Study On The Adsorption Of Four Kinds Of Materials To Heavy Metals And The Bioavailability Of The Adsorbed Heavy Metals

In recent years,heavy metals have been enormously introduced into the agricultural soil by improper irrigation with industrial,domestic,and mining waste water with high concentrations of heavy metals(HMs).The HM can enter the food chain and accumulate in human bodies by long-term consumption of HM-enriched foods.Since HM may have severe toxic effects on human health,the excessive HMs in soil has become an important environmental concern and social issue.The HMs have to be remediated in order to reduce the HM accumulation in plant tissues.One most common approach is to stablize the HM by using stablizing agents.However,many of previous work have focused on adsorption and desorption behaviors of HMs on sorbents in batch sorption experiments and HM uptake by plants in short-term growth periods.Thus,there is a need to investigate the long-term effects of potential stablizing agents on HM immobilization in soil in the long run.In this study,commercial biochar,diatomite,attapulgite,and water-retaining agents(WRAs)were tested for their sorptive removal of HMs in aqueous solutions,and stabilization of HM in soil in a pot experiment.To implement the experiment,the four stablizing agents were first saturated with HM solutions,and HM spent sorbents were then air-dried and mixed with soil prior to ryegrass planting.The ryegrass was consecutively harvested for six times,and accumulative HM removal by ryegrass were obtained by summing up HM uptake in plant tissues.In addition,the soil samples were also analyzed for the total HM concentrations as well as fractionation of HMs.Accordingly,results from HM accumulation in both plants and soil have been used to reveal HM release characteristics,HM bioavailability and transformation of different HM fractions after long-term incubation of HM saturated materials.The main results obtained were as follows:1.Batch sorption experiments indicated that the maximum adsorption capacities of Pb and Cu were in the order of WRAs>Attapulgite>Biochar>Diatomite,Cd was in the order of WRA>Biochar>Attapulgit>Diatomite.2.The desorption experiment revealed that sorption of Pb,Cu,and Cd ions by four agents was less attributed to ion exchange,most of them are combined by complexation,even chelation.3.Ryegrass uptake trials revealed that the HM concentrations in plant tissues were not significantly elevated by incorporation of pristine stabilizing agents in CKB,CKD,CKA,CKW treatments,compared to CK0.However,the application of HM-saturated agents increased HM accumulation in plants.Specifically,Pb-saturated WRAs(Pbw),Cu-saturated diatomite(CuD),and Cd-saturated WRAs(Cdw)increased Pb,Cu and Cd uptake to largest extent,respectively.4.The bioavailability of HMs can be inferred from HM concentrations in ryegrass.The results implied that plant uptake of Pb,Cu and Cd mainly occurred in the first four harvests,accounting for over 85%of HM uptaken during entire six harvests.The amounts of HM released into soil by the spent stablizing agents were not positively related to the sorptive capacity of HMs in batch sorption experiments.This could be associated with the bioavailability of sorbed HMs.The experimental results showed that Pb sorbed by biochar and WRAs was morebioavailable than diatomite and attapulgite,as suggested by greater proportion of Pb in plants in total Pb.Regarding Cu,diatomite-Cu was most bioavailability,and attapulgite-Cu was least.Moreover,WRAs-Cd tended to be released into soil for plant uptake,but diatomite-Cd was most difficult to be detached.5.The fractionation of HMs in the soil was performed based on the Community Bureau of Reference(BCR)procedure.With respect to sorbed Pb,reducible and residual fractions accounted for the greatest and least portion in all four stabilizing agents.Regarding sorbed Cu,least residual fractions were observed for biochar and WRAs-Cu,but the greatest for attapulgite-Cu.For biochar-Cd,Cd existed mainly as acid-soluble fraction,with small portion of oxidizable and residual fractions.For diatomite-Cd and WRAs-Cd,the four fraction decreased in the following order:the acid soluble>reducible>residual>oxidizable fraction.Attapulgite-Cd,the four fraction decreased in the following order:the acid soluble>reducible>oxidizable>residual fraction.6.The application of stabilizing agents affected soil physical and chemical properties and nutrient concentrations in soil.It was not significantly changed after treatments.