| Red mud, which is a by-product from alumina industry, is not only in large quantity but also causes serious pollution to the environment. For the purpose of treating waste with waste, red mud was used as the main raw material to produce red mud granular adsorbent (RMGA) in this study, and this novel adsorbent was applied for phosphate removal from aqueous solution. Some important parameters in the manufacturing process, which greatly affect the characteristics of RMGA, were investigated in details; RMGA was characterized by several analyse method; the mechanisms of phosphate removal and the reapplication property of RMGA was discussed further; a column study was also conducted, which is of great importance in the area of theoretical investigation and practical application.In the study of preparation of RMGA, red mud, bentonite and starch was used as the materials; the influences of some parameters, including mass ratio of three raw materials, preheating temperature, preheating time, sintering temperature and sintering time, were investigated. Mass ratio of raw materials and sintering temperature were important items that effected RMGA characteristic. Adsorption capacities for various RMGA were described by the removal capacity of phosphate from aqueous solution. The characteristic of RMGA was greatly affected by red mud ratio and sintering temperature. The optimum sintering temperature, at which the largest phosphate removal capacity could be achieved for RMGA, was much higher for RMGA with large red mud ratio, and it also varied with the different operation temperature in adsorption experiment. With the increase of sintering temperature, the physical and chemical character of RMGA changed, including the increase of surface area and the decrease of effective phosphate removal components (such as CaO, Fe2O3and γ-Al2O3). Based on the integrated result, RMGA with the mass ratio of90:5:5(red mud:bentonite:starch) that preatreated at400℃for20min and sintered at1000℃forlO min was selected for further study, and it was marked as RMGA-90%-1000℃.The selected RMGA-90%-1000℃was characterized and its phosphate removal performance was investigated. The influences of some operation parameters, like initial pH in solution, reaction time, adsorbent dosage and initial phosphate concentration, were discussed for phosphate removal, and the kinetics study was done. By the sintering process, the surface of RMGA could be optimized and components with functional groups of-OH and-SO4were formed, which enabled phosphate adsorption form aqueous solution through the ligand-exchange reaction. RMGA performed well in acidic solution, but the strong acid solution (with pH lower than1) would destroy the structure of RMGA, so an acceptable pH range was3.00-6.00. The removal of phosphate by RMGA was weakly affected by common coexisting ions in solution, and the selectivity of it for phosphate was62times to that of Cl-, NO3-and SO42-. A removal efficiency of89.23%could be achieved by RMGA dosage of10.0g/L in50mg/L solution for contact time of4h; and removal efficiency of91.18%could be achieved for20mg/L solution using4.0g/L of RMGA. The initial phosphate removal rate was faster at lower pH, since the electrostatic repulsion between RMGA and phosphate was enhanced as pH increased. During phosphate removal process, the pH in solution rose and the mechanism for phosphate removal could be divided into two stages:firstly, the removal of phosphate was mainly caused by adsorption; then, it was the combined effect of adsorption and precipitation with the increase of pH in solution. The kinetics studies presented that pseudo second-order model fit phosphate removal by RMGA well. However, the precipitation that attached on the surface of RMGA baffled adsorption reaction.The different phosphate removal behaviors of RMGA were investigated comparatively for different phosphate forms (orthophosphate form and pyrophosphate form). RMGA sintered below1030℃performed well for the both kinds of phosphates. At pH of3～4, the removal capacity of RMGA for orthophosphate was higher; and the removal capacity of RMGA for pyrophosphate was higher at pH of5～7. RMGA sintered at1010℃and1030℃could gain the largest removal capacity for orthophosphate and pyrophosphate, respectively. When orthophosphate and pyrophosphate coexisted in solutions, pyrophosphate was comparatively easily adsorbed because of the stronger electrostatic attraction effect. The total phosphate removal capacity was higher than that for pure orthophosphate or pyrophosphate removal, which implied that some effective sites on RMGA were able for orthophosphate or pyrophosphate adsorption selectively. The competitive adsorption experiment showed the mechanism for phosphates removal in this research was that precipitation affected orthophosphate removal greatly, while adsorption was the main reaction for pyrophosphate removal.In addition, in order to investigate the regeneration characteristics of RMGA manufactured at different sintering temperature, a systematic experiment was conducted by the adsorption, desorption and resorption tests. After being treated by different desorption reagents, RMGA could also remove phosphate from aqueous solution. It was assumed that the reductive release of CaO into solution during resorption process lead to a lower pH in solution, and this contributed to a higher resorption capacity for RMGA. When RMGA were treated by HC1solutions, although relatively higher desorption efficiencies were obtained, the acid erosion resulted in effective components extraction, and resorption capacities was smaller. A lower desorption efficiency was achieved when NaOH solution was applied to treat RMGA, and this was mainly based on ligand-exchange. While, because OH-could ameliorate the chemical composition on the surface of RMGA, RMGA treated by NaOH performed better than that treated by other desorption reagent. A ligand-exchange reaction between OH-in solution and the functional group of-SO4happened in NaOH desorption process. Since the crystal structure of RMGA manufactured under lower sintering temperature was comparatively unstable, the reactions above was easier to achieve, and RMGA sintered at temperature below1000℃could even obtained larger resorption capacities than their original adsorption capacities. Generally,0.01mol/L NaOH solution was a cost-effective desorption reagent for RMGA.Then, in order to reduce the cost of RMGA preparation, the raw material was improved by using sludge instead of starch. An orthogonal test with5factors and4 levels was designed and16kinds of RMGA were manufactured. The producing parameter for selected RMGA was as follows:mass ratio of red mud:bentonite: sludge was85:3:12, sintering temperature was900℃and sintering time was8min. The largest adsorption capacity of8.92mg/g could be achieved under the condition with pH of5.00, initial phosphate concentration of35mg/L, environmental temperature37℃and RMGA dosage4.0g/L after8h. Since metallic ions (such as Ca2+and Fe3+) would released from RMGA into acidic solution, the mechanism of phosphate removal was combined effect of adsorption and precipitation. The proportions of adsorption and precipitation in total removal efficiency were affected by several items, including reaction time, environment temperature and initial pH in solution, and precipitation was obvious in solution at initial pH of5.00.In the end of our research, the selected RMGA was applied in a column for phosphate removal. The result showed that RMGM column performed much better under the condition of lower flow rate, lower initial phosphate concentrations as well as larger RMGM dosage. In addition, the RMGM column can be regenerated automatically after suspend without treating wastewater for a short time, implying the effective component in RMGA could be sufficient used in dynamic test. Afterwards, the RMGM column which was saturated with phosphate is successfully applied in the treating of lead. According to the analysis of pH and Ca2+concentration in effluent solution during this process, it was known that the mechanism of lead removal was mainly based on ion exchange occurred on the surface of RMGA between Ca2+and Pb2+. For the toxicity analyse of RMGA, heavy metal contents in lixivium was lower than the thresholds determined by Hazardous Wastes Distinction Standard-Leaching Toxicity Distinction (GB5085.3-2007, China), and the treated water could meet level Ⅲ of the National Environmental Quality Standards for Surface Water (GB3838-2002, China). |
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