The effects of biochar on bacterial communities and geotechnical properties of landfill cover soils

Abstract

This study examined the applications of biochar to promote ecological restoration of sanitary landfills, and as a novel landfill final cover material. Ecological restoration is necessary to re–establish the ecosystems and minimise environmental disturbance. A compacted clay liner is often used as a landfill final cover system. However, clay is not widely available, and the shrinkage and crack formation will reduce its effectiveness. Compacted biochar–amended clay (BAC) is proposed as a potential effective sustainable and environmentally–friendly cover material in this study. The principle objectives of this study are to (1) investigate the effects of biochar on bacterial communities of landfill final cover topsoil, and (2) evaluate the performance of compacted BAC as a landfill final cover material. The establishment and development of plant and animal communities at the South East New Territories Landfill (SENT) in Hong Kong was investigated over a period of ten years (2003–2012). The long–term monitoring results show that flora and fauna were gradually restored with woodland established at the restored sites. The most abundant tree species was Acacia confusa, followed by Leucaena leucocephala. Both trees are exotic species; the former is more suitable to be used as pioneer species in restoring sanitary landfills. The bacterial communities in a newly established and aged final cover topsoil, and the effects of biochar were also studied at the Xiaping Landfill in Shenzhen using 16S rRNA–based T–RFLP methodology. Bacterial phyla including Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria were found. The most abundant bacterial phylum was Firmicutes, with constituted of more than 46% of bacteria found. Biochar application significantly (p<0.05) increased the bacterial diversity but no significant correlation (p>0.05) between bacterial communities and soil physicochemical properties were discovered. The geotechnical properties of compacted BAC were evaluated in a series of laboratory tests. Biochar used was derived from peanut–shell at 500°C by slow pyrolysis. The wetting and drying soil–water retention curves (SWRCs) and water permeability functions, k(ψ) and k(θ_w) of clay and BAC were examined using one–dimensional soil columns. The gas permeability of BAC was measured using a gas permeameter, and shrinkage properties were also investigated. The SWRCs show that biochar application increased the soil–water holding capacity of compacted clay. Adding 20% (w/w) of biochar reduced k(θ_w) of clay. For instance, addition of 20% (w/w) biochar reduced the k(θ_w) of clay from 1.64×10^-10 m s^-1 to 3.13×10^-11 m s^-1. Furthermore, adding 15% (w/w) biochar reduced the gas permeability of clay by two orders of magnitude from 3E-11 to 2E-13 m². In addition, application of 20% (w/w) biochar increased the shrinkage limit of clay by 24%, reducing the potential crack formation. This study demonstrated the beneficial usage of biochar in landfill cover topsoil and clay liners to restore ecological assemblages of plants, animals and microorganisms and to provide a novel cover material that increases soil–water retention, and reduces unsaturated gas and water permeabilities.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology