Synthesis of lanthanum and waste lye modified-zeolite from fly ash and its application in constructed wetland for nitrogen and phosphorus removal

Abstract

Excess inputs of nitrogen (N) and phosphorus (P) can lead to imbalance in water ecosystems and thus trigger eutrophication. In this study, a novel Lanthanum modified zeolite synthesized from fly ash (LZFA) was prepared and used as a modified substrate for constructed wetland (CW) to enhance N and P removal. Single-factor and surface methodology (RSM) were used to optimize the preparation. The results showed that the maximum adsorption capacities of N and P were 17.26 mg/g and 21.48 mg/g. The decline in sorption capacity or diffusion coefficients of LZFA for N compared to zeolite synthesized from fly ash (ZFA) is mainly due to the decrease in surface negative charge and cation exchange capacity. The mechanism of P adsorption is attributed to the formation of La-O-P monodentate, bidentate mononuclear or bidentate binuclear inner-sphere complexation. Meanwhile, the introduction of Ca in waste lye is also involved in the P reaction. The N and P removal rates of LZFA modified subsurface flow constructed wetland (SFCW) were 2.67% and 7.33% higher than SFCW modified with gravel. In practical production, if a circular chain from coal ash production to use in green plant fertilizer can be established, the cost of treating P can be significantly reduced.