Study On Effects Of Arsenic Removal In Yanzonghai Lake By Ferric Salt Coagulation And The Stability Of Arsenic-Bearing Precipitate In Sediments

In recent years,arsenic pollution in natural water bodies has become a severe issue among various global environmental issues and therefore,has attracted extensive research attention of scientists around the world.Arsenic(As)and arsenic based compounds have high toxicity,pathogenicity,carcinogenicity and teratogenicity.Due to these properties,such compounds can harm humans physically and mentally,while causing a danger to ecosystem too.In June of 2008,the arsenic concentration in Yangzonghai Lake showed an abnormal rise.On 16th of September,the arsenic concentration had reached as high as 0.128 mg·L-1.Meanwhile,water quality decreased from Category II(which had been so for the past 6 years)to Category V,and the lake water got polluted with large amounts of high arsenic pollutants within a short time period.If the arsenic pollutants are not controlled and removed effectively,these cannot only destroy the function of Yangzonghai ecosystem,but can also severely influence the normal production and living of surrounding living beings,thus hampering the social and economic development of Yangzonghai basin.Therefore,based upon literature review and previous experimental explorations,this paper directly selected the FeCl3 coagulation method to study the removal of As in Yangzonghai Lake water and spring water.Separated or sequential extraction method was employed to study the stability of iron-arsenic(Fe-As)co-precipitate.Modern instruments were used to analyze the morphology,composition and phase transformations of Fe-As.Additionally,the transformation rules of As(III)in lake water have also been investigated in the current work.The main research contents are listed as follows:1.Yangzonghai Lake is a large natural plateau lake.The total arsenic concentration of the lake water was not elevated,but As(III)and As(V)coexisted.As(III)is pre-oxidized first in the process of existing arsenic removal,the treatment process will become complicated and the costs will rise significantly.Further,such a process would introduce additional risks to the security of lake's ecosystem.As a result,under the condition of without any background electrolyte,this paper investigated the removal efficiency of FeCl3 and co-existence ion on low concentration of As(III)and As(V)and mixtures of As(III)/As(V)in a weak alkaline(pH 8.5)deionized water system.The results showed that the removal of As by FeCl3 coagulation method is a fast reaction.The majority of arsenic pollutants in the system can be removed within 5 min,while the removal efficiency of As(III),As(V),and mixed arsenic can reach 89.2%,99.3%and 89.2%respectively.As the FeCl3 dosage was raised,the removal efficiency of As(III)increased to 99.4%.Kinetic studies indicated that As adsorption of amorphous Fe(OH)3 follows the second-order kinetics,while the adsorption process is chemisorption.The effects of interfering ion such as HCO3-,Ca2+ and SO42-under low concentrations of As were very limited,and HCO3-played an important role in stabilizing the pH dynamics of the system.2.Yangzonghai Lake,its water reserves was 604 million m3.One of the difficulties in reducing arsenic pollution was impossible to pre-regulate pH for so large capacity water.The current study has focused upon pre-regulation of pH in the system for studying various arsenic removing techniques.Instead,FeCl3 was directly used to remove the arsenic in Yangzonghai Lake as well as the 3#spring water on the lakeside.Meanwhile,analysis methods such as BET,TEM,SEM and XPS were used to analyze the properties,morphology,constitution and valance states.The results showed that the removal efficiencies of FeCl3 coagulation method in lake water and 3#spring water were 95.0%and 98.0%,respectively.The concentration of As remaining in both the systems were lower than 0.050 mg·L-1,which meets the requirement of As for Category I-? surface water standard.At the same time,the study also found FeCl3 dosage was negatively correlated with the effect of initial As concentration on the stability of Fe-As co-precipitate.The adsorption step by step indicated that the As adsorption capacity of Fe(OH)3 in the lake water decreased gradually with an increase of sequences.Langmuir model can be fitted to As adsorption values with the correlation coefficient of R2>0.99,which showed that the process of As adsorption by Fe(OH)3 followed the monomolecular layer adsorption mode.Through colloidal chemistry,Fe(OH)3 colloidal particles made the coordination bond with As in lake water,while the active groups(-OH)on the surface of colloidal particles appeared and produced stable Fe-As co-precipitate.3.Expanded experimentation studies of As pollution control were conducted by FeCl3 flocculation method in Yangzonghai Lake.The changes in pH,iron concentration,chloride concentration,phytoplankton species as well as the spatial distribution of arsenic concentration was also discussed in the paper.The results showed that,FeCl3 coagulation method has advantages of high arsenic removal efficiency,easy operation,low costs,and ecological security.Direct atomizing and spraying FeCl3 flocculants can reduce the average As concentration of the whole lake from 0.117 mg·L-1 in November 2009.The lowest As concentrations recorded were 0.021 mg·L-1 in September 2010,which meets the requirement of As for Category I-III surface water standard.Amorphous Fe(OH)3 existing in the sediments still had adsorption effect on As in lake water.FeCl3 flocculation method did not cause the change of pH of the lake water.After the removal of arsenic using FeCl3 coagulation method,both the iron and chloride concentrations were below the allowable value of domestic drinking water in China.Strong convection currents inside the Yangzonghai Lake promote uniform distribution of As concentration in the whole lake.4.After the mixing of As in Yangzonghai Lake water with the bottom sediments through FeCl3 coagulation method,if there are drastic changes in the medium properties such as pH,Eh and so on,the As in sediments may be released back into the water bodies and hence,can cause secondary pollution.Therefore,the stability of Fe-As co-precipitate at the lake bottom is a crucial problem.This paper studied the stability,morphology and distribution of arsenic in different types of Fe-As co-precipitate by using immersing methods,separated or sequential extraction method.Geo-accumulation index(Igeo)method was also employed to assess the As pollution conditions in sediments.The results showed that,Fe-As co-precipitate can not only exist in the lake water for a long period,but can also remain stable in the sediments.In the 0-2 cm layer,the water soluble state and ion exchange state having high mobility accounted for only 0.3%,while the immobilized residual forms occupied 68.5%,which show little effect on the ecosystem at the bottom of lake.As concentrations in sediments decreased with increasing depth and the Igeo reached Level 2 in most of the area.This indicated medium pollution and therefore,no threat was posed to the lake.5.This paper adopted TEM and XRD methods to study the transformations of Fe and As in Fe-As co-precipitate.The results suggested that,during the coagulation of Fe(OH)3 micelles,numerous ions or molecules containing As were wrapped into the aggregates through trapping effect and a flocculent precipitate was formed.Moreover,the existence of As prevented the transformation of amorphous Fe(OH)3 to goethite and hematite phases.Under the condition of relatively stable quality of water,inner-orbital complex containing the Fe-O-As bond can remain stable for a long time period.6.And through expanded experimentation research,it was found that As(III)had a tendency to As(V)in the lake water.Therefore,this paper carried out the relevant studies on the morphology transformations of As(III)in the lake water through field investigation and sample analysis.The results showed that with time,the percentage of As(III)in lake water gradually decreased every year while that of As(V)increased every year.In April 2014,the percentage of As(III)was only 2%,while in May 2015,almost all of the As(III)had transformed into As(V).The oxidative and weak alkaline properties of Yangzonghai Lake water assisted the oxidation of As(III)by O2 into As(V)under ultraviolet radiations,which decreased the toxicity of As(III)to aquatic organisms as well as created favorable natural conditions for removing As by FeCl3 coagulation method.