Treatment Technology And Removal Mechanism For Wastewater Contaminated By Cr(â…¥) And As(â…¤)

Heavy metal pollution in water have posed a serious threat to our living environment and the wastewater contaminated by Cr(VI) and As(V) has become a hot problem in the field of environment. The chromium and arsenic contamination represent potential threats duo to their persistent, enrichment, strongly toxic and carcinogenic. Therefore, China has established the new maximum limited value of 0.05 mg/L and 0.01 mg/L for hexavalent chromium and arsenic in drinking water.Chromium is widely used in leather, textile production, dyeing and plating industries and the emission of industry three wastes is the main source of Cr(VI) pollution in water. Arsenic pollution comes mainly from volcanic activity, burning of fossil fuels, ore mining and the use of pesticides. Inadequate storage, transport and disposal of the Cr(VI) and As(V) wastes have caused great harm to the environmental balance and human health. Therefore, the treatment technology for water pollution of chromium or arsenic is needed to improve. Besides, Cr(VI) and As(V) have been identified as co-contaminants from wood preservative, petroleum refineries, paint and ink manufactures as well as some municipal wastewater, but few research have been conducted. Effective and economical methods for simultaneous removal of co-contaminants Cr(VI) and As(V) should be explored immediately.In treating the wastewater containing chromium and arsenic, there are many kinds of technologies including physical method, chemical method, and biological method. This work has studied the treatment of chromium water pollution by reduction and precipitation, the treatment of arsenic water pollution by new composite adsorbents and the removal of the Cr(VI) and As(V) co-contaminants by Zero-valent iron. Test contents and results are as follows:Reduction and precipitation method used a two-step treatment process involving (i) reduction of Cr(VI) by reductant, (ii) precipitation by alkali. Experiments were performed to compare the effect of four reducing agents (FeSO4, FeCl2, Na2SO3 and NaHSO3) and two precipitants (NaOH and Ca(OH)2). Results showed that the best removal efficiency of Cr(Ⅵ) can be achieved when FeSO4 was used as reductant and Ca(OH)2 was selected as precipitant. It took 20 min to reach the Cr(Ⅵ) removal equilibrium. When the molar ratios of Fe2+ to Cr6+ was between 2.7 and 5.1, the residual concentration of Cr(Ⅵ) and iron in the treated water could meet the National Environmental Quality Standards for Surface Water (GB3838-2002), under the experiment condition. It is found that the coagulation and adsorption of iron-chromium hydrate played an important role in the process of Cr(Ⅵ) removal by analyzing the FTIR spectra of precipitate gathered in experiment.Three groups of adsorbent including FCA、CeFA and CeAT were prepared, and their adsorption effects were compared. Results showed that the material named CeAT-4 composed by Ce, Al and Ti with the molar ratio of 3:1:3 had the highest removal capacity and the maximum adsorption capacity was 20.4 mg/g. The morphology characteristics of the adsorbent are determined by scanning electron microscope (SEM) method. The adsorbent surface was rough and gathered a large number of irregular micro-particles, extending their specific surface area. After absorption, the major structure of CeAT-4 remained intact which has a positive significance in As(Ⅴ) water pollution treatment.The experiment of Zero-valent iron investigated the removal efficacy and mechanism. The results showed that Cr(Ⅵ) and As(Ⅴ) removal was pH dependent and the maximum removal rate was both above 82.0% at pH range of 2.0-3.0. Removal efficiency was drastic enhanced with the dosage increasing from 0.1 to 4.0 g/L. The removal behave of Cr(Ⅵ) and As(Ⅴ) was better described by the pseudo first-order kinetic model than the pseudo zero-order kinetic model. The best removal efficiency was achieved when molar ratio of As/Cr is 2:1. SEM analysis showed that the removal of Cr(Ⅵ) and As(Ⅴ) was due to ZVI corrosion. It indicates that ZVI can be used as an effective media for the simultaneous removal of Cr(Ⅵ) and As(Ⅴ) from aqueous solutions. EDX element analysis revealed that the surface of reacted ZVI was adopt a large amount of oxygen.