Biochar obtained from spent coffee grounds: Evaluation of adsorption properties and its application in a voltammetric sensor for lead (II) ions

Biochar is a carbonaceous material obtained by controlled pyrolysis of biomass in a low amount of oxygen. This material has functional chemical groups on its surface, which can interact with organic and inorganic compounds. In the present work, biochar samples were prepared from spent coffee grounds using pyrolysis temperatures between 200 and 700 ?C. Biochar yielded at different conditions was activated by a chemical treatment using nitric acid (HNO3) as an oxidizing agent to increase surface groups. The characterizations of the pristine and chemically treated biochar samples showed a rise in functional groups, confirmed by FTIR with the appearance of new and intense carboxylic group bands. The variation of ID/IG ratio, calculated from the obtained Raman spectra, indicates an increase in defects and insertion of heteroatoms on the material surface after chemical treatment. Biochar samples were used as modifiers to prepare the carbon paste electrode (CPME) and evaluated lead ions? pre-concentration. The electroanalytical strategy was based on differential pulse adsorptive anodic stripping voltammetry (DPAdSV), performing the pre-concentration step under open circuit potential condition separately from the electrochemical cell. It was verified for samples submitted to chemical acid treatment, and, among them, the CPME prepared with the 300 ?C pyrolyzed biochar showed the best response. Under optimized conditions, it was observed a linear dynamic range (LDR) from 0.128 to 2.44 ?mol L-1, limits of detection (LOD) and quantification (LOQ) of 4.5 nmol L-1 and 15 nmol L-1, respectively. The effect of foreign ions on voltammetric response has been evaluated, and the proposed sensor was applied for the determination of lead ions present in gunshot residues and hair dye samples.

Automated summary

In this study, biochar samples were prepared from spent coffee grounds at different pyrolysis temperatures and activated using nitric acid treatment to increase surface groups. The chemically treated biochar showed enhanced functional groups, leading to improved response in pre-concentration of lead ions. Under optimized conditions, the biochar-based carbon paste electrode exhibited a linear dynamic range of 0.128 to 2.44 μmol L-1 for lead ions detection.