Coffee Waste Biochar: Characterization and Zinc Adsorption From Aqueous Solution

Abstract

The final disposal of organic wastes has become a major challenge with increasing industrialization and population growth. Coffee wastes are examples of this, thus the conversion of these biomasses into biochar through pyrolysis could provide economic and environmental benefits, such as remediation of heavy metal polluted water. Therefore, in this work, biochar produced at 700 °C from spent coffee grounds and coffee parchment were evaluated for Zn removal from aqueous solution. Batch adsorption tests were performed with six Zn concentrations and four replicates for each material. The desorption process was performed sequentially with a pH 4.9 buffer acetic acid solution. Langmuir and Freundlich isotherms were fitted to the adsorption data using non-linear models. Batch adsorption tests showed that the adsorption was strongly dependent on biochar properties. Biochar prepared from coffee parchment was more effective at Zn binding, showing the highest adsorption capacity (0.792 mg g<sup>-1</sup>). Nevertheless, both biochars bounded Zn strongly and the adsorption process was not easily reversed.

Cadmium and lead adsorption and desorption by coffee waste-derived biochars

“…In addition to the surface area size, the characteristics of this surface, such as the presence of chemical functional groups, have a substantial effect on the biochar sorption capacity for heavy metals. In our previous study (Sertoli et al 2019), we found that the both biochars have oxygenated groups, but CP showed more aromatic structures, which is consistent with its H/OC ratio. Therefore, these characteristics increase the stability and contribute to the resistance to the decomposition of this material (Sohi et al 2010).…”

“…The high degree of aromaticity and high point of zero charge (PZC) (Sertoli et al 2019) might explain the relatively low metal retention by these coffee waste biochars when compared with other biochars in the literature. The lower negative charge density due the aromatic structure of the biochars is supported by the high PZC values (7.7 and 9.2 for SCG and CP respectively) (Sertoli et al 2019).…”

“…The high degree of aromaticity and high point of zero charge (PZC) (Sertoli et al 2019) might explain the relatively low metal retention by these coffee waste biochars when compared with other biochars in the literature. The lower negative charge density due the aromatic structure of the biochars is supported by the high PZC values (7.7 and 9.2 for SCG and CP respectively) (Sertoli et al 2019). Such results imply a net negative charge on the biochars' surface only when the pH of the medium is higher than the PZC (Borba et al 2019), limiting the attraction and sorption of metallic ions.…”

“…Solutions containing 0, 1.0, 2.0, 4.0, 8.0 and 16.0 mg•L -1 Cd or 0, 20, 60, 100, 140 and 180 mg•L -1 Pb were prepared using the same electrolyte support solution and salts as described for the pH experiment. The sorption experiment was performed by the addition of 0.05 g of the biochars tested to 50 mL centrifuge tubes containing 40 mL of each metal solution (Sertoli et al 2019). Stirring, filtration and metal determination in the supernatants were performed as described before.…”

“…This neutralization effect was also observed by Huang et al (2018) with chicken manure biochar pyrolyzed at 700 °C, and they attributed the buffering capacity to the presence of alkaline ions PO 4 3and CO 3 2in the biochar. Given the aromaticity and other characteristics of these biochars in the implications for the availability of negative charge, mainly for CP (Sertoli et al 2019), when comparing with other sorption mechanisms, precipitation may have played a key role in the Cd and Pb sorption observed, which relates to solution speciation that is pH dependent. For cadmium, Cd 2+ ions are the predominant species below pH 8.0, when they start to precipitate as CdCO 3 and represent up to 60% of the Cd species in solution at pH 9.0 (Wu et al 2019).…”