Evaluation And Solidification Of Heavy Metals Contaminated Soils

Soil pollution is significantly increasing over the world and associated with massive urbanization.For past few decades,a rapid increase has been observed in the assessment of soil pollution to employ efficient environmental management measures.Among various soil pollution issues,heavy metal pollution is considered as one of the serious threats.Because heavy metals,namely lead(Pb),zinc(Zn),chromium(Cr),nickel(Ni),cadmium(Cd),manganese(Mn),and copper(Cu),are ubiquitous,toxic,and resistant to bioremediation and chemical transformation,consequently pose a high risk to the environment and human health.China has gained an amazing economical pace in last few decades but with the economic development few of its cities have been under rapid urbanization.For instance,Shanghai is listed among the most densely populated cities and it has been quickly urbanized in couple of decades.Similarly,Minhang campus of Shanghai Jiao Tong University(SJTU)also has a dense population.We investigated the heavy metal accumulation,magnetic susceptibility,and their correlation in topsoils of North Shanghai and the Minhang campus of SJTU in south-west of Shanghai.Following that,the level of heavy metal pollution in soil was assessed in the both regions.Moreover,the contribution of heavy metal pollution by different anthropogenic activities was studied by segregating the studied area based on land-use-pattern.Afterwards,the potential impact of soil pollution on environmental ecology and human health were analyzed.Last but not the least,a remediation treatment(Stabilization/Solidification)for heavy metal contaminated soil was applied to reduce the seepage of heavy metals to ground water and various factors were optimized to obtain the best results of the remediation technique.(1)A total of 127 topsoil samples were collected from industrial(n=28),roadside(n=38),residential(n=21),and agricultural(n=40)areas.The absolute concentration of heavy metals:Pb,Zn,Cr,Ni,Cd,and Cu,were measured using an Atomic Absorption Spectroscopy(AAS).Nemerow pollution index(NPI)was applied to estimate the pollution level in respective areas.The results showed an excess of heavy metals in the topsoil of North Shanghai,among which,Pb,Zn,and Cd were most prominent.The average values of Pb,Zn,and Cd were obtained as 118.5,228.6,and 0.56 mg/kg,respectively,which are5.6,3.0,and 2.8 times higher than the Shanghai soil background values.(2)In the second phase,enrichment factor(EF)and Kriging interpolation were used to determine the anthropogenic loadings and spatial dependency of heavy metal pollutions,respectively.Multivariate statistical analyses were used to describe the correlation among the sources of heavy metals.Moreover,the potential ecological risk to the environment was also assessed.Finally,the hazardous impact was evaluated based on human health risk assessment for the carcinogenic and non-carcinogenic impact of heavy metal pollution for children and adults.The EF values suggested anthropogenic loadings of the majority of heavy metals;spatial variability showed that pollution is more concentrated in Songnan town;Pearson's correlation,principal component analysis(PCA),and cluster analysis suggested different sources for the majority of the heavy metals.Risk index(RI)of Cr and Pb ranged between 2.8E-04 and 2.7E-07.However,hazard index(HI)was site specific(only for Cr,Pb,Mn),which were mostly in Songnan town.(3)After analyzing the heavy metal pollution level and understanding it's sever impacts on environment and humans,a lab-scale experimental study was designed to analyze the significance of a pollution remedial technique from the soil.A total of 10 soil samples were collected from the Minhang campus of SJTU.The absolute concentration of heavy metals:Cd,Cr,Cu,Pb,and Zn,were measured using an inductively coupled plasma atomic emission spectroscopy(ICP-AES).The soil samples taken in the vicinity of experimental laboratory of Material Science and Engineering showed excess of heavy metals among which Cr,Cu,and Zn were most prominent.The average values for Cd,Cr,Cu,Pb,and Zn were found as 0.164,157,376.25,28.41,222.66 mg/kg respectively,which are 1.22,2.45,16.01,1.33,and 3 times higher than the Shanghai soil background values.(4)We investigated the effect of Portland cement on solidification of heavy metal contaminated clayey and the change in microstructure features of soils that may give range of geoenvironemntal benefits.A series of tests including unconfined compression test(UCT),pH test,and synthetic precipitation leaching procedure(SPLP)were conducted in order to investigate the effects of curing time and cement content on the unconfined compressive strength(UCS)and leachability of heavy metals.Moreover,the test results also showed that the concentrations of leached heavy metals decrease with increasing curing time.After 21 d of curing,the Pb and Zn concentrations of C8 and C4 samples meet the environmental regulatory limits of class-II surface water(GB3838-2002).Test results showed that the dry density of both the samples with 4%cement(C4)and 8%cement(C8)increases with curing time;the increment of the dry density is within 1.2–1.6 g/cm~3.Similarly the compressive strength(qu)of C4 and C8 samples at 21 d of curing increases by 40%(200 kPa in average)and 15%(804 kPa in average)respectively,as compared to the 7 d of curing.The valuable outcomes are achieved from the evaluation of heavy metal contamination of soil and its solidification.The outcomes showed that highly urbanized areas of Shanghai need regular monitoring to protect environment and human health.This study also provide significant information for management purposes to prevent heavy metal pollution and to protect human health.Moreover,Stabilization/Solidification proved to be an effective remediation technique and can be applied to minimize the leaching of pollutants into ground water and to improve engineering properties of contaminated soil,wherever required.