Physico-chemical characterization of walnut shell biochar from uncontrolled pyrolysis in a garden oven and surface modification by ex-situ chemical magnetization

Abstract

The shells of walnuts (WS) are major refuse in the global fruits and nuts trade. This, otherwise discarded, lignin-rich material can be carbonized to biochar—a value-added product with environmental applications such as carbon sequestration, soil amelioration, and pollutant adsorption. These applications are dictated by structural and chemical characteristics of the biochar carbon. Conventional controlled pyrolysis (CPy) of biomass is cost-intensive and technically too complex for widespread adoption, especially in emerging economies. Here, walnut shell biochar (BWS0) is derived through uncontrolled pyrolysis (UCPy) in a pyrolysis oven and further hybridized as magnetic biochar through ex-situ chemical co-precipitation. The physico-chemical characteristics of biochar and its water-extractable fractions are comprehensively investigated to understand their carbon structure and environmental applicability. The sp2 amorphous carbon sequestered in BWS0 is 0.84 kgCO2/kgbiomass with a BET (N2) surface area of 292 m2/g and is comparable to biochar from CPy in terms of carbon structure. The polyaromatic hydrocarbons present are only trace amounts of naphthalene, biphenyl, and phenanthrene. The magnetization decreases porosity of BWS0 while greatly facilitating its separation from aqueous media. BWS0 is suitable for adsorption of cations (between pH 2.8 and 9.45) and hydrophobic pollutants with only 19 mg L−1 fouling from their intrinsic dissolved organic carbon. In combination with fast-release N, P fertilizers, BWS0 (C/N of 24.8) is suitable for application in hydrophilic soils at higher loading rates. The results suggest an avenue where WS biochar can also be prepared via UCPy for direct environmental applications. Future investigations into soil incubation and adsorption tests are recommended. Graphical abstract

The influence of sample mass (scaling effect) on the synthesis and structure of non-graphitizing carbon (biochar) during the analytical pyrolysis of biomass

“… 92 However, the BP was rich (6.53 dry wt%) in potassium, 93 an AAEM that catalyzes pyrolysis making it comparatively difficult to isolate scaling from AAEM influence as a cause of the scaling trend and the inflection point. Hence, such scaling studies are better performed with relatively pure (minimal AAEM and transition metals) biomass such as walnut shells 94 or wood after a comprehensive fiber and elemental analysis. For WS, the H/C molar ratio of ws_scl until 200 mg scale shows a strong correlation with (a) biochar properties of aryl-C% ( r = −0.972, p = 0.006), surface area ( r = −0.971, p = 0.028), (b) the pyrolysis Q DSC ( r = +0.981, p = 0.003), and (c) CO 2 emission ( r = −0.979, p < 0.004).…”