Key factors determining biochar sorption capacity for metal contaminants: a literature synthesis

Abstract

The sorption capacity and affinity of biochar for metals are both important attributes that determine biochar's suitability as a soil amendment for contaminant mitigation, yet few analyses have considered both characteristics simultaneously. We present a systematic review of literature published between 2010 and 2018 to test the hypothesis that sorption capacity and affinity are affected by biochar properties, attributes of the metal contaminant, and experimental conditions, in that order. We used random forest (RF) and multi-objective optimization to analyze data of 559 individual Langmuir adsorption isotherms extracted from 133 studies covering the sorption capacity (C max) and affinity (K L) of biochar for 17 different metals, elaborated from six different feedstock classes, three different types of feedstock pretreatment, and five types of post-pyrolysis treatment. Highest sorption values were obtained for Pb(II), Cr(IV), and Cd(II). The feedstock used was the key determinant influencing biochar's capacity and affinity to sorb metal contaminants (first and fourth most important variable in RF models for C max and K L , respectively) with best results obtained for biochar elaborated from nutrient-dense feedstocks (animal biowaste, sludge, and manure). Biochars that had both high sorption capacity and affinity were the result of a longer duration of pyrolysis; they had lower C and higher N and O content, as well as lower C/N and higher O/C and H/C ratios, higher pore volumes and higher pH. Applying some form of pretreatment was better than none, whereas chemical modification was the best of the post-treatment methods analyzed. This review demonstrates clearly that multiple parameters during the preparation process influence the effectiveness of biochar to immobilize metal contaminants. Future research that focuses on mechanisms and the underlying factors for the correlations observed should allow the development of biochar formulations that are even more effective at immobilizing metal contaminants in soils and sediments.

A Review of Intermediate Pyrolysis as a Technology of Biomass Conversion for Coproduction of Biooil and Adsorption Biochar

“…Always, a good adsorbent should have the high adsorptive capacity, adsorption rate, mechanical strength, and wear resistance, reactivation characteristics, and good particle size distribution [53]. Biochar to be used as an adsorbent with high absorption power and affinity to metals are those with lower carbon, high nitrogen and oxygen, as well as lower C/N and higher O/C and H/C ratios, higher pore volume, and higher pH [54]. It is always produced with low pyrolysis temperature at high pyrolysis time.…”

Enhancement of Cd(II) adsorption by rice straw biochar through oxidant and acid modifications

“…There are some oxygen-containing functional groups such as -COOH and -OH on the surface of biochar (Qian and Chen 2014), which make electrostatic adsorption and specific adsorption of heavy metals occur simultaneously on the biochar (Jiang et al 2012;Bashir et al 2018;Zhu et al 2020). There are great differences in the number of functional groups on the surfaces of the biochars prepared from different raw materials (Thomas et al 2020). In China, there are abundant crop straws such as rice straw, but the carboxyl content of the biochars prepared from these straws is not high.…”

Low tech biochar production could be a highly effective nature-based solution for climate change mitigation in the developing world

“…It is now clear that the production and use of biochar in agriculture and soil remediation has a clear economic value proposition (Thomas et al 2020;Ladd et al 2018). At the same time there is growing awareness among scientists and decision makers that simply curtailing greenhouse gas (GHG) emissions will be insu cient to avert a climate crisis (Ravi et al 2016;Li et al 2019;Rogelj et al 2021).…”