Removal Of Trace Heavy Metals In Water By Enhanced Coagulation And Adsorption

Both the emission of industrial wastewater and the happening of pollution accident can lead to heavy metal pollution to drinking water sources. The dissolved heavy metals have serious toxicity and can be enriched by organism. Different from organic pollutants, they can not be decomposed naturally. So it is necessary to study the methods and principle to remove them from drinking water.The removal effect and factors of conventional water treatment process to some typical heavy metals are studied firstly in the paper. The results showed that just by coagulation process Cu(â…¡),Pb(â…¡) and Cr(â…¥) can be effectively removed with the residual concentration lower than requirements in the drinking water standard while the removal effect of Cd(â…¡),Ni(â…¡)are very badly. Among the factors, pH is the most important one by affecting modality of the hydrolyzing, kinds and amounts of the hydrolysate. Generally speaking, the removal rate under alkalescence is higher than that under acidity. However, Cr (VI) can not be reduced to Cr (III) under the strong alkaline condition leading to the removal rate falling. Besides, the clay particles and organic matters in the water can also affect the removal rate by adsorption and complexation reactions with heavy metal.For Cd(â…¡)and Ni(â…¡)were difficult to be removed in the conventional treatment process, the removal effect and mechanism of ozone oxidation, potassium permanganate oxidation, powdered activated carbon adsorption and combine of potassium permanganate and powdered activated carbon were studied. Ozone oxidation can't increase the removal rate of Cd (â…¡),Ni (â…¡) effectively; the enhanced processes of potassium permanganate oxidation and powdered activated carbon adsorption can increase the removal rate of Cd (â…¡),Ni (â…¡) effectively, but the residual concentration is still higher than the national standard in drinking water when they are used alone. The combination process of potassium permanganate oxidantion and powdered activated carbon adsorption can improve the removal rate further making the residual heavy metal concentration up to the standard.The removal of heavy metals from water by potassium permanganate is mainly performed through hydrous manganese dioxide which can remove the trace heavy metals by specific adsorption, electronic adsorption and co-precipitation. Powdered activated carbon adsorbs heavy metals through complexation reaction of acidic functional groups such as carboxyl and sulfurous functional group on its surface. In the united system, powdered activated carbon can deoxidize the superfluous potassium permanganate to manganese dioxide by katalysis, and at the same time, the moderate oxidation of potassium permanganate has certain infection to molecular weight and chemical characters of organic matters, enhancing adsorption capacity of the latter powdered activated carbon and performing a certain cooperating function. In the study of factors, it is found that, the removal rate of the object metal will be affected through competing for being absorbed when there are several heavy metals or many alkaline metals existing together.In order to improve adsorption capability, the adsorption dynamics of Cd²⁺ by activated carbon modified by HCl,HNO₃ and tannic acid was studied. The results showed that the adsorption effect of powdered activated carbon is better than that of granular activated carbon. Compared with the original carbon, the adsorption capacity of activated carbon modified by concentrated HCl decreased while that of activated carbon modified by concentrated HNO₃ and tannic acid increased, and among them the modifying effect of concentrated HNO3 is the best. Through study of specific area,surface functional group and point of zero charge of activated carbon, it is found that infection of specific area of activated carbon on the adsorption of Cd²⁺ is very little while the increase of acid functional group such as carboxyl and phenolic hydroxyl can enhance its adsorption capacity significantly. Carboxyl content on the surface of activated carbon is greatly increased after being modified by concentrated HNO₃ and phenolic hydroxyl of tannic acid has strong complexation reaction with metal ions, both of which improve the adsorption capability of the activated carbon after being modified.Combined use of potassium permanganate and powdered activated carbon can remove trace heavy metals from drinking water effectively, safely and conveniently, easy to be joined with conventional water treatment process of water works in existence. Activated carbon could significantly improve its metal ion adsorption capacity after being modified by appropriate method, but still needs further study if going to be applied to water treatment.