Novel lanthanum and waste lye modified fly ash zeolite for nitrogen and phosphorus removal: mechanisms and application in the constructed wetland

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 and waste lye 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 response surface methodology (RSM) were used to optimize the preparation process. The results showed that the maximum adsorption capacities of N and P were 17.26 mg/g and 21.48 mg/g, respectively. Compared to solely zeolite synthesized from fly ash (ZFA), the decreased N sorption capacity in LZFA was attributed to a reduction in the surface negative charge and cation exchange capacity. The mechanism of P adsorption was attributed to the formation of La-O-P monodentate, bidentate mononuclear or bidentate binuclear inner-sphere complexation. Meanwhile, the introduction of Ca from waste lye was 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 for green plant fertilizer can be established, the cost of treating P can be significantly reduced.