Optimising production of a biochar made from conifer brash and investigation of its potential for phosphate and ammonia removal

Abstract

As a lignocellulose biomass, waste conifer brash (waste treetops and branches) from felled afforested peatland sites can be converted to biochar through pyrolysis, thus creating a potentially useful product. Here, we propose that brash from ‘forest-to-bog’ peatland restoration sites through conversion to biochar, could be utilised for nutrient (PO43−-P and NH4+-N) removal at such restoration sites, or within the water sector. As a first step, we explore the factors involved in biochar production that will result in high nutrient adsorption efficiency and pyrolysis yield and low production cost (using a Plackett-Burman experimental design (PBD)). Central composite design (CCD) was used for further optimisation of pyrolysis time and temperature as the two most significant factors. Model predictions for an optimised biochar (OB) suggested pyrolysis conditions of 500 °C for 30 min could achieve the highest yield of 34.75 %, the lowest cost of 0.37 £ /kg, and the highest PO43−-P and NH4+-N removal of 9.9 % and 65.2 %, respectively. Additionally, the OB morphology, structure and surface chemistry were characterised using different instrumental techniques which showed typical features for a wood-based biochar. While the OB did remove NH4+-N from solution (due to the presence of negatively charged functional groups), it did not remove significant amounts of PO43--P, indeed it leached PO43--P back into solution. Therefore, an unmodified biochar produced from conifer brash did not fulfil the aim of removing these two key nutrient pollutants for use in improving water quality at restoration sites. To address this challenge, the surface chemistry of the OB could be functionalised to increase its affinity toward both PO43−-P and NH4+-N ions.