Study On The Pollution Characteristics And Environmental Effects Of Heavy Metals In Soil-crop Systems In Typical Black Rock Series With High Geological Background

The geochemical weathering of black shale is a vital contributor to geogenic enrichment of heavy metals(HMs)in soils from southwestern China.Recently,research on enrichment of HMs in soil from black shale region and its potential eco-environmental risks is of increasing concern.However,the processes and mechanism of enrichment of HMs in soil in black shale are still unclear.Additionally,few study focused on the transfer and accumulation of HMs in soil-plant systems,and on the risks of HMs mobilization.In the present study,rocks,soils and maize were collected from a typical black shale area with high geochemical background in Chengkou County of Chongqing.The transfer,accumulation and environmental effects of HMs in rock-soil-plant system was systematically investigated.The main research understanding are as follows:(1)In the study area,black shales are enriched in HMs,with Cd and Zn up to 52.7 mg/kg and 2363 mg/kg,respectively.Compared with the world shale,average enrichment factors descend in the order of Cd(29.1)> Zn(4.79)> Cr(2.11)> Cu(1.49)> Ni(1.10)> Pb(0.91).HMs in overlying soils have geochemical inheritance with the parent rocks.Clay minerals and Fe-Mn oxides formed during weathering processes of parent rocks play an important role on the enrichment of HMs in soils.(2)The natural soils enriched in HMs due to weathering of HMs-rich parent rocks.The average concentrations of Cd,Zn,Cu,Ni,Cr and Pb are 2.73 mg/kg,433 mg/kg,140 mg/kg,94.5 mg/kg,88.0 mg/kg and 23.0 mg/kg,respectively.Compared with the background value of Chinese soil,average enrichment factors of HMs in agricultural soils descend in an order of Cd(51.1)> Zn(6.58)> Cu(3.90)> Ni(3.54)> Cr(2.57)> Pb(0.79),in which average concentrations of Cd(5.11 mg/kg)and Zn(488 mg/kg)are much higher than those in Chinese soils.High p H,ECEC and Ca CO3 are vital contributors to the enrichment of HMs in soil of the study area.Compared with the “Soil environmental quality Risk control standard for soil contamination of agricultural land”,the samples out of limits followed an order of Cd(100%)>Zn(78%)> Cu(66%)= Ni(66%)> Cr(38%),in which 72% of samples with Cd concentrations exceed the intervention value.Cd was the major pollutant in collected soil and presents the highest pollution level(average Igeo of 4.46)and potential ecological risk(average Er of 1534).(3)In the collected soils,Zn,Cu,Ni,Cr and Pb mainly exist in residual fraction(All proportion were upper than 48.9%),suggesting weak ability of migration of these metals.On contrast,Cd in soils mainly residing in ion exchange fraction(18.4%-50.9%),followed by FeMn binding(14.8%-38.9%)and residual fractions(9.34%-48.6%),indicated the high ability of migration for Cd.The bioavailable concentrations of HMs,extracted by 0.01 M Ca Cl2,descend in an order of Cu(5.43 mg/kg)> Zn(5.22 mg/kg)> Ni(0.43 mg/kg)> Cd(0.25 mg/kg)≈ Pb(0.04 mg/kg).While Cu(average 9.87%)and Cd(average 6.14%)present high extractability.Soil p H is the key factor affecting bioavailability of Cd and Ni.When p H < 6.5,the extractability of Ca Cl2 decrease rapidly with the increasing of p H,but it not obviously changed with p H at near neutral condition because of the strong buffer capacity of Ca CO3.(4)The ability of HMs accumulation in different parts of maize fruit in the study area was descend in an order of corncob > bractea > grain.The average concentrations of HMs in grain descend in the order of Zn(30.0 mg/kg)> Cu(1.96 mg/kg)> Ni(0.95 mg/kg)> Cr(0.70mg/kg)> Cd(0.13 mg/kg)> Pb(0.02 mg/kg).Compared with the limits of HMs in “the National Standard Food for Food Safety,the over-standard rate of grain(fresh weight)was Cr(32%)>Cd(14%)> Pb(8%).The transfer factor of HMs in grains followed an order of Zn(0.07)> Cd(0.05)> Cu(0.03)> Cr ≈ Ni(0.01)> Pb(0.004).Low p H could promote the transfer of HMs from soil to maize.However,the distribution of micronutrient elements(e.g.,Zn and Cu)and toxic elements(e.g.,Cd and Pb)in maize are different due to the blocking effect of corn bractea on toxic elements.The results showed that Zn and Cu are enriched in grains,but Cd and Pb in grains are much lower than those in bractea(0.99 mg/kg for Cd and 0.15 mg/kg for Pb).(5)In the study area,most of soils with HMs exceed the risk control values but only few of maize grain with HMs out of the safety.These results suggested that the present standard may not suitable for the study area.Therefore,combined with p H grading and soil Cd median content,we proposed suggestions for risk control of HMs in soils of study area: When soil p H≤ 6.5,Cd ≤ 0.3mg/kg(risk screening value),maize can be planted safely;when soil p H ≤ 6.5,0.3mg/kg < Cd content ≤ 3.45mg/kg,there are potential risks in the soil,it is necessary to conduct agronomic regulation and soil amendment;when soil p H ≤ 6.5,Cd content >3.45mg/kg,it is not suitable to plant maize.When p H > 6.5,it is safe for maize.This study revealed the source,enrichment and bioavailability of HMs in soils from a typical black shale area.The outcomings are expected to provide scientific guidelines for the utilization of land resources and adjustment of planting structure in the study area.Additionally,it is also expected to provide basic data for pollution prevention and control of HMs in area with high geological background in southwestern China.