Research On Arsenic Removal By Biological Technology From Natural Water

Arsenic is a toxic element, its excessive exposure causes harm to ecology and human health. In recent years, domestic and foreign arsenic water pollution accident occurred frequently which has brought negative effects on the local economy and environmental security. The chemisorbents can remove arsenic effectively from water with high arsenic concentrations, whereas these methods were limited at low arsenic concentrations due to the high cost of the media and the excessive sorbents dose needed. However, biosorbents can resolve this problem very well. Recently, studies on As5+ removal by biosorbents from water with high arsenic concentrations had been widely reported, but the use to treat As3+ of low concentrations were few referred. As3+ and its compounds are more toxic than that of As5+ and difficult to remove from water. Preoxidation process can be avoided through effective directly adsorption by biological materials which help reduce costs and improve water security. In this study, eight kinds of biosorbents(P.vittata L., Ceratophyllum demersum L., Waste Crab Shells (WCS), Herb of Creeping Falsepimpernel, Potamogeton maackianus A. Bennett, Potamogeton crispus, Myriophyllum verticillatum and Hydrilla verticillata, respectively) are chosen for remove arsenic, the reaction characteristics are described as well last the best worked material will be considered for practical application. All test results shows as follows:(1) From the comparison of the ability to adsorb As3+ by six submerged plants, P.vittata was the best, Myriophyllum verticillatum was the minimum, Herb of Creeping Falsepimpernel, Potamogeton maackianus A. Bennett and Hydrilla verticillata was roughly equal. Linear models were the best fits for all isotherm data, and Freundlich models were also satisfied to the data except for P.vittata. The D-R models can only show the Potamogeton crispus and P.vittata L. reaction characteristics. From kinetics, the whole process can be described well by Pseudo-second-order model, the Intra-particle diffusion was not the only rate-limiting step, and the reaction rate was controlled by several separate steps.(2) It is toxic for P.vittata and Ceratophyllum demersum by high concentration As3+ (10.0、100.0mg/L) which will stress tissues’growth and lead to the final death. Alternatively, the two materials increase and show no toxicity symptom under low concentration level(0.1、0.5 and 1.0 mg/L). The maximum As3+ concentration of the two species of plants could bear is 10 mg/L, but P.vittata. L could stay alive for a longer time than Ceratophyllum demersum L. The maximum As bioaccumulation and short-term uptake concentration of P.vittata were separately 5 and 12 times higher than Ceratophyllum demersum L., and the main As3+ bioaccumulation tissue parts are their fronds.(3) It was concluded that the As3+ adsorption capacity (Qe) by WCS increased with the increasing initial As3+concentration, however, As3+ removal efficiency decreased from 64.9% to 35.1% at equilibrium. Increasing temperature did little effect on Qe but Qe increased obviously with the increasing initial solution pH. What’s more, this value slightly decreased from 162.70 mg/kg to 151.83 mg/kg with the biomass dose increased from 2.0 g/L to 10.0 g/L. The maximum As3+ adsorption was 165.78 mg/kg in this study(0.928 mg/L initial As3+concentration, pH=7.0, T=40℃,2.0 g/L biomass dose).Freundlich and Linear models were the best fits for all isotherm experimental data, but the latter was better. Kinetic reached equilibrium at 10 h and can be divided into three stages from the curves, and the process can be described well by Pseudo-second-order model. The Intra-particle diffusion model curve obtained did not pass through the origin, indicating that Intra-particle diffusion was not the only rate-limiting step. Analysis of thermodynamic data suggested that the reaction, which was non-spontaneous and had a low adsorption activation energy, was favored at higher temperatures, and the degree of disorder was reduced throughout the whole reaction system.Above all, P.vittata L. is high tolerance and considerable bioaccumulation amount to As3+ press, its initial As3+ adsorption rate is speed and the adsorption reaches equilibrium shortly, so it can be considered as the best biosorbents for As3+ removal from natural water in this study.