Performance Study Of Arsenic Removal During The Titanium Salt Coagulation And Titanium Hydroxide Adsorption Process

Arsenic contamination in natural water poses a great threat to millions of people in many regions of the world. Arsenic exists in water primarily as the inorganic oxyanions of arsenite(As(III)) and arsenate(As(V)), with As(III) predominating in anaerobic waters and As(V) prevailing in oxic waters. Arsenite is more toxic(25–60 times), mobile and less efficiently removed than arsenate at the pH range of most natural and wastewater sources,since the dominant arsenite species is the nonionic H3AsO3 at neutral and acidic pH, while the dominant arsenate species are in the form of anions such as H2AsO4- and HAsO42-.Therefore, there is a great need to develop efficient, feasible methods to remove both arsenic species from water, particularly As(III). Recently, Titanium compounds showed good adsorption properties for arsenic especially for As(III) by previous research. Many technologies have been developed to remove arsenic including coagulation/precipitation,adsorption, ion exchange, membrane separation and so on. Titanium compounds mainly be used to coagulation/ precipitation and adsorption among these technologies. This research mainly divided into the following two parts.1. Binary mixed hydroxide of titanium and calcium(Ti/Ca hydroxide) were synthesized by a low-cost coprecipitation process. The first part of this study is aimed to present its study results on the factors influencing As(III) and As(V) removing efficiency from the drinking water by synthetical Ti/Ca hydroxide. We investigated the effects of such factors, as different preparation methods, Ti/Ca ratio, the solution’s pH value, calcium ion,phosphate anion through batch experiments. The adsorption thermodynamics properties of As(III) and As(V) on materials were also studied, and discussed its adsorption mechanisms.Results of experiments show that the As(III) removed ratio can reached 95.5% with the adsorbent dosage of 0.2g/L at 30℃ and As(III) concentration of 1mg/ L at initial pH value after 3 hours of adsorption, whereas the contrast experiment only reached 73.3% of As(V).It proves that Ti/Ca hydroxide have adsorption selectivity on As(III). In addition, the adsorbent could be successfully applied for As(III) and As(V) removal in natural pH ranges.The solution’s pH value at 5~9 and calcium ion had no obvious effect on As(III) adsorption,while presence of phosphate anion in the solution would somewhat inhibit the As(III)adsorption. As for As(V), acidic conditions and calcium ions can significantly improve its removal efficiency but phosphate anion show serious negative results. The adsorptionisotherm data could be well described by Langmuir and Freundlich isotherm model, and the adsorption capacities for arsenate and arsenite removal were 16.6mg/g and 30.2mg/g at30℃, respectively. Calculated the parameters of the D-R isotherm, it indicate that adsorption of As(III) on the Ti/Ca hydroxide may a physical adsorption processes.Accordingly, it is believed that the adsorbent developed in this study is effective for arsenic polluted groundwater treatment.2. The second part of this study investigated the effects of such factors, as different coagulant dosages, the solution’s pH value, Ca(OH)2 as regulators, combined use of FeCl3 and TiCl3/TiCl4,KMnO4 as oxidant, hydraulic condition, co-existing ions through batch experiments and FeCl3 was used as a reference. The results of experiments show that the arsenic removal with Ti(III) was much better than that with Fe(III) and Ti(IV). Compared to the FeCl3, titanium salt flocculation exhibited faster floc growth rate and larger floc size,a short retention time coupled with larger floc size formation lead to smaller and more compact flocculation and sedimentation units. Chromaticity of water significantly better than FeCl3 after precipitation. Under the condition of unchange ph in the coagulation process, arsenic removal efficiency increased with increasing dose of TiCl4, and arsenic removal by TiCl4 was great at mild alkaline conditions. Hydraulic condition effects arsenic removal by TiCl3 relatively minimal, the most influential factor is fast-stirring speed and time. There were an obvious improving role for arsenic removal by TiCl3 and TiCl4 with KMnO4 preoxidation at 300r/min for 2 min. when permanganate was applied at 0.9mg/L,arsenic removal up to nearly 100%. Calcium ion had no obvious effect on arsenic removal,while presence of phosphate and silicate anion in the solution would significantly inhibit the arsenic removal.The Ti/Ca hydroxide preparation are operated simply and raw materials achieved easily, and synthesized materials have excellent adsorption properties, especially for As(III). The study of adsorption isotherm shows that Ti/Ca hydroxide adsorbed As(III) mainly via physical adsorption, therefore the adsorption process has less susceptible to solution’s pH and co-existing ions. At present, it’s rarely reported about titanium salt coagulation for arsenic removal. This paper conducts a comprehensive study on it, the result shows that Ti(III) coagulation was much more efficient for As(III) removal than Fe(III) and Ti(IV) under neutral conditions and in the presence of co-existing anions,the permanganate pre-oxidation process had significant promoting effect on As(III)removal by TiCl3 andTiCl4.