Innovative characterization and application of biochar for sustainable water treatment

Biochar is the carbon based material rising from thermo-chemical conversion of biomass in an oxygen limited environment. Biochar’s production process affords for thermal degradation of biomass, enhancing its recalcitrance, as a consequence of carbon condensation. This transformation can establish biochar as a mean of stable carbon storage. Biochar’s physical-chemical properties also make it compelling for many applications, both novel and as an alternative to conventional carbon based materials. Among these, biochar gained traction as water sorbent on account of shared beneficial characteristics with activated carbon in high surface area, extensive surface functionality and affinity for common contaminants. Real-world, large scale deployment of such a technology is currently hindered by two key challenges: biochar variability and lack of real case studies. The aim of this Ph.D. thesis is to bridge the gap between laboratory and real world through characterization and use of commercial biochar for water treatment in real case studies. In particular, the objective was to provide evidence and best practices for use of widely available biochar in real-world scenarios. This provides support to biochar’s legacy as a sustainable material and directly contributes to Goal 6, 7, 12 and 13 of the 2030 Agenda for sustainable development. Nine commercial biochar types have been characterized by means of elemental analysis, scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX), x-ray diffraction (XRD), thermogravimetric analysis (TGA-DTG), surface analysis (BET-BJH) and Fourier-transform infrared spectroscopy (FT-IR). Moreover, micro-Raman spectroscopy and organic petrography have been employed as novel methods for biochar characterisation, on the base of analogies of biochar’s production process to natural degradation pathways of organic matter dispersed in sediments. Supporting techniques in ion coupled plasma mass spectroscopy (ICP-MS) and ion coupled plasma optical emission spectroscopy (ICP-OES) have been used to survey changes in contaminant concentrations in both biochar and aqueous solutions. Characterization results underscored commercial biochar’s heterogeneity, concerning both physical-chemical structure and available market products. Such an heterogeneity stems from feedstock and production process and must be addressed in order to make biochar use consistent and reliable. It is suggested that this can be achieved through implementation of standardized, application-focused characterization. In this regard, micro-Raman spectroscopy and organic petrography through reflectance analysis, can be instrumental for process optimization, quality control and evaluation of carbon sequestration potential. Case studies dealt with both real surface waters and industrial wastewaters. Surface waters did not allow testing of biochar’s sorption potential as a consequence of low contaminant concentrations, in contrast with published data from the local environmental authority. This fact has highlighted the presence of a lower contaminant threshold for detection of sorption through biochar sorbents. In this context, leaching experiments on biochar were conducted to ensure its safety in presence of unpolluted matrixes. Leaching of chemicals from biochar into solution was recorded, but did not impact water quality. This reinforces biochar use for water treatment as a safe practice. Real industrial wastewaters were characterized by concurrent presence of multiple competing contaminants at varying concentrations, ranging from mg/L down to µg/L. Results of sorption experiments testify biochar’s viability for metal removal both at high and low concentration. Nevertheless, ion competition and biochar’s affinity towards a few species may skew sorption in favour of specific contaminants. Additionally, sorption experiments conducted with all of the nine commercial biochar types and synthetic solutions of Pb2+ and Cr3+, have highlighted the role of biochar’s porosity structure in the sorption process, potentially inhibiting it at high concentrations. The Ph.D. project provides a hands-on approach to biochar use for water treatment by employing widely available materials in real case studies, providing results instrumental to scale-up of biochar technologies. Characterization fills in the current lack of data of commercial biochar for water treatment purposes and proposes ways to standardize biochar production and classification through novel methods, also for the sake of carbon sequestration. Real case studies and synthetic follow-ups testify biochar’s potential for water treatment and point out constraints for the achievement of efficient contaminant removal. Therefore, the present study can serve as a stepping stone for real-world deployment of biochar technologies for water treatment, fostering sustainable development.