Retarding performance of the vadose zone for nitrogen pollutants derived from municipal solid waste landfills in the red bed zone

Abstract

Nitrogen pollutants such as ammonia and nitrates cause soil and groundwater contamination at municipal solid waste landfill (MSWL) sites due to leachate leakage. Here, the migration of nitrogen pollutants in the vadose zone of the red bed (VZRB) at a MSWL site was studied by static adsorption batch experiments and one-dimensional simulated migration experiments. The results indicated that the soil in the red bed did not adsorb nitrates. Chemical adsorption and monolayer adsorption of the soil played dominant roles during ammonia adsorption in the VZRB, which was best fitted by the pseudo-second-order kinetic equation (RNH4+2 = 0.99) and the Langmuir model. The ammonia adsorption capacity of the soil was the highest (Qm = 2.041 mg·g−1) at 318 K. It was due to the endothermic and non-spontaneous chemical adsorption of ammonia, whose enthalpy change (ΔH) reached 20.995 kJ·mol−1 and Gibbs free energy ranged from 8.469 to 8.706 kJ·mol−1. Chloride penetration tests indicated that the diffusion coefficient and migration speed reached 0.0515 cm2·h−1 and 0.0833 cm·h−1, respectively, in the clay layer under the MSWL sites. The average hysteresis diffusion coefficients of ammonia in the simulated soil columns leached by ammonium chloride solution (SSCAC) and by the leachate from MSWL (SSCL) were 1.129 and 1.400, respectively. After the leaching experiments, the clay pore structure was saturated, and the specific surface area decreased. The absorption peak intensities of clay functional groups, including carboxyl, alkyne, and hydroxyl groups, were reduced. The ammonia content in the soil of SSCAC decreased from the top (14.51 mg·kg−1) to the bottom (3.14 mg·kg−1) and in the SSCL from 24.96 mg·kg−1 to 5.05 mg·kg−1, respectively. Thus, the impermeable clay layer and VZRB helped in retardation of ammonia leakage from MSWL sites. This was due to the blockage of seepage, ammonia mechanical filtration, ammonia monolayer chemical adsorption, and the reaction between the functional groups and ammonia in the VZRB underneath the MSWL sites.