# Almond Shell-Derived Biochar for Lead Adsorption: Comparative Study of Pyrolysis Techniques and Sorption Capacities

## TL;DR

This study compares different methods of making biochar from almond shells to remove lead from water, finding that non-activated slow pyrolysis biochar is the most effective and sustainable option.

## Contribution

The study introduces a comparative analysis of pyrolysis techniques and activation methods for almond shell-derived biochar in lead adsorption.

## Key findings

- Non-activated slow pyrolysis biochar showed the highest surface area and best adsorption performance.
- KOH activation increased hydroxyl content but reduced textural properties and caused pore collapse in microwave pyrolysis samples.
- Optimal lead removal occurred at pH 4 with no leaching, and Freundlich and Redlich–Peterson models best described equilibrium data.

## Abstract

Lead (Pb(II)) contamination in water poses severe environmental and health risks due to its toxicity and persistence. This study compares almond shell-derived biochars produced by slow pyrolysis (SP) and microwave pyrolysis (MW), with and without KOH activation, focusing on structural properties and Pb(II) adsorption performance. Biochars were characterized by proximate and elemental analysis, BET surface area, FTIR spectroscopy, and adsorption experiments including pH dependence, kinetics, and equilibrium isotherms. Non-activated SP exhibited the highest surface area (SBET = 693 m2·g−1), pronounced mesoporosity (≈73% of total pore volume), and the largest observed equilibrium capacities. KOH activation increased surface hydroxyl content but degraded textural properties; in MW samples, it induced severe pore collapse. Given the very fast uptake, kinetic modeling was treated cautiously: for non-activated biochars, Elovich adequately captured the time-course trend, whereas activated samples returned non-physical kinetic constants (e.g., negative k2) likely due to high post-adsorption pH (>11) and probable Pb(OH)2 precipitation. Equilibrium data (fitted over 50–500 mg·L−1) were better captured by the Freundlich and Redlich–Peterson models, indicating a mixed adsorption behaviour with contributions from heterogeneous site distribution and site-specific interactions. Optimal Pb(II) removal occurred at pH 4, with no measurable leaching from the biochar matrix. Overall, non-activated SP biochar is the most effective, sustainable and low-cost option among the tested materials for Pb(II) removal from water, avoiding aggressive chemical activation while maximizing adsorption performance.