ABSTRACT
A novel sulfate removal process via ettringite precipitation was developed by dissolving ettringite and recycling Al3+ under low pH condition. Effects of solid to liquid ratios, pH and temperature on ettringite dissolution, Al recovery and transformation of precipitates were investigated by batch experiments. The optimum condition for Al recovery is pH =3.0, suspended solid of 9.8 g/L and temperature below 303 K. Ettringite dissolution consists of two stages, (i) rapid but inconsistent dissolution with the fastest release of sulfate, followed by calcium, and then Al(OH)63-; (ii) slow dissolution of Al(OH)63- core and gypsum precipitation. Dissolution of Al(OH)63- core follows the first-order kinetics with activation energy of 41.18 kJ/mol, while gypsum re-precipitation follows the second-order kinetics with activation energy of 26.36 kJ/mol. Long-term results of pilot-scale systems for treatment of real flue gas desulfurization wastewater showed that the process achieved sulfate removal of 98.3%-99.5% and Al recovery above 98.4%, and converted 98.8% sulfate in ettringite to CaSO4, which resulted in 66.0% of sludge reduction and improved sludge dewaterability. Economic evaluation shows that the process with Al recovery reduces cost of ettringite precipitation by 35.1%, and is highly feasible and cost-effective for industrial application of high-sulfate content wastewater treatment.
ABSTRACT
One of the main challenges in industrial wastewater treatment and recovery is the removal of sulfate, which usually coexists with Ca2+ and Mg2+. The effect of Mg2+ on sulfate removal by ettringite precipitation was investigated, and the process was optimized in the absence and presence of Mg2+. In the absence of Mg2+, the optimum conditions with sulfate removal of 99.7% were obtained at calcium-to-sulfate ratio of 3.20, aluminum-to-sulfate ratio of 1.25 and pH of 11.3 using response surface methodology. In the presence of Mg2+, sulfate removal efficiency decreased with increasing Mg2+ concentration, and the inhibitory effect of Mg2+ matched the competitive inhibition Monod model with half maximum inhibition concentration of 57.4 mmol/L. X-ray diffraction and Fourier transform infrared spectroscopy analyses of precipitates revealed that ettringite was converted to hydrotalcite-type (HT) compound in the presence of Mg2+. The morphology of precipitates was transformed from prismatic crystals to stacked layered crystals, which confirmed that Mg2+ competes with Ca2+ for Al3+ to form HT compound. A two-stage process was designed with Mg2+ removal before ettringite precipitation to eliminate the inhibitory effect, and is potential to realize sludge recovery at the same time of effective removal of sulfate and hardness.
Subject(s)
Magnesium , Sulfates , Wastewater , Chemical Precipitation , Hydrogen-Ion Concentration , Minerals , X-Ray DiffractionABSTRACT
Simultaneous zeolites regeneration and nitrogen removal were investigated by using a mixed solution of NaClO and NaCl (NaClO-NaCl solution), and effects of the regenerant on ammonium removal performance and textural properties of zeolites were analyzed by long-term adsorption and regeneration operations. Mixed NaClO-NaCl solution removed more NH4+ exchanged on zeolites and converted more of them to nitrogen than using NaClO or NaCl solution alone. Response surface methodological analysis indicated that molar ratio of hypochlorite and nitrogen (ClO-/N), NaCl concentration and pH value all had significant effects on zeolites regeneration and NH4+ conversion to nitrogen, and the optimum condition was obtained at ClO-/N of 1.75, NaCl concentration of 20 g/L and pH of 10.0. Zeolites regenerated by mixed NaClO-NaCl solution showed higher ammonium adsorption rate and lower capacity than unused zeolites. Zeolites and the regeneration solution were both effective even after 20 cycles of use. Composition and morphological analysis revealed that the main mineral species and surface morphology of zeolites before and after NaClO-NaCl regeneration were unchanged. Textural analysis indicated that NaClO-NaCl regeneration leads to an increased surface area of zeolites, especially the microporosity. The results indicated that NaClO-NaCl regeneration is an attractive method to achieve sustainable removal of nitrogen from wastewater through zeolite.
