Preparation Of A Novel Fe-Ti-Mn Composite Oxide And The Evaluation Of Its Effectiveness And Mechanisms For The Removal Of Arsenic Contaminant

At present,arsenic contamination is increasingly serious in China.Especially,groundwater and surface water are often applied as direct water sources in the rural and remote region of our country.A series of natural and anthropogenic factors have caused significant arsenic contamination problems in the provision of safe drinking water.The commonly existing species of As in natural waters are inorganic arsenic and organic arsenic.Inorganic arsenic(As(Ⅴ)and As(Ⅲ))is the predominant form of arsenic.Compared to inorganic As(Ⅴ),inorganic As(Ⅲ)is more mobile,toxic and difficult to be removed.p-arsanilic acid(p-AsA)is a typical phenylarsonic acid compound,which is widely used as an animal feed additive.The compound is barely metabolized in the animal bodies and is almost completely excreted into the natural water via manure and urine,causing concerns of environmental pollution and posing human health risk.Adsorption is considered to be the most economical and efficient water purification technology for the treatment of rural dispersed arsenic pollution.These traditional adsorbents are highly effective for As(Ⅴ)and p-AsA removal,but fail in the case of As(Ⅲ).To achieve higher removal efficiency,an oxidation pretreatment is adopted prior to adsorption,which results in a complicated operation with high cost.In addition,the difficulties encountered in separating spent adsorbent and propensity to aggregate limit the application of power materials in water treatment under some conditions and ranges of water quality.Development of a novel adsorbent which could efficiently remove As(Ⅴ),As(Ⅲ)and p-AsA and overcoming the difficulties encountered in separating and regenerating spent powdered materials in water treatment were the primary objectives of this study.Moreover,a systematic investigations including optimization of preparation,characterization of adsorbent,evaluation of adsorption performance,and arsenic removal mechanisms were investigated in the current work.In the first section of this work,a novel nanostructured Fe-Ti-Mn composite oxide(FTMO)was synthesized via a simultaneous oxidation and co-precipitation method and was systematically characterized by various techniques.Meanwhile,we evaluated the arsenic removal performance of the FTMO adsorbent.The results showed that the adsorbent(10-30 nm)was amorphous or poorly crystallized with a high specific surface area of 434.3 m~2/g.The adsorbent could effectively and rapidly remove inorganic arsenic,and the maximal adsorption capacities for As(Ⅴ)and As(Ⅲ)at 25°C were 74.4mg/g and 122.3mg/g,respectively.The arsenic removal process was exothermic and spontaneous,and the adsorbent was effective for As(Ⅴ)and As(Ⅲ)removal across a wide pH range.Besides,more than 50%of its original arsenic removal percentage was still maintained for both As(Ⅴ)and As(Ⅲ)after 8 times of regeneration,and the FTMO material could be effectively regenerated and used repeatedly.The FTMO material had an excellent arsenic adsorption performance.In the second section,batch control experiments were carried out with the use of series of spectroscopy detection technologies to investigate the oxidation state change of arsenic and material elements and the respective role of Fe,Ti and Mn content in the arsenic adsorption process.The results showed that the material could effectively oxidize As(Ⅲ)to As(Ⅴ)and light illumination could further apparently enhance the oxidation and removal efficiency of As(Ⅲ).The As(Ⅴ)adsorption mechanism was mainly achieved through the replacement of the surface hydroxyl group and the formation of inner-sphere surface complexes at the water/oxide interface.However,the As(Ⅲ)removal mechanism was a photooxidation,oxidation coupled with adsorption approach;the Ti and Mn contents were dominant for the As(Ⅲ)oxidation,while the Fe content mainly played an important role for the adsorption of newly formed As(Ⅴ).Furthermore,some synergistic effect in the composite oxide system might also enhance both the performance of component materials and the overall arsenic adsorption capacity.Except for inorganic arsenic,organic arsenic is the commonly available species of arsenic in natural waters.p-arsanilic acid(p-AsA)is a typical phenylarsonic acid compound,which is widely used as an animal feed additive.The third section of the current work focused on evaluating the p-AsA removal performance of the FTMO adsorbent.In addition,series of spectroscopy detection technologies such as Zeta,FTIR,HPLC/ESI-QqQMS,EPR,XPS and XANES,were used to investigate the respective role of Fe,Ti and Mn content and the removal mechanism in the p-AsA adsorption process.The results showed that the FTMO adsorbent could effectively and rapidly remove p-AsA,and the maximal adsorption capacity for p-AsA was 45.6 mg/g.The p-AsA removal mechanism was an adsorption coupled with oxidation approach.The p-AsA molecules were primarily adsorbed onto the surface of the adsorbent via replacing the surface hydroxyl group,and then forming the inner-sphere surface complexes.Parts of adsorbed p-AsA molecules were oxidized by Mn and Ti content to produce a p-AsA radical intermediate.The p-AsA radical intermediate could be further transformed through radical-to-radical couping to form a hydrazo intermediate and other oxidation products.The Fe and Ti content mainly played an important role for the p-AsA adsorption,while the Ti and Mn contents were dominant for the p-AsA oxidation.Powdered FTMO material has an excellent adsorption performance for arsenic pollutants,but the difficulties encountered in separating spent adsorbent and propensity to aggregate limit its applications under some ranges of water quality.In addition,the traditional loading granular materials have the problem of secondary pollution and no alkali resistance.In the fourth section of this research,a novel adsorbent of FTMO impregnated chitosan bead(FTMO/chitosan)was fabricated and was systematically evaluated the arsenic removal performance.The results showed that the FTMO/chitosan material was sphere-like and was aggregated from heterogeneous nanosized particles.The prepared FTMO/chitosan was effective at removing both As(Ⅴ)and As(Ⅲ)and the maximal adsorption capacities are 16.2 and 22.6 mg/g,respectively.The arsenic-loaded FTMO/chitosan could be effectively regenerated and reused.In dynamic continuous flow experiments,about 2000 and 1650 bed volumes of simulated groundwater containing about 200μg/L As(Ⅲ)and As(Ⅴ)were respectively treated before breakthrough.The FTMO/chitosan material had complete appearance and stable mechanical strength in more than 20 days of continuous operation tests,indicating that it was a promising candidate for arsenic removal from real drinking water.