Enrichment Mechanism And Bioavailability Of Heavy Metals In Soils With High Geochemical Background In The Karst Region Of Guangxi Province,China

Heavy metals are typical cumulative pollutants with the characteristics of non-degradability,biological toxicity and persistence.The enrichment of heavy metals in soils poses a potential threat to soil,crop and ecological environment.In southwestern China,the soils have a high geochemical background for Cd and other heavy metals,which was naturally induced by the weathering of carbonate rocks and the subsequent soil formation processes.However,mechanism of geogenic heavy metal enrichments in these areas were poorly understood,and the origin of heavy metals in the soils have been controversial.Meanwhile,the bioavailability and ecological risk of heavy metals in the soil-rice system in karst areas with high geochemical background are urgent problems to be resolved.Guangxi is one of the most extensively distributed karst landforms in China.It is also with the highest degree of naturally-induced heavy metal enrichments in China,thus typical high geochemical background dominates this area.In this thesis,the karst region of Guangxi is selected as the research area,and samples of rocks,saprolites,soils,rice,etc.have been systematically collected.In-depth research has been conducted on the distribution,enrichment mechanisms,inheritance relationships,migration characteristics and ecological risks in the rock-soil-rice system.The main research results obtained are as follows:（1）The contents of As and heavy metals in soils from the karst region of Guangxi were significantly higher than the background values of soils in China.The proportions of samples with Cd content exceeding the background value of soils in Guangxi and China reached 84.2%and 100%,respectively.The proportion of samples exceeding the risk screening value and risk control value reached and 76.3%and 7.6%.The contents of As and heavy metals in the vertical soil profiles varied slightly,and even tends to be enriched in the deep horizon.This phenomenon is significantly different from that in anthropogenically contaminated areas where heavy metals are mainly enriched in the surface horizon.In various types of soils,heavy metal contents are disctinct,for instance,Cambisols and Ferralsols have the highest content of heavy metals,and Anthrosols has the lowest content of heavy metals.（2）The elevated Cd in local carbonate parent rocks and the residual accumulation during the soil formation processes control the enrichment of Cd in soils,while the main reason for the enrichment of As,Cu,Hg,Ni,Pb,Zn is their residual accumulation.Multiple factors might affect the heavy metal enrichments in soil:the higher content of Al,Fe,and Mn is associated with the higher enrichment of heavy metals;the increase of soil p H and TOC may promote the enrichment of heavy metals;the higher degree of soil weathering is associated with the higher enrichment of heavy metals;clay minerals,iron and manganese oxides in the soil are principal carriers of heavy metals,and play an important role in the enrichment of heavy metals in the soil.（3）The inheritance of heavy metals in the carbonate bedrock-saprolite-soil weathering profile and the Pb isotopic compositions show that the soils in the karst region mainly originate from the in-situ weathering of the underlying carbonate rock.During the pedogenesis,there is also an external input of Pb,the transportation and deposition of non-carbonate materials（clastic rock/granite derived soils）made a relatively low contribution to Pb input in the soil,while the input of Pb from anthropogenic activities is negligible.The ^206/207Pb and^δ114/110Cd compositions of carbonate rocks and soils derived from carbonate rocks are significantly higher than other natural samples and are far away from anthropogenic end members.Because the initial U/Pb ratio formed in the marine sedimentary environment were very high,resulted in high ²⁰⁶Pb and low ²⁰⁸Pb in the carbonate rocks.During pedogenesis,heavy Pb were lost with carbonate leaching during weathering and the composition of Pb isotopes in the soil is mainly controlled by the components of saprolites.（4）Most of the heavy metals in the soil exist in the residue phase,which have poor migration ability and low ecological risk.For instance,most Pb exist in the residue phase（average 55.7%）and humic acid phase（average 27.7%）,while the ratio of water-soluble phase and ion-exchangeable phase（average 0.4%）is very low.Compared with the anthropogenically contaminated areas,the labile phases of heavy metals in soils with high geochemical background account for a much smaller proportion.The geographical distribution of heavy metals such as Zn,Cr,and Ni in the soil are consistent with their microscopic distribution obtained by SRXRF.XANES results show that Zn in soils mainly exists in a relatively stable speciation that bound to clay minerals.The speciation of heavy metals at the molecular level is also consistent with the macroscopic chemical phases obtained by sequential extraction procedures.（5）The bioavailability and BCF of heavy metals in soils is generally low and much lower than those in the anthropogenically contaminated ares.The exceedance rates of Cd and As are 10.8%and 0.3%,respectively,while other heavy metals do not exceed the FQS.We conclude this phenomenon as"high geochemical background,low ecological risk".The Ca Cl₂-extractable metals vary greatly in different types of soils.The correlations between Ca Cl₂-extractable metals and their concentrations in rice grains are significant but the prediction is not ideal.The DGT technique provide better performance of the Cd content in rice grains than chemical extraction.Soil p H and Ca content were screened to be important indicators and reliable soil-rice transfer models were established.It delivered poignant advice to improve rice quality and control ecological risk in karst region with high geochemical background.