Enviromental Geochemistry Of Heavy Metal And Modelling Analysis Of Their Speciation For Wuliangsuhai Lake In Inner Mongolia

The heavy metal pollution in lakes is a serious problem, which has been payinggrowing attention because of their toxicity and subsequent accumulation in aquatichabitats. This paper studied the speciation and the environmental geochemicalcharacteristics of seven heavy metals including mercury （Hg）, zinc （Zn）, chromium （Cr）,copper （Cu）, lead （Pb）, cadmium （Cd）, and arsenic （As） in water and sediment inWuliangsuhai Lake （WL） of Inner Mongolia. The pollution sources were analyzed byPrincipal component analysis method and the speciation of these metals and the factorsimpacting their existing status were analyzed by PHREEQC model. The main findingsare listed as below:Firstly, environmental geochemistry of heavy metal of WL is studied and analyzed.The water quality data combined with national standard value are analyzed. For themetals of Cu, Zn, Pb, Cr, Cd, and As, most of the samples collected for the seven metalsmeet the national surface water quality standard first level and the national fisherystandard。The pollution of Hg did not meet the national surface water quality standard Iand the national fishery standard, and50%of the samples did not meet the surface waterquality standard IV. The spatial estimate and analysis on variability of heavy metal areanalyzed by Kriging. It shows the spatial distribution of the heavy metals with highconcentrations can be classified as the point pattern of Hg-Zn-Cr at the inlet of the lake,the strip pattern of Cu-Pb-Cd-As at the outlet, and the strip pattern of Cu-As at both inletand outlet. The environmental pollution is analyzed using the method of IntegratedPollution Index and the health risk model. The water heavy metal pollution of WL isslightly and the carcinogen risk level is low.In order to explain the driving factors for seasonal difference, selected sediment datato research environmental geochemistry of sediments. Based on the geoaccumulationindex, i) Cu, Zn, Pb, Cr, and Cd were more polluted in winter than in summer; ii) Hg andAs were more polluted in summer than in winter. Based on the correlation analysis,temperature, hydrodynamic conditions, organic matter concentration, and particle sizewere the main control factors for seasonal distribution pattern and Hg and As were themain control factors for redox and salinity properties. Principal component analysis（PCA） was used to identify the different pollutant sources. The results demonstrated that the heavy metal sources are the parent rocks, industrial pollution, agriculturalnon-point-source pollution and atmospheric deposition. The chemical speciation of Cu,Zn, Pb, Cr, Cd, Hg, and As in sediment was analyzed based on the EuropeanCommunities Bureau of Reference （BCR） fractionation method. The correlation of thechemical speciation and biological activity among these metals were analyzed based ontheir chemical and physical characteristics, and the ratio of secondary phase to primaryphase （KRSP） was employed to assess the heavy metal pollution in sediments.The resultsshow that i) total concentration is the most important factor affecting the residual fraction;ii) organic matter, pH, and salinity are the primary factors affecting the acid extractabilityand the adsorption of sulfur compound by organic matter; iii) total nitrogen and totalphosphorus are the main factors affecting the adsorption of sulfur compound on organicmatter. Iiii) based on the total concentration and bioavailability, Cd is the most risk metal,followed by Hg. So, more attention should be paid to Cd and Hg pollution in WL.As different chemical species in water have different toxity to organism, thespeciation of the studied metals in WL was analyzed with the PHREEQC model. TheHREEQC database for these heavy metals was established with HSC Chemistry v5.0software. The chemical species of Hg, Cr, As, Cu, Zn, Pb and Cd in water solution werefound from literature. The results indicate that CuCl₂^-, CuCl₃^2-, CuCO₃, Cu（OH）₂, Zn²⁺,Zn （OH）₂, ZnCO₃, ZnSO₄, PbCO₃, PbOH⁺, Pb（CO₃）₂^2-, Cr（OH）₃, Cr（OH）₂⁺, CdCl⁺, Cd²⁺,CdSO₄, CdCl₂, Hg（OH）₂, HgCl₃^-, HgCl₂, HAsO₄^2-, and H3AsO₃are the dominant speciesin WL. As HgCl₂is very toxic and mercury pollution did not meet the national standard,Hg pollution should be paid more attention. With the change of seasonal weather, thetemperature, redox, and pH of the lake will be changed. The impact of temperature, redoxand pH on the heavy metal species in WL was simulated with the PHREEQC software.The results are as follow: i) the change of temperature have minor impact on thespeciation of this heavy metals.ii) pH influences greatly the speciation of these heavymetals. In the condition of low pH, the counter hydrolysis reaction of heavy metals isdominant, which is intermediated by Cl-and SO₄^2-ions. In the condition of high pH, thehydrolysis reaction of heavy metals is dominant. iii) Water redox condition in the lakehas different effects on heavy metal speciation. In the reduction condition, CuCl₂^-,H3AsO₃, and H2AsO₃-are the dominant species for Cu and As metals. In the oxidationcondition, Cu（OH）₂, HAsO₄^2-, and H2AsO₄-are dominant species. At the weak alkalinestate, redox potential change has minor impact on Cr speciation. Hg, Zn, Pb, Cd, and Crspecies do not change while the redox condition changes in lake.At last, based on the findings of this study, the following suggestions to control WL pollution are provided; i) The toxity of the studied heavy metals depends on thespeciation in water solution, which can be controlled by the external environmentalconditions, such as pH, redox of the lake; ii) The external pollutant emitted from industryshould be controlled;3) The internal pollutant from sediment should be controlled byreducing disturbance to the bed of WL.