| With the development of the modern steel industry, the high-grade iron ore recourses are exhausted, and the exploitation and utilization of the high-phosphorus iron ore has become an urgent task. Reducing the phosphorus content of the ore is the key to effectively using the high-phosphorus iron ore. The frequently-used phosphate removal method for iron ore include magnetic separation, flotation, acid leaching, microbe leaching, and etc.. Of these methods, acid leaching is an effective phosphate removal method. Unfortunately, a large amount of wastewater with phosphate is produced after dephosphorization, which will cause serious environmental problems. At the same time, the treatment of the municipal wastewater with phosphate is also an important problem to be solved promptly. At present, the main methods of phosphate removal from wastewater include chemical method, biological method and adsorption method. Due to the fact that the adsorption method possesses a lot of merits such as ease of operation, low amount of sludge produced in the process, it become the most widely used technique for phosphate removal. The low-cost and high-efficiency adsorbent is the key to the application of adsorption method. A large amount of low-grade iron ore mine wastes are generated in mining high-phosphorus iron, which not only occupy the land resources, but cause landslide and dust pollution. In order to effectively utilizing the wasted mineral resources, the low-grade iron ores with phosphorus (LOGIOP) was used as adsorbent for the phosphate removal in wastewater in present work, which is helpful for the effective utilization of high-phosphorus iron ore and LOGIOP resources, and has significant meanings for the treatment of wastewater.The adsorption process of phosphate onto the minerals involves a complicated system. During the adsorption process, not only the simple physical adsorption but also the interface adsorption reactions including surface complexation adsorption, adsorption coagulating, etc. between the minerals and water take effect. At present, the studies of phosphate removal using minerals are focused on investigating the phosphate removal efficiencies of the adsorbents. However, the researches on the interface adsorption between the minerals and wastewater with phosphate are not in-depth enough, and the theoretical models which can systematically describes the surface adsorption characteristics of minerals, phosphate removal mechanism, interface adsorption, and interface reactions between the minerals and phosphate in wastewater are absent. In present work, LOGIOP were used as the adsorbent for the phosphate removal in high-phosphate acid wastewater and municipal wastewater with phosphate. The knowledge and methods about the disciplines of surface chemistry, colloid chemistry, mineral processing and so on were comprehensively used to evaluate the surface characteristics of LOGIOP and to determine the interface adsorption behavior of phosphate onto LOGIOP in wastewater. The effects of the components of LOGIOP on the phosphate removal were investigated, and the phosphate removal mechanism was clarified. The research outcomes have significant science and engineering meanings for the application of LOGIOP in the phosphate removal of high-phosphate acid wastewater and municipal wastewater.The surface characteristics of LOGIOP were studied by multiple characterization method including scanning electron microscopy (SEM), X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR) and acid-base titration method.①The isoelectric point (IEPpH) of LOGIOP was about4.80-5.20, and the surface of LOGIOP characterized with hydroxylation.②The proton adsorption on the surface of LOGIOP at different pH values were described using single-and double-site non-electrostatic models (NEM). Through the non-linear fit based on the acid-base titration data, the total surface site densities (N) and protonation constants (K) of the LOGIOP surface in single-site and double-site models were obtained as follows:NT=1.64×10-4mol/g, Ka1=2.20×10-4, Ka2=6.82×10-9, and NT,Fe=1.60×10-4mol/g, Kal,Fe=1.90×10-4, Ka2,Fe=1.74×10-9, NT,Si=2.77×10-6mol/g, Ka2,Si=8.32×10-2.③The chemical combination state of the LOGIOP surface changes with pH value. In single-site acid-base model, the positive and negative charges were produced by the protonation and deprotonation of>SOH (the hydroxylated single surface site). At pH<5.68, the surface of LOGIOP is positively charged and exhibits Lewis acids. At pH>5.68, the surface of LOGIOP is negatively charged and exhibits Lewis bases. In double-site acid-base model, the production of positive charge is only related to the protonation of> FeOH (the hydroxylated Fe2O3surface site) on the surface of LOGIOP, and has nothing to do with> SiOH (the hydroxylated SiO2surface site).The interface reactions between the minerals and phosphate in wastewater, and the phosphate removal behavior by adsorption were systematically studied from multiple aspects, such as adsorption isotherm, adsorption kinetics and surface complexation.