The mechanisms of conventional pollutants adsorption by modified granular steel slag

This study was performed to investigate the removal of phosphorus, nitrogen, ammonia and dissolved organic matter (DOM) from domestic wastewater using modified granular steel slag (GSS) as adsorbent. The modification methods include acid modification, alkalinity modification and thermal modification. The GSS modified at 800°C for 1 h was easier to hydrolyze Ca²⁺, which could promote the precipitation of phosphates and the ion exchange process of ammonia. Therefore, the adsorptive capacity of GSS for phosphates and ammonia could be significantly improved by the thermal treatment. The acid-modified GSS promoted the adsorption capacity of nitrates by increasing surface protonation, specific surface area and pore size. The highest nitrates adsorption capacity was obtained when GSS was immersed in 1 mol/L HCl solution for 24 h. The presence of nitrates inhibited the adsorption of phosphates by GSS because the adsorption of nitrates and phosphates by GSS depended largely on electrostatic attraction and intermolecular force, and the competition between them reduced the adsorption capacity. Ammonia can promote the hydrolysis of metal ions on the surface of GSS, increase the concentration of metal ions in solution and promote the formation of phosphate precipitation, but ammonia also competed with phosphate for active sites on the surface of GSS. The effect of ammonia nitrogen on phosphate adsorption was the result of the interaction of the two mechanisms. For domestic wastewater, the thermally modified GSS showed the best adsorption rate of total phosphorus and total nitrogen, and the acid-modified GSS had better adsorption capacity for organic matter. The thermally modified GSS had a good application effect in laboratory subsurface flow constructed wetlands.