Impact of Pyrolysis Temperature and Feedstock on Surface Charge and Functional Group Chemistry of Biochars

Abstract

The capacity of biochars to adsorb ionic contaminants is strongly influenced by biochar surface chemistry. We studied the effects of biomass feedstock type, pyrolysis temperature, reaction media pH, and AlCl pre-pyrolysis feedstock treatments on biochar anion exchange capacity (AEC), cation exchange capacity (CEC), point of zero net charge (PZNC), and point of zero salt effect (PZSE). We used the relationship between PZNC and PZSE to probe biochar surfaces for the presence of unstable (hydrolyzable) surface charge functional groups. The results indicate that biochars produced at ≤500°C have high CECs and low AEC, PZSE, and PZNC values due to the dominance of negative surface charge arising from carboxylate and phenolate functional groups. Biochars produced at ≥700°C have low CEC and high AEC, PZSE, and PZNC values, consistent with a dominance of positive surface charge arising from nonhydrolyzable bridging oxonium (oxygen heterocycles) groups. However, biochars produced at moderate temperatures (500-700°C) have high PZSE and low PZNC values, indicating the presence of nonbridging oxonium groups, which are rapidly degraded under alkaline conditions by OH attack on the oxonium α-C. Biochars treated with AlCl have high AEC, PZSE, and PZNC values due to variably charged aluminol groups on biochar surfaces. The results provide support for the presence of both hydrolyzable and nonhydrolyzable oxonium groups on biochar surfaces. They also demonstrate that biochars produced at high pyrolysis temperatures (>700°C) or those receiving pre-pyrolysis treatments with AlCl are optimized for anionic contaminant adsorption, whereas biochars produced at low pyrolysis temperatures (400°C) are optimized for cationic contaminant adsorption.

Coffee Waste Biochar: Characterization and Zinc Adsorption From Aqueous Solution

“…In this work, both biochar have high pH PZC (9.2 and 7.7 for coffee parchment and spent coffee grounds biochar respectively) ( Figure 3). These data are consistent with the results reported by Banik et al, (2018), who evaluated influence pyrolysis temperature on surface charge and functional group chemistry of biochars. They showed that biochars pyrolyzed at higher temperatures (≥ 700) presented consequently higher pH PZC .…”

Biochar physicochemical properties: pyrolysis temperature and feedstock kind effects

“…The detectable CEC suggested that when biochar was produced at temperatures up to 480°C, some acidic oxygenated functional groups such as phenolic acid and carboxyl groups were retained (Mitchell et al 2013). Banik et al (2018) reported that the CEC of biochar is dependent on the nature and distribution of O-containing functional groups on the biochar surface. The negative charge sites on biochar surfaces are attributed to carboxylate and phenolate functional groups (Mia et al 2017).…”

“…The negative charge sites on biochar surfaces are attributed to carboxylate and phenolate functional groups (Mia et al 2017). They assume that negative surface charge can only come from carboxylate and phenolate groups and positive charge from oxonium groups (heteroatoms in aromatic rings) (Banik et al 2018). However, other studies have found that biochars with higher specific surface area (obtained at temperatures above 600°C) have greater surface microporosity and increased CEC (Gomez-Eyles et al 2013; Kasozi et al 2010).…”

Biochar-Facilitated Soil Remediation: Mechanisms and Efficacy Variations

“…The zeta potential of canola-strawand peanut-straw-derived biochars became more negative as the solution pH increased between 3.0 and 8.0, indicating a greater negative charge density on the biochar surface at a higher pH (Xu et al, 2011). Biochars derived from corn stover, red oak, cottonseed hull, pecan shell, and pure cellulose through 200-900 • C slow pyrolysis showed pH ZNC (pH at which biochar has zero net charge) in the range of 3.1-8.5, increasing as the pyrolysis temperature was raised (Uchimiya et al, 2011a;Banik et al, 2018). This corroborates that biochars produced at higher temperatures had a lower CEC value owing to further losses of surface functional groups (Figure 2), as a higher pH ZNC indicates fewer negative charges on the surface.…”