An investigation of hydraulic and adsorption behaviour of biochar-sand mixtures

Abstract

Biochar, an engineered material from pyrolysis of biomass, is widely used in agriculture. Recent studies have demonstrated extensive potential for biochar to be utilised as an adsorbent in environmental engineering applications. The research presented in this thesis focuses on the application of biochar as part of the fixed bed filtration in water and wastewater treatment systems. The study aims to develop new insights into the key engineering properties of biochar-sand mixtures, including the hydraulic behaviour, adsorption/retardation properties and erosion-assisted release of contaminants. The results of a series of columns tests, chemical analysis and microscopic examinations on a range of biochars and sand-biochar mixtures. The investigations included i) the hydraulic conductivity of mixtures, ii) the adsorption capacity of biochar for the removal of microplastic and iii) the erosion of colloidal sized particles from biochar and co-transport of ionic species. The feedstocks of biochars used in this study include hardwood, corn straw and wheat straw, and the silica sand used vary from course to medium and fine sizes. Microscopic examinations and elemental analysis included SEM, EDX, XRD, DLS, etc., by which the mechanisms of retardations of microplastic spheres and properties of eroded biochar were investigated. In order to evaluate the erosion of biochar, a bespoke experimental system was designed and manufactured. The results of permeability tests, alongside the 3D modelling of sand-biochar systems, revealed that a dual-porosity concept in which there is negligible water flow in the intraparticle pores of biochar can describe much more accurately the hydraulic behaviour than the single porosity concept. The results of experiments and microscopic examinations highlight the strong ability of biochar in immobilising the microplastic spheres, which are the most mobile type of microplastic pollution. The dominant trapping mechanisms identified are Stuck, Entangled and Trapped. The colloidal particles detected in effluent unveil the continuous erosion in the biochar filter. The results of co-transport experiments showed that the eroded adsorbent carrying contaminants can escape the filter. It proved the necessity of post-filtration for ensuring the removal of hazardous chemicals transported by the eroded particles. This thesis provides an improved understanding of key physical and chemical properties of biochar and its behaviour as a filter for developing low-cost localised water and wastewater treatment systems. This study also highlights the extensive potential for the removal of microplastic by biochar and opens a new horizon for the research on biochar for tackling the emerging contaminants.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Majid Sedighi (Supervisor) & Amanda Lea-Langton (Supervisor)