Steel slag, which makes up a gigantic amount of metallurgical industrial solid waste, was in this experiment successfully synthesized an inexpensive adsorbent used to remove nitrate pollution from aqueous solution. This adsorbent was obtained by mixing steel slag, aluminium hydroxide and deionized water, and aging this at a mass ratio of 3:0.45:2, and then activating it at 800 • C. The physicochemical characteristics of the steel slag before and after modification were investigated to compare

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... e effect of their surface properties on the adsorption behaviour of nitrate. The effects of adsorbent dosage, pH, and contact time on the adsorption process were investigated. The results showed that an increase in specific surface area and the formation of a positive surface of the modified steel slag (MSS) compared with the original steel slag (OSS) could effectively increase the number of the active adsorption sites and nitrate removal ability. The optimum parameters for nitrate removal were as follows: the concentration of nitrate was 20 mg/L, the dosage was 1 g/100 mL, the pH was four, and the reaction time was 180 min. The adsorption capacity of the MSS was approximately 1.9 times that of the OSS. The nitrate adsorption of the MSS was in accordance with the pseudo-second-order model and the Freundlich model, which indicated that the adsorption of nitrate on the MSS was mainly single layer chemical adsorption. The mechanism of nitrate removal mainly included ion exchange, hydrogen bonding, electrostatic interactions and intermolecular interactions. In addition, regeneration experiments indicated that the MSS after regeneration still had the capacity to remove nitrate. Water 2017, 9, 757 2 of 17 metallurgical industry. It has a loose and porous structure, high density, settles fast in water, a short cycle of solid-liquid separation, and contains alkaline oxide and a large amount of iron and silicon. These features make steel slag a possibility for the adsorption of pollutants in wastewater [14] . A great deal of research has shown that steel slag is an inexpensive wastewater adsorbent that can effectively remove phosphate, ammonia, harmful metal and other pollutants in aqueous solution by chemical reaction and adsorption [15] [16] [17] . However, nitrates are almost completely soluble in water, compared with phosphate ions, ammonium ions and fluoride ions, etc., and it is difficult to form various insoluble substances attached to the surface of the adsorbent and there is some difficulty in removing them [18, 19] . Compared with some industrial synthetic adsorbents, steel slag is a type of complex system of CaO-MgO-Al 2 O 3 -SiO 2 whose surface combines with some minor elements in the process of crystallization or glass transition that can affect its adsorption function [20] , so that the internal chemical composition cannot be effectively used. Therefore, the adsorption efficiency of steel slag to nitrate is relatively limited [7, 21] . Compared with Fe 3+ loaded chitosan (8.35 mg/g), nano alumina (4 mg/g) and activated carbon (1.22 mg/g) [22] [23] [24] , the maximum adsorption capacity of steel slag on nitrate is 2.83 mg/g in aqueous solution based on previous research [7] . Therefore, it is necessary to improve the removal efficiency of nitrate with a modified method. Researchers have applied different activation techniques to improve the adsorption capacity of steel slag to other pollutants [25] [26] [27] , making steel slag an inexpensive wastewater adsorbent. According to previous studies, aluminium hydroxide (Al(OH) 3 ) has been successfully used as a modifier for adsorbents in wastewater treatment to improve its adsorption efficiency [19, 20, 26, 28] . Duan et al. [19] showed that the removal efficiency of nitrogen and phosphorus was significantly improved by using Al(OH) 3 to modify steel slag, and the activated alumina (Al 2 O 3 ) generated in the process of modification played an important role in the removal of pollutants. Activated alumina (Al 2 O 3 ) has high specific surface area and amphoteric properties, which show good performance in removing nitrate from water [18, 29, 30] . However, the removal efficiency of nitrate in aqueous solution by MSS has not been studied. Therefore, this study intended to modify the steel slag to prepare a new type of low-cost adsorbent to reduce the nitrate pollution in water. In this study, steel slag, Al(OH) 3 and deionized water were mixed in a certain proportion, and activated at a high temperature to improve the nitrate removal efficiency of steel slag in aqueous solution. It has been found that this modification method could improve the nitrate removal efficiency by enlarging the porosity and surface properties, the amount of active substances and the positive charge on the surface of the steel slag to a large extent. In addition, regeneration experiments indicated that the MSS after regeneration still had the capacity to remove nitrate.