Valorization of chestnut shell into biochar and activated carbon for Pb(II) adsorption: comparative performance, material characterization, and adsorption mechanisms

Abstract

Agricultural chestnut shell waste was valorized into three structurally distinct carbonaceous adsorbents, namely raw chestnut shell (RCS), chestnut shell biochar (BCS), and chestnut shell-derived activated carbon (ACCS), to comparatively evaluate their effectiveness for Pb & sup2;(+) removal and to clarify how thermochemical conversion and activation influence adsorption performance. Differences in the structural, morphological, and surface characteristics of RCS, BCS, and ACCS were identified using FTIR, SEM-EDX, BET surface area measurements, and pHPZC analysis. Their Pb2+ uptake behavior was subsequently examined through batch adsorption experiments conducted over an initial concentration range of 50-600 mg/L under varying operating conditions. Maximum Pb2+ uptake was obtained under optimized operating conditions, namely an initial solution pH of 5.51, a contact time of 120 min, and adsorbent dosages of 0.10 g (RCS), 0.05 g (BCS), and 0.02 g (ACCS). At 298 K, these conditions resulted in adsorption capacities of 22.44 +/- 0.56, 75.96 +/- 1.15, and 206.35 +/- 3.18 mg/g, respectively. The progressive increase in adsorption capacity from RCS to BCS and ACCS demonstrates the benefit of converting low-value chestnut shell waste into porous carbonaceous materials, particularly activated carbon, which provides a potentially low-cost, renewable, and structurally tunable platform for heavy-metal removal. The substantial enhancement observed following thermochemical conversion and surface modification was associated with the development of a more developed porous structure and changes in surface-active functionalities, as supported by the characterization results. Mechanistic interpretation based on the combined FTIR, pHPZC, SEM-EDX, and adsorption results suggested that Pb2+ uptake involved multiple interactions, including surface complexation with oxygen-containing functional groups, electrostatic attraction, and pore-filling within the developed porous structure, with the relative importance of these interactions depending on the physicochemical characteristics of RCS, BCS, and ACCS. Experimental kinetic data were best represented by the pseudo-second-order model. In addition, thermodynamic findings suggested that increasing temperature enhanced Pb & sup2;(+) adsorption and that the process occurred spontaneously. Overall, the results highlight chestnut shell-derived activated carbon as an effective and promising adsorbent for Pb2+ removal from aqueous solutions, while demonstrating that chestnut shell biochar also offers a renewable and low-cost alternative to conventional adsorbents for heavy-metal removal.