Study On The Removal Of Arsenite From Water Through Synergistic Oxidation-Adsorption By Iron-Based Adsorbent

Arsenic is one of the most toxic and non-degradable contaminants in water.Elevated levels of arsenic-contamination in water from natural sources and anthropogenic activities not only cause serious drinking water safety problems,but also raise concerns about food safety.Arsenic is usually present as two species in aquatic environment:arsenate?As????and arsenite?As????.In oxidizing and alkaline environment,As???is the predominant arsenic species and present as anions(H₂AsO₄^-or HAsO₄^2-),while non-ionic H₃AsO₃ is the dominant form in reducing conditions and acid environment,e.g.groundwater.Compared with As???,As???is more toxic,mobile and difficult to be removed because non-ionic H₃AsO₃ has low affinity for various adsorbents.Adsorption is the most widely used technique for arsenic removal.However,the direct adsorption and removal of non-ionic As???is far from satisfaction.Therefore,pre-oxidation of As???to As???is generally regarded as a requisite step in practical application.In this thesis,four practically enhanced iron-based adsorbents have been prepared,and heterogeneous catalytic process of adsorbents with oxidants has been established for As???simultaneous oxidation and adsorption of As???.The removal performance and mechanism were studied.The main findings are as follows:?1?Titanium dioxide nanoparticle dotted superparamagnetic Fe-Cu binary oxide?Fe-Cu/TiO₂?adsorbent has been prepared by evaporation and solvothermal method.The surface-dotted TiO₂ nanoparticles can highly oxidize As???to As???under UV irradiation,while the Fe-Cu binary oxide can effectively adsorb As???and produced As???.Due to the reasonable distribution of adsorption and oxidation sites.5.2 mg/L As???can be completely removed using 1 g/L of Fe-Cu/TiO₂ under UV irradiation,which is better than that in dark conditions or using homogeneous mixed ternary oxide?Fe-Cu-Ti?under UV irradiation.Under UV irradiation,the adsorption capacity of Fe-Cu/TiO₂ for As???is 17.49 mg/g,and the adsorption can work effectively in the range of pH 3-9.Meanwhile,the adsorption of As???by Fe-Cu/TiO₂has a strong anti-interference to the aquatic environment.The adsorbent has excellent reusability,and the removal efficiency of As???after five runs is still maintains at 91.7%.The adsorbent can reduce the As concentration in the As???-spiked natural groundwater to less than10?g/L.Additionally,the adsorbent can be easily separated from the solution using a magnet.?2?CuFe₂O₄ magnetic nanoadsorbent has been prepared using citrate combustion method,and the integration of CuFe₂O₄ and PMS constructs an advanced oxidation process for the oxidation and adsorption of As???.PMS can be induced by CuFe₂O₄ to generate radical species for As???oxidation.CuFe₂O₄/PMS has a stronger affinity for arsenic than CuFe₂O₄ alone.Oxidation and adsorption promote each other.As a result,the heterogeneous catalytic process is more efficient for As???removal than a preoxidation of As???followed by adsorption.The adsorption capacity of As???on CuFe₂O₄/PMS reaches up to 63.9 mg/g,which is much higher than that of As????36.9 mg/g?or As????45.4 mg/g?on CuFe₂O₄ alone.The process can work effectively over a wide range of pH values?3⁹?and temperatures?10⁴0°C?.The coexisting substances like sulfate,carbonate,silicate and humic acid have an insignificant effect on As???removal.As???levels?1415?g/L?can be completely oxidized to As???and rapidly removed to below 10?g/L?less than 15 min?using CuFe₂O₄?0.2 g/L?/PMS?100?M?.Moreover,50?g/L of As???can be completely oxidized and removed within 1 min.The proposed treatment process is highly feasible for the remediation of As???-contaminated water under ambient conditions.?3?Radially porous Fe-chitosan beads?P/Fe-CB?have been prepared using freeze-casting technique.The P/Fe-CB particles possess radially aligned micron-sized tunnels from the surface to the inside as well as excellent acid resistance.Kinetic studies show that the adsorption equilibrium time of P/Fe-CB to 0.975 mg/L As????within 240 min?is considerably shorter than that of compact Fe-chitosan beads?over 600 min?.The maximal adsorption capacity of P/Fe-CB for As???is 52.7 mg/g.It can work effectively in a wide pH range from 3 to 9,and the coexisting sulfate,carbonate,silicate and humic acid have no significant effect on As???removal.The addition of H₂O₂ can further accelerate and promote the As???removal except at high pH?11?and phosphate concentration?50 mg/L?.The fixed-bed experiments demonstrate that the P/Fe-CB column can effectively treat about 3000 bed volume?BV?of simulated As???-containing groundwater to meet the drinking water standard?<10?g As/L?.This study would extend the potential applicability of porous Fe based chitosan adsorbent and millimeter-sized adsorbent combined with H₂O₂ to a great extent.?4?The Fe-NN/BFs adsorbent has been prepared by hydrothermal method.The akaganeite nanoneedles are grown vertically on the surface of the biochar fiber,which ensures that the adsorption sites can be fully exposed and the diffusion distance of arsenic can be shortened.For As???adsorption,the adsorption equilibrium is reached within 15 minutes.For As???adsorption,the adsorption equilibrium can also be reached within 2 hours.In addition,the adsorbent has a very high iron utilization efficiency,and its adsorption capacities for As???and As???are 93.94 and 70.22 mg/g-Fe,respectively.The Fe-NN/BFs packed column has an excellent reusability,and two packed columns in tandem can handle 2900 and 2500 BV of As???and As???spiked natural groundwater,respectively.The surface Fenton system constructed by Fe-NN/BFs with hydroxylamine and hydrogen peroxide can significantly promote the removal of As???.In addition,the proposed surface Fenton system can be effectively applied over a wide pH range?4-9?.