| In recent years, the pollution of water resource has become a significant problemin China. Livestock and poultry, pharmaceutical, chemical and other industries haveemitted large amounts of wastewater which contains phosphate, sulfate and nitrateanions mainly and works as a direct threat to the environment and human health.Therefore, effective measures should be taken to reduce the concentration ofinorganic anions in sewage as well as the amount of discharged to the water body.And it has an important and practical significance.In many ways which can reduce the inorganic anions in sewage, the adsorptionmethod is highlighted for its simple process, low operation cost and its function ofregeneration and recycling of resources. Among all the adsorption materials, the LDHcompounds become a spot in research because of their excellent anion adsorptivefunction. Three calcium aluminate (3CaO Al2O3, abbreviated as C3A) is one of themain minerals of ordinary Portland cement clinker. A remarkable feature of it is theformation of calcium based LDH in the hydration process, the interlayer OH can beeasily exchanged by other anions. Therefore, C3A has great potential in the aspect ofreduce inorganic anions in sewage.This subject takes NO3-、SO42and PO43-as representatives, with the goal ofremoval in C3A hydrate layer structure efficiently, through the methods by changingthe temperature and initial PH of solution and so on, and explore the effect casts bythe medium environment on the C3A’s anion adsorption capacity, adsorption rate andso on. Meanwhile, through isothermal adsorption experiments and kineticexperiments as well as the solution and solid-state component determination in theresearch, and discover the reaction mechanism of C3A and NO3-、SO42and PO43-, thenprovide the theory basis for the application of C3A as adsorption material in thetreatment of inorganic anion pollution in water bodies. Therefore, main conclusionsas follows are obtained in this study.1. High purity C3A can be obtained through solid phase sintering method.According to XRD map of C3A, its characteristic peak is identical with the standard XRD map in the database of EVA. What’s more, the phases of the product are all thesame; it means it is pure single mineral. The infrared spectra of C3A displayed that theabsorption peaks of vibration of metal oxygen (the Al-O and the Ca-O) is located onthe900-400cm-1. Also it can be seen from the result that C3A belongs to cubic crystal,and has a non lamellar structure before hydration. The analysis result of thermalcharacteristics of TG-DTA suggests that the synthesis of C3A contains water and asmall amount of CO2in this experiment.2. Research on the sorption kinetics of C3A on NO3-showed that: the sorption ofC3A on NO3-was about4.1mmol/g at25℃. And the absorption equilibrium wasreached at about90min later. And its adsorption behavior was accorded with thequasi two stage reaction equation (R2=0.9766). Research on the isothermal adsorptionof C3A on NO3-indicated that: the maximum absorption of C3A on NO3-was about4.1mmol/g at25℃, and the isothermal adsorption curve was accorded with theLangmuir mode (R2=0.9848). In addition, though the absorption quantity of C3A onNO3-increased with the initial PH of solution, it had little effect on the result. Whenthe PH was at10~12, the adsorption effect was the best and had the maximumcapacity of4.2mmol/g. At the same time, the adsorption of C3A on NO3-was affectedby temperature, and the adsorption quantity increased with the increase oftemperature at25℃~35℃. While at35℃~45℃, the adsorption capacity of C3A onNO3-decreased with the increase of temperature and reached its maximum,4.3mmol/g, at35℃.3. According to the kinetic study of C3A on SO42-, when the temperature was at25℃, the adsorption effect had the maximum capacity of3.7mmol/g. and it took20min to reach the adsorption equilibrium. And its adsorption behavior was accordedwith the quasi two stage reaction equation (R2=0.9951). Research on the isothermaladsorption of C3A on SO42-indicated that: the maximum absorption of C3Aon SO42-was about3.8mmol/g at25℃, and the isothermal adsorption curve was accorded withthe Langmuir mode (R2=0.9441). In addition, though the absorption quantity of C3Aon SO42-increased with the initial PH of solution, it had little connection with theinitial PH. When the PH was at10~12, the adsorption effect was the best and had themaximum capacity of3.8mmol/g. At the same time, the adsorption of C3A on SO42- was affected greatly by temperature, and the adsorption quantity increased with theincrease of temperature at25℃~45℃and reached its maximum,4.75mmol/g, at45℃.4. According to the kinetic study of C3A on PO43-, when the temperature was at25℃, the adsorption effect had the maximum capacity of3.9mmol/g. and it took12min to reach the adsorption equilibrium. And its adsorption behavior was accordedwith the quasi two stage reaction equation (R2=0.9616). Research on the isothermaladsorption of C3A on PO43-indicated that: the maximum absorption of C3A on PO43-was about3.9mmol/g at25℃, and the isothermal adsorption curve was accorded withthe Langmuir mode (R2=0.9977). In addition, though the absorption quantity of C3Aon PO43-increased with the initial PH of solution, it had little connection with theinitial PH. When the PH was at10~12, the adsorption effect was the best and had themaximum capacity of3.9mmol/g. At the same time, the adsorption of C3A on PO43-was affected greatly by temperature, and the adsorption quantity increased with theincrease of temperature at25℃~45℃and reached its maximum,4.1mmol/g, at45℃.5. The removal mechanism of C3A on NO3-, SO42-, and PO43-: the removalmechanism of C3A on NO3-is the intercalation of calcium based LDH. It is removedthrough the formation of CaAl-N O3-LDH by NO3-inserted in the hydrate layer ofC3A. There are two methods to remove the SO42-by C3A. First one, C3A dissolvesand releases Ca2+which could formulate indissoluble sediment of CaSO4with SO42-.Second one; it is removed through the formation of CaAl-SO4-LDH by SO42-inserted in the hydrate layer of C3A. The main adsorption mechanism of C3A on PO43-is the dissolution-precipitation effect. C3A dissolves and releases Ca2+which couldformulate indissoluble sediment of hydroxyapatite with PO43-. It means that theformation of hydroxyapatite controls the removal of PO43-. And the exchangecomplexation reaction between aluminum hydroxide surface hydroxyl and PO43-enhanced the removal process of PO43-.6. C3A has a faster removal rate and higher removal percentage upon differentinorganic anions under different initial solution PH and concentration of inorganicanions than other adsorption materials. What’s more, its adsorption capacity is much larger. Thus, C3A has good adaptability and strong removal effect towards differentinorganic anions in different kinds of sewage. |
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