Bamboo Biochar Boosted by KOH Activation Strips Cadmium from Water

Cadmium is one of the most insidious contaminants in the modern water cycle. It leaches from metal plating shops, pigment factories, phosphate fertilizer fields, mining operations, and sewage sludge, and once it enters a river or aquifer it simply stays there. Unlike organic pollutants, cadmium does not degrade, and even trace exposures over time can damage kidneys and bones. The World Health Organization sets the acceptable concentration of cadmium in drinking water and industrial wastewater at just 0.003 mg/L, a threshold that conventional treatment trains often struggle to meet cheaply. Against that backdrop, a research team from Mizoram University and Government Zirtiri Residential Science College in Aizawl, India, has reported a strikingly simple answer hiding in one of the region’s most abundant and underutilized plants: the bamboo species Melocanna baccifera.

The study, published in Frontiers in Carbon, evaluated biochar made from M. baccifera both in its pristine form and after chemical activation, asking a deceptively simple question: how much does activation actually improve cadmium uptake, and what mechanisms drive the binding? The answer, quantified through isotherm and kinetic modeling, is that a potassium hydroxide treatment lifts the maximum adsorption capacity from 15.15 mg/g to 24.39 mg/g, a gain of roughly 60 percent, while the underlying surface area grows by only about 34 percent. That mismatch between porosity and performance is one of the most technically interesting findings of the work, because it points to surface chemistry, not just surface area, as the decisive factor.

To make the adsorbent, the researchers collected dried culms of M. baccifera, washed them, cut them into pieces, and carbonized them in a brick kiln under oxygen-limited conditions. The temperature was ramped gradually to 500 degrees Celsius and held there for four hours before the chamber was allowed to cool while still sealed, preventing oxidation of the freshly formed char. The resulting pristine biochar, designated MPB, was ground and sieved to a uniform 100 mesh, corresponding to particles of roughly 149 micrometers. This is deliberately low-tech pyrolysis, the kind of process that could plausibly be replicated in communities where the bamboo grows, which matters for any technology pitched as a cost-effective water treatment option.

Activation was where the chemistry got aggressive. The team treated the biochar with potassium hydroxide and phosphoric acid at concentrations of 20, 40, 60, and 80 percent, soaking 40 grams of biochar per liter of reagent for 24 hours on an orbital shaker at 150 rpm. The treated material was then heated to 800 degrees Celsius for 30 minutes in a muffle furnace, washed repeatedly with deionized water until the pH was neutral, and dried. When the variants were screened against a 10 mg/L cadmium solution, the 60 percent KOH treatment stood out decisively, achieving 97.89 percent removal. Phosphoric acid activation reached 73.93 percent, while the untreated biochar managed only 51.39 percent. KOH activation was selected for all subsequent experiments.

The reason KOH performs so well is partly textural and partly chemical, and the study disentangles both. During high-temperature carbonization, KOH reacts with carbon in a reaction that produces metallic potassium, hydrogen gas, and potassium carbonate. These products etch the carbon matrix, carving out new pores and exposing edge sites that oxidize on contact with air to form hydroxyl, carbonyl, and carboxyl groups. Nitrogen adsorption-desorption analysis confirmed the textural transformation: the BET surface area rose from 322 to 432 square meters per gram, average pore volume climbed from 0.154 to 0.212 cubic centimeters per gram, and micropore volume grew from 0.084 to 0.108 cubic centimeters per gram. Scanning electron micrographs showed correspondingly more developed pore structures and larger pore openings in the activated material.

But the numbers reveal that texture alone cannot explain the performance jump. If surface area were the whole story, a 34 percent increase in BET area should yield roughly a 34 percent capacity gain. Instead, capacity rose about 60 percent. The missing ingredient is functionalization. Fourier-transform infrared spectroscopy of the activated biochar showed intensified or shifted bands near 3,395 cm⁻¹ for O-H stretching, between 1,684 and 1,589 cm⁻¹ for C=O and C=C vibrations, and between 1,016 and 1,114 cm⁻¹ for C-O stretching. These oxygen-containing groups act as ligands for cadmium ions through metal-oxygen coordination and ion exchange, effectively adding chemical binding sites on top of the physical ones.

The FTIR evidence becomes even more compelling when spectra are compared before and after cadmium loading. The broad O-H/N-H envelope near 3,395 cm⁻¹ decreased in intensity and shifted slightly after cadmium adsorption, consistent with deprotonation and coordination of surface hydroxyl and phenolic groups. The carboxyl C=O band shifted from 1,685 to 1,687 cm⁻¹ in cadmium-bound pristine biochar and to 1,684 cm⁻¹ in the activated material, while the C=O stretching band between 1,572 and 1,589 cm⁻¹ moved to 1,583 and 1,589 cm⁻¹ respectively. The C-O region also shifted. Taken together, these spectral changes point to inner-sphere complexation of cadmium with carboxylate, carbonyl, and phenolic oxygen donors, with concurrent ligand and ion exchange at those sites, as the dominant binding mechanism.

