Removal Of Cadmium From Micro-polluted Raw Water By Pretreatment Of Adsorption

Cadmium is one of the five main heavy metal contaminants, which is hazardousto human health. In recent years cadmium pollution cases have erupted frequently inour country. A number of watersheds, such as the Yangtze River, the Pearl River,Xiangjiang River and so on, have hidden danger of cadmium pollution. Therefore, tofind some effective agents and materials to removal trace cadmium from water and tostudy their removal efficiency, principles and feasibility in practical application hasbecome very urgent.Nowadays, reports about cadmium removal in water at home and abroad weremainly focused on industrial wastewater. Studies showed that activated carbon andclay minerals have good adsorption effect on high concentration of cadmium, butits application in the pretreatment of potable water resource were reported scarcely.This paper researched the cadmium efficiency in micro-polluted raw waterof usingpowder activated carbon, bentonite, sepiolite and three coagulants (Polymericaluminum chloride, Polymeric ferric sulfate, Ferric chloride) by captive test.According to its removal efficiency, found out the optimal using conditions,including the optimal dosage, combinationgroup, adsorption time, initial pH and soon,then the principles of cadmium removal were discussed and analyzed.The water samples were compoumded by Xiangjiang water and Cadmiumstandard solution,the concentration of cadmium were about50μg/L. The resultsindicated that all these three adsorbents were effective on removing cadmium,theefficiency of bentonite> sepiolite> powder activated carbon.Comparing thecadmium removal performance of the combation with three coagulants, the optimalone was Polymeric aluminum chloride (PAC), when bentonite dosage was1g/L, pre-treatment time was10min, PAC dosage was20mg/L (measured by goods weight),without adjustment of the pH, the removal efficiency of cadmium was about90%,and the residual cadmium concentration after sidiment was about5μg/L. In the sameconditions increasing PAC dosage to30mg/L or adjusting the pH of raw water to8.0can ensure the residual cadmium concentration after sidiment acheive thestate water quality standards. The orthogonal experiment of bentonite and PACindicated that the effect of the initial pH> bentonite dosage> raw water temperature> PAC dosage. Studies of adsorption dynamics, adsorption isothermal model and adsorptionthermodynamics showned that the dsorption dynamicsof trace cadmium with the threeadsorbents all corresponded with the second-order eqution of Lagergren; theadsorption isothermal of trace cadmium were better agreement with Langmuireqution;The adsorption mechanism of bentonite on tace cadmium was mainlyphysical adsorption, sepiolite and powder activated carbon were chemical adsorptionin the initial state, then were mainly physical adsorption when it increased to acertain concentration.