Utilizing Waste Materials in Permeable Reactive Barriers (PRBs) for Groundwater Remediation in Landfill Areas

Landfill leachate has the potential to contaminate ground and surface water. Permeable reactive barriers (PRBs) are potential in situ treatment systems for contaminated groundwater. Hence, the aim of this research was to investigate the potential of mixtures of selected waste materials to act as composite reactive media in PRBs for treating groundwater contaminated by landfill leachate. The treatment efficiencies of the reactive media for heavy metals (Pb and Fe), chemical oxygen demand (COD), and total dissolved solids (TDS), as well as the changes of the reactive media bed when exposed to contaminated groundwater, were investigated. The experiment involved both laboratory-scale column and bench-scale experimental runs. The column and the bench-scale PRB reactor were filled with composite reactive media primarily made of dewatered alum sludge (DAS), natural zeolites (NZ), building waste (BW), zero valant iron (ZVI), biochar (BC), sawdust (SD), and sugar cane bagasse (SCB). In the column experiment, all the selected materials were mixed according to pre-determined proportions. The bench-scale reactor had two reactive media beds connected in series. In the first reactive media bed, high-density reactive materials were composited, while the second reactive media bed was a composite of low-density reactive materials. The average removal efficiencies of the reactive media in the column for Pb, Fe, COD, and TDS were 86.16 ± 7.51%, 51.33 ± 4.80%, 94.41 ± 1.60%, and 57.92 ± 2.07%, respectively, at the average influent concentrations of 67.5 × 10⁻³ (Pb), 15.0 (Fe), 1323.3 (COD), and 4207.5 (TDS) mg/L. In the PRB reactor, these efficiencies were 92.19 ± 1.84%, 49.52 ± 6.09%, 92.58 ± 1.71%, and 75.68 ± 3.04%, respectively, at the average influent concentrations of 62.2 × 10⁻³ (Pb), 15.1 (Fe), 1259.9 (COD), and 3921.0 (TDS) mg/L. In all three tested reactive media, the particle size distribution became more well-graded, and the hydraulic conductivity decreased slightly. The cohesion decreased slightly while the friction angle increased notably in all three media. The shear strength of the column medium increased, whereas it decreased in the bench-scale media. It can be concluded that the high-density reactive media bed alone had a much better treatment potential compared to the low-density media bed. However, mixing the high- and low-density media could enhance the structural integrity of the PRB, which is an important factor for its longevity.