Lignin cationization for the removal of phosphates and nitrates from effluents of wastewater treatment plants†

The removal of phosphates and nitrates from wastewater treatment plant (WWTP) effluents is important for preventing pollution of receiving waters. In this study, we chemically modified alkaline lignin (aLN) with quaternary ammonium groups to obtain biodegradable cationic lignin (cLN). We characterized the cLN and tested its efficacy for removing phosphates and nitrates in a lab setting and on field-collected WWTP samples. Adsorption isotherm and kinetic studies were performed in aqueous media, and the effects of several variables (contact time, pH, initial concentration, and adsorbent dose) were investigated. The Langmuir isotherm described phosphate and nitrate adsorption well, with R² values of 0.97 and 0.84, and maximum adsorption capacities of 0.59 mg g⁻¹ and 2 mg g⁻¹ respectively. For phosphate, the data fit the Freundlich isotherm model with an R² of 0.95, suggesting that both homogenous and heterogeneous adsorbent surfaces were involved in phosphate adsorption. Adsorption kinetics revealed that both phosphate and nitrate sorption onto cLN was better described by the pseudo-second-order model, with a correlation coefficient of 1. Furthermore, a 2-dimension Doehlert matrix was used to model the effect of initial concentration and adsorbent dose on the phosphate and nitrate removal. The results showed that cLN 1516 mol% was most effective for low phosphate and nitrate concentrations. With an obtained optimum adsorbent dose of 10 mg mL⁻¹, we achieved a successful reduction of nutrient loads of WWTP effluent from 0.42 mg L⁻¹ to 0.18 mg L⁻¹ (adsorption capacity of 0.6 mg g⁻¹) and from 4.1 mg L⁻¹ to 2.3 mg L⁻¹ (adsorption capacity of 4.5 mg g⁻¹), corresponding to the removal of 57.7% and 43.9% for phosphates and nitrates respectively.