Arsenic Removal By Modified Activated Carbon Fibers

Arsenic is harmful to human health ranging from acute lethality to chronic andcarcinogenic effects. Estimate of the rural population exposed to unsafe As levelsfrom drinking untreated groundwater in the world has grown to over100million.Coagulation precipitation technique was widely used, but the coagulationwould produced waste toxic sludge producing secondary pollution. Membraneseparation technique is effective in removing the As but it is a high-technologyoperation requiring frequent maintenance. Bioremediation and ion-exchangetechnique are not suitable for removal of co-existing ions.Solvent extractiontechnique leaves very little by-product but is not applicable due to its characteristic.Among various treatment processes reported in the literatures, adsorption is a simpleto operated method for ground water treatment, especially for removal of low-levelcontaminants.Activated carbon fibers （ACF） are especially interesting since they havefaster adsorption kinetics, higher adsorption capacity and strongermechanicalstrengthcompared with other adsorbents.The manganese-modified activated carbon fibers and iron-modified activatedcarbon fiber （Fe-ACF and Mn-ACF）were characterized by BET analysis, transmissionelectron microscopy （TEM）, scanning electron microscopy（SEM） techniques, X-rayphotoelectron spectroscopy （XPS） and Boehm titration analysis. The analysis resultsindicate that the pore volume and surface area of Fe-ACF and Mn-ACF weredecreased whileoxygen-containing groups on the carbon surface were increased. Itcan be seen that the surface of ACF is smooth. However, the surface of Fe-ACF isclearly covered by floccules. Many needle-like crystals are found to grow on thesurface of Mn-ACF.The dominant iron species deposited onto the surface of Fe-ACFare iron oxides such as Fe₂O₃and Fe₃O₄while oxidation of surface Mn-ACF speciesresulted in MnO₂.Static adsorption experiments were carried out to investigate the effect of the pH,dosage, As（Ⅴ）initial concentration, adsorption time and co-existed ions on theadsorption. The results indicated that with Fe-ACF, the residual concentration ofAs（Ⅴ）was below the tolerance level (0.05mg·L^-1) at pH=7, dosage=2.0g·L^-1, initialconcentration=3.5mg L^-1, adsorption time=180min. while with Mn-ACF, theresidual concentration of As（Ⅴ）was below the tolerance level (0.05mg·L^-1) at pH=7,dosage=1.5g·L^-1, initial concentration=10.0mg L^-1, adsorption time=135min.The experimental results indicate that the adsorption kinetics could be described by pseudo-second-order kinetic equation. Further, adsorption equilibrium data ofAs（Ⅴ）on both the Fe-ACF and Mn-ACF adsorbent followed the Langmuir andFreundlich models, while the Langmuir model provided a better correlation of theexperimental data. The adsorption capacities （qmax） of As（Ⅴ）on Fe-ACF andMn-ACF were18.34mg·g^-1,36.53mg·g^-1respectively. The thermodynamicparameters including the adsorption on Fe-ACF and Mn-ACF showed that theadsorptions were chemical, spontaneous and endothermic adsorptions. Themechanism of As（Ⅴ）removal by Fe-ACF was found to chemical coagulation ofFe（OH）₃（s） followed by adsorption of As（Ⅴ）onto the Fe-ACF. The removal of As （V）by Mn-ACF could be described by the following two possible processes, i.e. reactionbetween Mn²⁺and As （V）, followed by adsorption of As （Ⅴ）onto the surface of theMn （IV） sites.The residual concentration of As（Ⅴ）adsorption on the Fe-ACF and Mn-ACFwas below the tolerance level (0.05mg·L^-1) under the dynamic adsorption conditionswithin2h. The regeneration experiment of adsorbing Fe-ACF and Mn-ACF wasstudied using HCl solution and NaOH solution. The results showed that regenerationof adsorbing Fe-ACF by NaOH was the best, the residual concentration of As（Ⅴ）wasapproximately0.05mg·L^-1. The regeneration of Mn-ACF by treatment withH2C2O4was effecftive.