Batch experiments mapped how the process behaves under different conditions. Solution pH proved to be the most influential variable: adsorption capacity increased with rising pH and peaked at pH 5. At lower pH, the abundance of hydrogen ions competes directly with cadmium ions for the same surface sites, suppressing uptake. As pH rises, that competition fades and cadmium can access the hydroxyl, carboxyl, and carbonyl groups identified by FTIR. The researchers deliberately stopped at pH 5 to avoid pushing the chemistry into a regime where cadmium hydroxide would precipitate, which would confound adsorption measurements with simple precipitation. Contact time experiments showed rapid initial uptake, with a substantial fraction of the total adsorption occurring within the first 30 minutes as vacant sites were filled, followed by a slowing rate that approached equilibrium after about 360 minutes. Increasing the initial cadmium concentration from 5 to 30 mg/L progressively saturated the finite adsorption sites, flattening the removal percentage at higher concentrations.

The equilibrium data were then fitted to a battery of isotherm models, and the results tell a coherent mechanistic story. The Langmuir model, which assumes a finite number of energetically uniform sites forming a monolayer, gave the best description, with correlation coefficients of 0.983 for pristine and 0.990 for activated biochar and maximum capacities of 15.151 and 24.39 mg/g respectively. The Sips model, a hybrid of Langmuir and Freundlich, yielded heterogeneity indices of 0.863 and 0.891, values close enough to unity to confirm near-Langmuir behavior. The Fowler-Guggenheim model, which probes lateral interactions between adsorbed molecules, returned negative interaction energies of −15.554 and −36.884 kJ/mol, indicating weak repulsive interactions among adsorbed ions at high surface coverage, though not strong enough to deviate significantly from Langmuir behavior. The Harkins-Jura multilayer model fit poorly, with correlation coefficients of 0.887 and 0.543, ruling out multilayer adsorption. For context, the 24.39 mg/g capacity of the activated bamboo biochar compares favorably with KOH-activated Enteromorpha prolifera biochar at 5.84 mg/g and pyrolytic carbon at 21.7 mg/g, and sits close to activated rice straw biochar at 24.7 mg/g.

Kinetic analysis completed the picture. The pseudo-first-order model produced moderate correlation coefficients but predicted equilibrium capacities far below the measured values, a well-known artifact of fitting only the initial adsorption phase. The pseudo-second-order model, by contrast, matched the experimental capacities almost exactly, with correlation coefficients above 0.999 for both materials and rate constants of 19.37 × 10⁻³ g/mg/min for pristine and 25.64 × 10⁻³ g/mg/min for activated biochar. This indicates that the rate-limiting step involves interactions between two species, typically the adsorbate and an adsorption site, rather than simple first-order decay of available sites. Intraparticle diffusion plots revealed two distinct stages, with a fast initial rate constant of 1.678 mg/g/min⁰·⁵ for the activated biochar dropping to 0.012 mg/g/min⁰·⁵ in the second stage, showing that diffusion inside particles contributes but is not the sole rate-controlling step. Liquid film diffusion, with rate constants of 0.015 and 0.02 cm/s, also plays a role, meaning the overall process is governed by a combination of film transport and intraparticle diffusion alongside the surface reaction.

What makes this study more than another adsorption data point is its grounding in a specific regional resource. Melocanna baccifera dominates large swaths of Northeast India, grows rapidly, and is often treated as a low-value lignocellulosic residue despite its high cellulose and hemicellulose content, which is precisely what makes it a good precursor for porous, oxygen-rich carbon after activation. The authors are candid about the limits of the work: the experiments were conducted in batch mode with synthetic cadmium solutions, and real wastewater carries competing ions, organic matter, and variable pH that could all erode performance. They call for continuous-flow column studies and testing with actual contaminated water as the necessary next steps. Still, the core demonstration stands. A locally abundant bamboo, a kiln, and a potassium hydroxide soak produce an adsorbent that removes nearly 98 percent of cadmium from a 10 mg/L solution and binds it through well-characterized surface chemistry. In a world where water treatment budgets are thin and heavy metal contamination is persistent, that combination of simplicity, low cost, and mechanistic understanding is exactly what scalable remediation requires.

Subject of Research: Cadmium ion adsorption from water using pristine and KOH-activated Melocanna baccifera bamboo biochar, analyzed through isotherm and kinetic modeling

Article Title: Cd(II) adsorption from aqueous solution using pristine and activated Melocanna baccifera biochar: isotherm and kinetics studies

Article References: Cd(II) adsorption from aqueous solution using pristine and activated Melocanna baccifera biochar: isotherm and kinetics studies. (n.d.). https://doi.org/10.3389/frcrb.2026.1720673

Image Credits: AI Generated

DOI: 10.3389/frcrb.2026.1720673

Keywords: cadmium, biochar, Melocanna baccifera, KOH activation, water treatment, adsorption isotherm, adsorption kinetics, Langmuir model, pseudo-second-order, FTIR spectroscopy, heavy metal removal, sustainable adsorbent