Human Health Risk Assessment Of Heavy Metals In Industrial Contaminated Soil Based On Bioaccessibility

Soil Contamination in Northeast China’s heavy industrial region has become an environmental problem of widespread concern because of its harm to human health and the surrounding environment.At present,the risk control of soil pollution in China is mainly based on the total concentration of heavy metals.However,after the soil particles enter the mouth,the heavy metals in the human body can not be completely absorbed by the human body.Therefore,clarification of the coupling effect of the influencing factors of heavy metal bioaccessibility,optimization of the in vitro simulation method of heavy metals in industrial polluted soil are essential to human health risk assessment,which has great significance on cost-effectiveness.The study area is a non-ferrous smelting and processing plant,which is one of the typical heavy industrial venues in northeast China characterized with long pollution cycle,compound pollutants.According to the pollution of deployed points,6 points were selected to collect 21 soil samples within 0～2.5m depth,pollution level and spatial distribution characteristics of Cd,Pb,As and Cu were analyzed.For an estimation of the intrinsic characteristics and oral bioaccessibility of heavy metal contaminants in soil,the soil physicochemical properties,the total concentration of heavy metals,and the content of anion and cation in soil solution were measured;the continuous extraction experiment of arsenic（shiowatana method）and in vitro gastrointestinal simulation experiment（SBRC,PBET,Chinese Pharmacopoeia,UBM method）in soil were carried out;the speciation of arsenic was verified with Visual MINTEQ model.25environmental factors that may affect heavy metals bioaccessibility（including soil physicochemical properties,total concentration and form of heavy metals in soil）are screened through correlation analysis and redundancy analysis.Then prediction models of heavy metals bioaccessibility are established according to the stepwise multiple regression method,so as to reveal the health risk of typical heavy metal pollutants in the soil in study area.Some concluding remarks are obtained:（1）The elements exceeding standard level in soil included Cu,Pb and As,and the over-limit rate was low on the whole;Cd concentrations did not exceed the standard.The polluted areas were mainly distributed in the production workshop and heating power workshop,with compound pollution of heavy metals.Vertically,the soil heavy metal concentration,X1 water-soluble arsenic and X2 surface adsorbed arsenic content first increased and then decreased with the increase of soil depth.The heavy metal pollution was mainly within the depth of 0～1.3m,and the total amount and active forms of heavy metals were accumulated in the deepest layer of mixed soil that was mainly sandy（0.8～1.3m）.（2）The main influencing factors of water-soluble arsenic and surface adsorbed arsenic are particle size,conductivity,p H,Ca²⁺,TAs,Na⁺of soil.The Gaussian DOM model in Visual MINTEQ can accurately predict and verify the morphological distribution of water-soluble arsenic and surface adsorbed arsenic in soil of the study area,and it finds that the chemical form of As is As（V）arsenate.The main factors affecting the bioaccessibility of As in gastrointestinal tract are X2,（TAs）,extreme fine sand and the copper total concentration（TCu）;The main factors affecting the bioaccessibility of Pb in gastrointestinal tract are the total Pb concentration（TPb）,TCu,conductivity and Ca²⁺;The main influencing factors of Cd bioaccessibility in gastrointestinal tract are the total Cd concentration（TCd）,TPb,organic matter,p H and cation exchange content（CEC）,and the total interpretation rate of the main influencing factors amounts to 84.9%.（3）The bioaccessibility of As,Pb and Cd in polluted soil is large.The total concentrations of Cd,Pb and As heavy metals have the most significant impacts on their bioaccessibility.Heavy metals bioaccessibility is greatly affected by the particle size,conductivity and p H of soil physicochemical properties,as well as the antagonism and synergy between heavy metals.When p H increases,the bioaccessibility of Cd,Pb and Cu decreases and the bioaccessibility of arsenic increases;Soil salinity is positively correlated with the bioaccessibility of Cd,Pb and Cu,and negatively correlated with the bioaccessibility of As;The active metal content is mostly in extremely fine sand.The bioaccessibility prediction models of Pb,Cd and As in human gastric（BCG）and intestinal（BCGI）tract are as follows:BCG（Pb）=0.247 TPb-0.021 conductivity+0.934,BCGI（Pb）=0.010 TPb+0.201;BCG（Cd）=0.83 TCd-0.002 conductivity-0.436 medium powder+5.332 coarse powder-1.738,BCGI（Cd）=0.066 TCd+0.02TAs-0.196;BCG（As）=-0.160+4.220 extremely fine sand+0.156 X2,BCGI（As）=0.076 TAs+0.719 X5-0.383.In the in vitro simulation method,the p H is the main controlling factor of As bioaccessibility,and the organic component is the controlling factor of Pb and Cd bioaccessibility.The regression model obtained by UBM method represents Pb and Cd bioaccessibility,and the regression model obtained by Chinese Pharmacopoeia and PBET method represent As bioaccessibility.Chinese Pharmacopoeia method simulates local human gastrointestinal environment,and Cd,Pb,and As bioaccessibility concentrations are low.（4）There is a high carcinogenic risk of metalloid As,carcinogenic risk of Cd and non-carcinogenic risk of Cu in soil of the study area.Therefore,it is necessary to repair As,Cu and Cd in soil before development and utilization,and it is not necessary to carry out Pb remediation and treatment on soil of the study area.The human risk assessment based on the total concentrations of heavy metals overestimates the cancer risk and hazard quotient,which will result in over remediation;While the health risk assessment considers the bioaccessibility of heavy metals,the average values ofCR_oisandHQ_ois are reduced by 30%—86%.Cd and Pb bioaccessibility evaluated by UBM method are safer and more accurate.And As bioaccessibility is safer and more accurate in PBET method.Alternatively,the maximum bioaccessibility value can be conservatively used to assess human health risks.The total concentration of water-soluble arsenic and surface adsorbed arsenic can also replace As bioaccessibility in human health risk assessment.To sum up,the coupling effects of various in vitro simulation methods,soil properties,As forms and combined heavy metals pollution are systematically investigated in this study to the bioaccessibility of As,Pb and Cd in industrial polluted soil,and bioaccessibility prediction models are constructed,which is of great significance to accurately evaluate human health risks.