①The adsorption of phosphate onto LOGIOP belongs to the specific adsorption. The phosphate adsorption kinetics of LOGIOP can be well described by pseudo-second-order kinetic model. The adsorption isotherm of phosphate onto LOGIOP follows Langmuir isotherm model. Single molecular layer adsorption prevails in the adsorption of phosphate onto LOGIOP, and the value of the maximum adsorption capacity was found to be11.44mg/g.②In KH2PO4-LOGIOP-H2O system, the plot of adsorption efficiency of phosphate vs. initial pH value appears to the pattern "M" and the initial pH values corresponding to the two peaks were5.60and10.36, respectively. The theoretical single-and double-site surface complexation models were established were established using the non-electrostatic surface complexation model (NEM). The models indicated that the combination of phosphate in wastewater with the hydroxyl of Fe2O3surface site was stronger than that with the hydroxyl of SiO2surface site.③The theoretical single-and double-site surface complexation models were used to predict the adsorption of phosphate onto LOGIOP for municipal wastewater, and the absolute deviations were both below8%, which indicate that the two models both can well describe the phosphate removal behavior by adsorption of LOGIOP.LOGIOP was used as the adsorbent for the phosphate removal in high-phosphate acid wastewater and municipal wastewater with phosphate. Effects of adsorption time, adsorption temperature, initial phosphate concentration of wastewater, initial pH value and adsorbent dosage on the phosphate removal were investigated, the optimum process parameters were obtained and the phosphate removal mechanisms were clarified.①The phosphate removal in acid leaching wastewater with high phosphate by LOGIOP is mainly fulfilled by the surface complexation adsorption and chemical coagulating adsorption of Fe3+and Ca2+. At the optimum process parameters of pH value about5.90, adsorption time40min, adsorbent dosage40g/L, adsorption temperature25℃, and initial phosphate concentration50mg/L, the phosphate removal efficiency exceeds99.65%, which satisfies the requirement of TP<0.5mg/L in the discharge standards of water pollutes in steel industry (GB13456-2012).②Fe3+and Ca2+can promote phosphate adsorption in wastewater, Cl-and NO3-have not obvious effect on phosphate adsorption, while Na+and SO42-show depressing action on phosphate adsorption.③The phosphate adsorption onto LOGIOP in municipal wastewater is mainly the surface complexation adsorption. At the optimum process parameters as following:initial pH value about5.80, adsorption time60min, adsorption temperature70℃, adsorbent dosage20g/L, and the initial phosphate concentration10mg/L, The phosphate removal efficiency is above96%. The phosphate concentration of wastewater after phosphate removal is lower than0.5mg/L, which achieves the A level of the discharge standards of pollutes of wastewater treating plant in towns (GB18918-2002).④LOGIOP also shows good adsorptive abilities for Mn2+, Pb2+and Cu2+in municipal wastewater. The adsorptive rate of Mn2+is about96%, and the adsorptive rates of Pb2+and Cu2+are both above99%. The concentrations of Pb2+and Cu2+in the solution after adsorption are both lower than0.5mg/L, which satisfies the lead and copper discharge requirement of the discharge standards of water pollutes in steel industry (GB13456-2012).In summary, LOGIOP possesses good adsorption abilities for the removal of phosphate and heavy metal ions in high-phosphate acid wastewater and municipal wastewater with phosphate, and the compositions of FeCO3and CaMg(CO3)2in LOGIOP can be effectively used to neutralize the acid in the wastewater, so the process treatment of adding alkali is saved. Due to the abundant recourses, low cost and simple process, using LOGIOP as adsorbent for the wastewater treatment can not only realize the low-cost treatment of wastewater, but benefit the comprehensive utilization of resources and enhance the value of LOGIOP. Therefore, LOGIOP has a broad application prospect in the phosphate removal of high-phosphate acid wastewater and municipal wastewater with phosphate. |
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