Mechanisms And Effects Of Combined Amendment Regulating Paddy Soil Complexly Contaminated With Cadmium And Arsenic

With the rapid development of China's industry and agriculture,the pollution problem of cultivated land with heavy metals like Cd and As becomes increasingly grim.As "town of nonferrous metals" and "town of agriculture and fishery",Hunan Province is suffering from the severe pollution and destruction by large-scale mining to adjacent ecological environment and agricultural land,which seriously restricts the rice production and edible safety.How to manage the farmland suffering Cd-As complex pollution and control Cd-As translocation to rice turns to be an urgent environmental task.Surrounding the remediation of paddy soil suffering Cd-As complex pollution,a combined amendment QFJ was developed in this paper through investigating the paddy soil with different Cd-As pollution levels in Mine A(Shizhuyuan Mining Area in Chenzhou)and Mine B(the mining area in Fuxing Village,Baojing County,XiangXi Autonomous Prefecture)based on amendment screening experiment,aqueous solution adsorption experiment,rice pot experiment and field experiment.Furthermore,the mechanisms of QFJ to reduce Cd and As bioavailability in soil as well as their concentrations in brown rice were systematically described,thus providing an effective technique for controlling the bioavailability and translocation of Cd and As in paddy field.Our findings have been expatiated below.(1)The action mechanisms of QFJ,an amendment developed by combining hydroxyapatite,zeolite and biochar according to definite ratios,were determined in our study.Owing to large specific surface area,all these 3 substances could adhere Cd2+ and As5+ in the solution through surface adsorption and channel adsorption.The exchange adsorption and covalent binding of-OH,-COOH,-Si-O-Si and CO32-groups with Cd2+ and As5+ ions would produce carboxylates,silicates and carbonates,thereby realizing the situ immobilization of Cd2+,As3+ and As5+ freeing in the soil solution.(2)The rice pot experiment to verify QFJ improving paddy soil suffering Cd-As complex pollution demonstrated that pH values,CEC and OM contents in the rhizosphere soil followed an increasing trend.QFJ application reduced exchangeable Cd concentrations in the rhizosphere soil of Huanghuazhan and T You 272 by 12.6%?39.8%and 20.6%?70.6%respectively;QFJ application amounts between 0.5 and 2.0 g·kg-1 exerted a lowering effect on exchangeable As by 12.2%?55.1%and 7.0%?39.5%respectively,while increased application levels were associated with dramatically elevated exchangeable As concentrations.With the increase of QFJ application,the TCLP extracted Cd level in soil was gradually reduced,whereas that of As was decreased first and then increased.Soil pH value and CEC were found in a positive linear relationship with the levels of exchangeable Cd,TCLP extracted Cd and acid extractable Cd in soil,while soil pH value exhibited a negative linear relationship with the levels of extractable As and TCLP extracted As.(3)In the rice pot experiment,applying QFJ changed the chemical speciation fraction of Cd and As in soil.It reduced acid extractable Cd concentrations,and promoted transformation of soil Cd from soluble to insoluble ones.As a result,the concentrations of soil organic Cd increased by 6.8%?49.5%and 2.5%?56.5%,respectively.It also promoted transformation of soil As from soluble to insoluble,thus increasing calcium-bonded As concentrations by 34.1%?93.4%and 2.8%?53.3%respectively,and reducing exchangeable As concentrations.However,when the application rate of QFJ reached beyond 4.0 g kg-1,the soil exchangeable As concentrations turned upwards.(4)According to the rice pot experiment,the Cd concentrations in all rice parts were lower than the control group CK,and the Cd concentrations in brown rice were lessened from 0.49 mg·kg-1 to 0.11 mg·kg-1;the Cd concentrations in different rice parts followed the rank of roots>straws>brown rice>husks.QFJ application of 0.5?2.0 g·kg-1 could significantly reduced the As concentrations in each overground rice part.With QFJ application at 2.0 g·kg-1,the As concentrations in brown rice of two varieties were decreased from 0.27 mg·kg-1 and 0.28 mg·kg-1 to the minimum values of 0.11 mg·kg-1 and 0.16 mg·kg-1,respectively.However,with the increase of application amount of QFJ,the As concentrations in all rice parts began to rise.The As concentrations in different rice parts followed the rank of roots>straws>husks>brown rice.(5)The rice pot experiment showed that Cd bioaccumulation factor(BAF)(of rice roots varied in the range of 0.65?1.26,while As BAF in the range of 0.024?0.061,showing obviously that bioaccumulation capacity of the rice roots was greater for Cd than for As.The Cd translocation factors(TF)of rice parts were in the order of TFCd-hb>TFCd-sh>TFCS-rs,whereas As TFs were in the order of TFAs-rs>TFAs-hb>TFAs-sh.These orders suggested that among various rice organs,rice husk was the highest in Cd transferring capacity,while root was the highest in As transferring capacity.Application of QFJ also decreased the rice translocation quantity factor(TQF)of Cd and As,and then decreased the total accumulation quantity of rice plants of Cd and As in the overground.(6)The field experiment showed that the pH value,CEC and OM contents in the rhizosphere soil at different growing stages were increased with QFJ applying amounts in both mining areas.The QFJ application could significantly reduce the exchangeable Cd concentrations at different rice growing stages in both mining areas,with the highest reduction amplitude reported at the tillering stage.Contrary results were observed between two mining areas in terms of exchangeable As in soil,where Mine Area A exhibited increased risk while Mine Area B showed a risk-reducing tendency.QFJ application could also significantly decrease the TCLP extracted Cd levels in soil at different rice growing stages in both mining areas;the similar function was observed in the TCLP extracted As levels in Mining Area B,but the application amount of 4.0 g·kg-1 was associated with increased risk in Mining Area A.(7)The field experiment demonstrated that QFJ application could not only effectively reduce the Cd concentrations in all rice parts at different growing stages,but also lessen the As concentrations in all rice parts at the maturing stage.At QFJ application amount of 4.0 g·kg-1,the Cd concentration in brown rice was decreased from 0.46 mg·kg-1 to 0.18 mg·kg-1,whereas inorganic As from 0.22 mg·kg-1 to 0.18 mg·kg-1;both the Cd and inorganic As concentrations in brown rice were less than 0.2 mg·kg-1,which met the requirement set in the National Food Standards(GB 2762-2012).