Phosphorous recovery from treated wastewater by salt activated biochar

Phosphorus (P), an essential element for life and an irreplaceable component of modern agriculture, suffers losses at every stage of its biogeochemical cycle, raising concerns about future supplies as well as water and soil pollution [1]. Furthermore, P is a non-renewable resource so much so that it is estimated that its reserves will be depleted within 50-400 years [2]. Europe, in order to meet its need for P, is entirely dependent on imports from other countries. The unbalanced global distribution of P is increasingly becoming a geopolitical problem for Europe that can only be overcome by finding new and sustainable sources of supply. By virtue of the above, a more rational use of P fertilisers and the recovery of P from agro-food products, urban and industrial wastewater are desirable [3]. An efficient and economical method to recover P from wastewater is by its sorption onto a solid phase that, in turn, can applied to soil as slow-release fertiliser. Biochar is among the most widely used solid materials for nutrient recovery. It is a material obtained by pyrolysis of biomass, usually wastes, at high temperatures (300-800 °C) and in absence of oxygen [4]. Biochar consists mainly of aromatic-type carbon and is characterised by a large specific surface area (200-1000 m2 g-1), low density and high porosity [5]. Studies conducted to investigate the potential of biochar as adsorbent of P from aqueous solutions are few and, often, conflicting. This is probably due to the performance and properties of biochar being highly influenced by many factors such as temperature, heating rate and pyrolysis duration, feedstock used and particle size [6]. Studies have shown that biochar has mainly a negative surface charge, which causes non-attraction or only very weak interactions with negative ions such as phosphate. For this reason, a modification of the surface and structure of the biochar is required if it is thought to be used for phosphate sorption. Zheng et al., (2019) [10] found that biochar modified with Mg and Al contains more functional groups than natural or treated biochar with only Mg or Al, moreover, it shows a larger surface area [10]. Zhong et al., 2019 [9], for example, impregnated coconut shell biochar with Fe. The results showed that Fe-biochar adsorbed 36 mg of P g-1, which was 2.4 times higher than the untreated biochar. Although the results are encouraging, it is important to emphasise that the search for biochar with a higher phosphate adsorption capacity is in its early stages, so further studies are needed. The purpose of this study was to assess the ability of two biochar, pre-treated or not with HCl and, subsequently, treated with 0.5 M and 2 M of one of the following salts, CaCl2, MgCl2, AlCl3, FeCl3, to adsorb P from a mono-component solution.