Bean Husk Waste Transformed into Biochar That Strips Lead and Chromium from Water

Every year, the global harvest of common beans generates mountains of discarded husks, a fibrous byproduct that most processors burn, dump, or leave to rot. A research team led by Hussein K. Okoro and Adewale George Adeniyi, working across institutions in Nigeria and South Africa, has now shown that this humble agricultural waste can be transformed into a remarkably effective water purifier. In a study published in Discover Chemistry, the researchers converted bean husks into biochar and demonstrated that the resulting material can strip two of the world’s most dangerous water contaminants, lead and hexavalent chromium, from aqueous solutions with efficiencies exceeding ninety percent, all without the chemical activation steps that make many competing adsorbents expensive.

The stakes could hardly be higher. Lead is a bluish-grey heavy metal that enters water supplies through industrial operations, smelting, lead-based paints, and legacy leaded fuels. It ranks first on the U.S. Agency for Toxic Substances and Disease Registry’s list of hazardous compounds, and even trace exposures impair the reproductive, neurological, hematologic, and cardiovascular systems. Hexavalent chromium, meanwhile, is a potent oxidizer used in metal finishing, leather processing, textile printing, and steel manufacturing. It is carcinogenic and mutagenic, roughly a hundred times more dangerous than its trivalent counterpart, and irritates plant and animal tissues at extremely low concentrations. The U.S. Environmental Protection Agency has set maximum contamination levels of just 0.015 milligrams per liter for lead and 0.1 milligrams per liter for hexavalent chromium in drinking water.

To build their adsorbent, the team collected bean husks from a marketplace in Ogbomosho, Nigeria, washed and sun-dried them, and loaded one hundred grams into a top-lit updraft reactor, a device in which air enters through perforations at the base and feeds a combustion zone that indirectly heats the biomass chamber. The carbonization ran for 110 minutes, peaking at just 320 degrees Celsius, a relatively low temperature that favors high biochar yield. The process converted 38.4 percent of the feedstock into biochar, a figure consistent with yields from corn husk, sugarcane bagasse, and coconut fibre reported in earlier studies. Crucially, no chemical activating agents were used at any stage, keeping production costs and environmental footprint to a minimum.

Characterization revealed why the material performs so well. Brunauer-Emmett-Teller analysis showed that carbonization boosted the specific surface area more than sevenfold, from 3.36 to 24.28 square meters per gram, while pore volume surged nearly 140-fold, from 0.0037 to 0.5105 cubic centimeters per gram. Scanning electron microscopy confirmed a well-developed, irregular porous network ideal for trapping dissolved ions. Fourier transform infrared spectroscopy identified a rich inventory of oxygen-bearing functional groups, including hydroxyl, carbonyl, and carboxylic moieties, and energy-dispersive X-ray analysis found carbon making up about 67.6 percent of the material. X-ray fluorescence detected abundant potassium, calcium, silicon, and magnesium oxides, with calcium oxide in particular contributing alkalinity that enhances heavy metal capture.

Batch adsorption experiments, monitored with atomic absorption spectrophotometry under rigorously calibrated conditions, probed how pH, dosage, contact time, concentration, and temperature govern performance. The results hinged on the biochar’s point of zero charge, measured at pH 6.8. Above this threshold the surface is negatively charged, attracting positively charged lead ions; below it, protonation creates positive sites that electrostatically capture chromate anions such as HCrO4− and Cr2O7²−. Accordingly, lead removal climbed from 54.2 to 94.7 percent as pH rose from 2 to 8, while chromium removal peaked at 91.5 percent at pH 3 and collapsed under alkaline conditions. Optimal dosing was 0.5 grams of biochar per 20 milliliters of solution, with efficiency plateauing after roughly 40 minutes of contact.

Equilibrium modeling showed the Freundlich isotherm fit best for both metals, with correlation coefficients of 0.9843 for lead and 0.9395 for chromium, indicating multilayer adsorption on a heterogeneous surface with sites of varying energy. Maximum uptake capacities reached 18.38 milligrams per gram for lead and 22.78 milligrams per gram for chromium. Separation factors between zero and one, and Freundlich constants greater than one, confirmed favorable, intense adsorption. Kinetic analysis told a subtler story: lead uptake followed the pseudo-second-order model with an R² of 0.9956, pointing to chemisorption through electron exchange, whereas chromium aligned with the pseudo-first-order model, suggesting weaker physical interactions dominated by van der Waals forces.

Thermodynamic analysis added further depth. Gibbs free energy values were negative across the tested range of 25 to 60 degrees Celsius, becoming more negative as temperature rose, from −60.47 to −68.93 kilojoules per mole for lead and −32.66 to −37.05 kilojoules per mole for chromium. Positive enthalpy and entropy changes confirmed the process is spontaneous, endothermic, and accompanied by increasing disorder at the biochar-water interface. Spectroscopy provided direct mechanistic evidence: after adsorption, the hydroxyl band shifted from 3435 to 3305 wavenumbers, carbonyl and aromatic bands displaced slightly, and new low-frequency bands at 593 and 529 wavenumbers appeared, signatures of metal-oxygen vibrations proving the formation of surface-bound Pb-O and Cr-O species.

The proposed mechanism is a coordinated ensemble rather than a single pathway. Lead is captured primarily through ion exchange and surface complexation with oxygenated functional groups, reinforced by electrostatic attraction when the surface is deprotonated. Hexavalent chromium follows a different route: electrostatic adsorption onto protonated sites under acidic conditions, followed by reduction to the far less toxic trivalent form, which then forms stable surface complexes. Physical adsorption via van der Waals forces contributes to both. This multiplicity of mechanisms, the authors argue, explains why an unmodified, low-temperature biochar can rival materials that require costly chemical treatment.

Reusability, the make-or-break criterion for real-world deployment, delivered encouraging results. Nitric acid desorbed 87.9 percent of lead and 89.4 percent of chromium in the first regeneration cycle, as competitive hydrogen ions displaced metal ions from the biochar’s surface. Over three full adsorption-desorption cycles, the material removed 88.5 percent of lead and 91 percent of chromium initially, still retaining 63 and 70.1 percent removal respectively by the third round. For a non-activated, agriculturally derived adsorbent, that durability is notable and suggests the material could survive the economics of repeated use in treatment systems.

The study does not claim record-breaking capacity; chemically modified biochars with engineered microporosity and extra functional groups can adsorb more per gram. But those materials demand additional reagents, higher energy input, and greater production cost, barriers that matter most in the low-resource settings where heavy metal contamination is often worst. With global bean production approaching 27 million tonnes annually, the husk feedstock is abundant, essentially free, and currently a disposal liability. By closing the loop between agricultural waste and clean water, the work offers a template for affordable, sustainable remediation, and a reminder that sometimes the solution to industrial pollution is sitting in the compost heap.

Subject of Research: Use of bean husk-derived biochar as a low-cost adsorbent for removing lead and hexavalent chromium from water

Article Title: Biochar derived from bean husk for the removal of lead and hexavalent chromium from aqueous solutions

Article References: Okoro, H. K., Emenike, E. C., Iwuozor, K. O., Emeghai, J., Zvinowada, C., Ngila, C. J., & Adeniyi, A. G. (2026). Biochar derived from bean husk for the removal of lead and hexavalent chromium from aqueous solutions. Discover Chemistry, 3(1), Article 500. https://doi.org/10.1007/s44371-026-00930-z

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00930-z

Keywords: biochar, bean husk, heavy metals, lead removal, hexavalent chromium, water treatment, adsorption, agricultural waste, isotherm modeling, adsorption kinetics, regeneration, wastewater remediation