Study On Arsenic Removal By Ferric Chloride From High-arsenic Water

Arsenic is a ubiquitous, toxic element occurring in all naturally contaminated environmental compartments. Although exposure to high levels of arsenic and acute poisonings are not common, chronical exposure to low concentrations of arsenic can cause serious health problems, such as skin cancer, skin thickening, lung, neurological disorders, muscular weakness, loss of appetite and nausea. In recent years, It is universally acknowledged that arsenic pollution is associated with the geochemical environment uncontrolled anthropogenic activities, such as mining, the burning of fossil fuels, smelting of metal ores, using of wood preservatives agents and arsenical pesticide application, can also release high-arsenic directly to environment, which have been aroused great attention in worldwide. The treatment of water with arsenic pollution has become a global environmental problem.In this study, Ferric Chloride was applied to investigate arsenic removal from high arsenic water, which provided some theoretical basses and references for the treatment of industrial wastewater with arsenic. The main conclusions are as follows:(1) Effects of the main physical and chemical factors on arsenic removal with Ferric ChlorideFerric Chloride was employed to remove arsenic in the arsenic contaminated water, this paper studied the effect of the main physical and chemical factors on arsenic (V) removal with Ferric Chloride by one factor and orthogonal tests. The experiment results showed that the important extent of the factors to arsenic removal by Ferric Chloride is described by the raw water pH value>the reaction temperature>the mass ratio of the Fe/As>the stirring parameters. The optimum reaction conditions of the arsenic (V) removal by Ferric Chloride are as follows: the raw water pH value9, the reaction temperature30℃, the mass ratio of the Fe/As1.0, the speed of agitator consisted of rapid mixing for1min at300rpm, mixing for19min at100rpm, slow mixing for10min at50rpm and settling for90min.(2) Study the effect of pH on the distribution of arsenic speciation in arsenic-borne coagulates The distribution of arsenic speciation can be applied to elucidate the coagulation mechanism during the sorption of the arsenate. In this study, the effect of initial pH on the distribution of arsenic speciation in arsenic-ferric coagulates was determined by a five-step sequential extraction procedure. The experimental results showed that the As (V) specifically sorbed and the As (V) associated with coagulant floe were57.0%,41.1%;41.8%,57.7%;39.9%,59.9%at different initial pH values (5,7and9), respectively. This result manifest that the main mechanisms governing the arsenic removal with ferric chloride were specifically adsorption and association. Besides, as pH increased from5to9, the association played a more and more important role in arsenic removal. At the same time, the As (V) soluble and the As (V) non-specifically sorbed were barely detectable, which suggested that physically sorbed had almost no effect on arsenic removal. Moreover, the FT-IR spectra were recorded from As-Fe sediments to confirm the oxidation state of the sorbed speciation, which were in an excellent agreement with arsenic speciation analysis.(3) Study the effect of dosing modes on arsenic removal from high-arsenic waterIn this study, we evaluated arsenic removal with the same dosage of ferric chloride, in different dosing modes (single dosing mode, sequential dosing mode). Arsenic removal efficiencies in different dosing modes were showed in the following order:the sequential dosing mode [dosing mode C> dosing mode B]> the single dosing mode [dosing mode A]. The results showed that arsenic removal efficiency in sequential dosing mode was over99.0%, which was higher than that in single dosing mode. The arsenic speciation of the arsenic-borne coagulates in different dosing modes was analyzed by a five-step extraction procedure. The results indicated that the main arsenic speciation were the As (V) specifically sorbed and the As (V) associated with poorly crystalline hydrous oxides of Fe in single or sequential dosing mode, which proved that the main mechanisms of arsenic removal in single or sequential dosing mode were specifically adsorption and the formation of chemical compounds, which can be related to the formation of crystalline ferric arsenate (i.e. scorodite). Besides, the non-specifically sorbed mechanism played a more important role in sequential dosing mode than that in single dosing mode, which can be further conclude that the arsenic-borne coagulates in sequential dosing mode than that in single dosing mode.