 and the adsorbent concentration was 2 g·L^-1,the adsorption removal rates of Sch@BC for As（Ⅲ）and Cd（Ⅱ）can reach 97.1%and 40.35%,respectively.The adsorption kinetics data of As（Ⅲ）and Cd（Ⅱ）by Sch+BC and Sch@BC fitted well with the pseudo-second-order kinetics model,indicating that the adsorption process belongs to chemical adsorption.The maximum adsorption capacities of Sch+BC for As（Ⅲ）and Cd（Ⅱ）were 14.28 mg·g^-1 and 1.323 mg·g^-1,respectively,while the maximum adsorption capacities of Sch@BC for As（Ⅲ）and Cd（Ⅱ）were 8.64 mg·g^-1 and 3.643 mg·g^-1,respectively.The adsorption of Cd（Ⅱ）on the surface of biochar was affected by the change of physical and chemical properties of biochar loaded with schwertmannite.3.The immobilization of As and Cd in contaminated paddy soil by Sch+BC and Sch@BC was further studied.The results showed that the addition of schwertmannite would decrease the soil p H value,while the addition of biochar would increase the soil p H value.The addition of schwertmannite can effectively decrease the content of As in soil pore water,significantly decreased the contents of non-obligate adsorbed As and obligate adsorbed As in soil,but increased the contents of non-obligate adsorbed Cd and obligate adsorbed Cd in soil.The addition of biochar has a high adsorption and fixation ability for non-obligate and obligate adsorbed Cd in soil particles,but it can improve the migration and transformation ability of As in soil.The Sch+BC can simultaneously immobilized As and Cd in the investigated paddy soil,while Sch@BC significantly reduced the migration capacity of As in the soil but improved the migration of Cd in the soil.It can be concluded from the present study that the Sch+BC can realize the synchronous immobilization of As and Cd in the contaminated paddy soil,while the Sch@BC would enhance the mobility of Cd in the soil.Therefore,the physical mixing of biochar and schwertmannite at the weight ratio of 10:1 is a novel and effective method to immobilize As and Cd in the contamined paddy soils.