Study On The Stability Of Arsenic In Sediments Of Yangzonghai Lake

Yangzonghai Lake,area of 31 km²,the average depth of 20 m,the storage capacity of 604 million m3,is the third largest plateau lakes.Since 2008,the arsenic concentration of the lake was seriously increased from 0.006 mg L^-1 to the highest value of 0.134 mg L’1 due to the illegal discharge of the enterprise.The water quality was reduced from Grade II to Grade V for many years.After arsenic pollution incident,experts and government departments repeatly argued and decided to use the program proposed by the group.They do not pre-oxidize trivalent arsenic and control the pH value,directly spraying ferric chloride solution into Yangzonghai Lake,From 2009,the arsenic concentration of water continued to decline after more than two years of implementation of the project,reaching the target of the concentration of arsenic in the lake less than 50 μg/L,and the water quality recovered from class V to class Ⅲ.Fermentation of ferric chloride solution to remove arsenic pollution in Yangzonghai Lake is effective,then the stability of sediments deposited at the bottom of the lake need to evaluate.This problem has caused widespread concern.Therefore,the study of arsenic stability in the sediments of Yangzonghai Lake can provide scientific basis and theoretical guidance for the protection and management of Yangzonghai Lake.The main contents and results of this paper are as follows:1.Study on the stability of As in the sediments of Yangzonghai Lake by Single extraction methodThe stability of arsenic was investigated by separated extraction Yangzonghai sediments under different conditions.The results showed that the desorption rates of NaH₂PO₄ and Na₂HPO₄ were 9.47%and 7.28%,respectively,due to their acidic dissolution and phosphorus and arsenic competitive adsorption.With leaching concentration of 200 mg L^-1,oscillation time of 16 h,solid-liquid ratio of 1:100,and oscillation speed of 300 r min^-1,the desorption rate was 0.728%.It can promote the release of arsenic from the hydroxide,but also with arsenate to form arsenic sulfide precipitation.The desorption rate increased significantly when pH<5.5 or pH>8.5.The lower pH is less favorable to the formation of Fe and A1 colloid,which is not conducive to the adsorption of sediments.With the increase of pH,the negative charge of the colloid increases and OH^-concentration increases.And the effect of sodium sulfide solution on the desorption of arsenic in sediments was lower than that of the control group.In contrast,other leaching agents have little effect on arsenic desorption.2.Studies on the stability of As in Fe-As（Ⅲ）,Ca-As（Ⅲ）,Al-As（Ⅲ）coprecipitateThe experimental method was used to simulate the Fe-As（Ⅲ）coprecipitate formed by Yangzonghai sediments.The structural characteristics of Fe-As（III）coprecipitates were analyzed and the residual was obtained by Tessier seven-step method.The coprecipitates were obtained by using the same molar ratio of aluminum salts and lime,and we compare the advantages and disadvantages of different adsorbents.The arsenic（III）removal rate of iron,aluminum and calcium were more than 90%,and the higher the molar ratio,the higher the arsenic removal rate.Iron salts removal arsenic and arsenic adsorb in ferrihydrite with the formation of internal complexes.After the seven-step leaching,the residual state is still the poorly crystal structure of ferrihydrite.The arsenic is deep into the pore structure of ferrihydrite,and the adsorption of arsenic prevents the conversion of ferrihydrite to other minerals.Ferrihydrite has a strong ability to removal arsenic,and the residual state was highly stable（residual arsenic was more than 90%）.Calcium hydroxide removal arsenic.At pH 13,AsO₃^3-,HAsO₃^2-and Ca²⁺ form calcium arsenite,and calcium-arsenic（Ⅲ）coprecipitate stability is poor.The molar ratio of 10 to calcium-arsenic coprecipitate dissolved in the fifth step.The molar ratio of 20,30 to calcium-arsenic coprecipitate stability is slightly higher,and residual arsenic only accounting for 6%of the total arsenic content.Aluminum salts removal arsenic.Arsenous acid and aluminum chloride form AlAs.The crystal structure damaged after multi-stage leaching,and the residual state convered into a poorly crystalline pseudohyorite α’-AlOOH.Residual arsenic accounted for 10%of total arsenic.Water-soluble arsenic,ion exchange arsenic accounted for 20%of the total arsenic.3.Study on the stability of As in sediments of Yangzonghai Lake by seven-step extraction methodArsenic is present as various combinations in sediments.The extraction of arsenic from different forms can be used to calculate the origin,stability and biotoxicity of arsenic.The results showed that the arsenic content of each state is in the order of the active state（water-soluble,ion-exchanged and carbonate-bound）<strong organic binding state<humic acid-bound state<iron-manganese oxidation state<residual state.The results showed that the available arsenic content was 0.113 mg/kg,0.330 mg/kg,0.125 mg/kg,which is 0.277%of the total arsenic content.The Iron and manganese oxidation state accounted for 24.54%,and only reducing condition can promote iron and manganese oxidation state convert to effective arsenic.Residual arsenic is highest content,accounting for 57.26%of total arsenic.Residual arsenic in natural conditions is not easy to release,and it is high stable,which also shows that Yangzonghai sediments will not cause secondary pollution after the engineering treatment.