Accelerated aging of biochars: Impact on anion exchange capacity

Abstract

Little is known about the stability of biochar anion exchange capacity (AEC) and by what mechanisms AEC changes as biochar ages and weathers in soil environments. The goal of this study was to investigate chemical changes that may occur during ageing of biochar in neutral or alkaline soils and to assess the impact of ageing on AEC. To simulate and accelerate ageing, biochars were oxidized in alkaline hydrogen peroxide for 4 months. Spectroscopic evidence (FTIR, XPS and 13 C-NMR) revealed that ageing increased carbonyl and alcoholic character in biochars produced at 500 °C and effected endoperoxide formation in biochars produced at 700 °C; the latter exhibited greater arene carbon character. Ageing caused biochar AEC to decline on average by 54% with greater decreases in biochars produced at 500 °C in contrast to biochars produced at 700 °C. The AEC of biochar derived from alfalfa meal and cellulose produced at 700 °C did not change significantly (p = 0.20 and p = 0.50, respectively) with ageing. Stability of AEC in the high temperature biochars is attributed to the presence of oxonium groups in bridging positions of arene carbon, which are sterically resistant to nucleophilic attack.

Effect of aging on surface chemistry of rice husk‐derived biochar

“…It is likely that aging can alter the biochar properties and affect its biogeochemical properties. Chemical oxidation agent was used to simulate and accelerate aging, for example, Lawrinenko et al [20] oxidized biochars in alkaline hydrogen peroxide for 4 months and found that aging increased carbonyl and alcoholic character in biochars produced at 5008C and anion exchange capacity of most biochars declined with aging; Ghaffar et al [21] found that oxidation biochar produced by peanut-shell with HNO 3 / H 2 SO 4 increased oxygen and thus increased PAEs sorption on oxidized biochar surfaces compared to their precursors. However, this kind of hard oxidation using chemical agent may not happen in the initial phase of biochar environmental exposure, but the results provide a basis to discuss the property changes of biochar after long-term environmental aging.…”

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

“…They measured AEC values that ranged from 0.94 to 27.8 cmol c kg −1 for biochars produced from albumin, cellulose (CE), corn ( Zea mays L.) stover (CS), and alfalfa ( Medicago sativa L.) and reported that AEC decreased with increasing pH and was higher for biochars produced at 700 than 500°C. Later, Lawrinenko et al (2016) reported that nonbridging oxonium groups are degraded at high pH by nucleophilic attack of OH − on exposed oxonium α‐C atoms, which transforms positively charged oxonium groups into neutral hydroxy ether groups. By contrast, bridging oxonium groups, which do not have exposed α‐C atoms, are sterically protected from nucleophilic attack and hence are stable even at high pH.…”

“…The recent discovery that positive charge sites on biochar surfaces are partly due to oxonium groups (Lawrinenko and Laird, 2015) was a major advance in understanding biochar surface chemistry and opens the possibility of engineering biochars specifically for adsorption of anionic contaminants. However, the vulnerability of nonbridging oxoniums to hydrolysis under alkaline conditions (Lawrinenko et al, 2016) would make biochars dominated by nonbridging oxoniums unsuitable as adsorbents of anionic contaminates. We hypothesize that increasing ionic strength will protect nonbridging oxonium groups from degradation because high concentrations of anions larger than OH − , such as Cl − , will effectively shield the oxonium α‐C from nucleophilic attack by OH − .…”

Effects of three different biochars on aggregate stability, organic carbon mobility and micronutrient bioavailability

“…1) appear to work in combination to improve aggregation processes. Nevertheless, the surface chemistry and physicochemical properties of biochar are understood to change with time in soil environments (Lawrinenko et al, 2016). Cheng et al, (2008) for example, using FT-IR, showed increases in hydroxyl and carbonyl groups from biochars incubated for 12 months relative to freshly prepared biochars; due to ageing they reported increased carboxylation of biochar surfaces.…”

“…Despite that, it is still unknown whether complexation by organic carbon functional groups, are from labile or refractory parts of the biochar (Mukherjee and Lal, 2013). Biochar application to agricultural soils may be affected by tillage processes, being brought to the surface and reacting with O2 which may enhance its oxidation (Lawrinenko et al, 2016). Clearly, further work in this area may lead to advancement of knowledge in terms of biochars ability to enhance soil aggregation and the effect ageing has on its surface chemistry.…”