[Paper Review] Decoupling Precipitation and Surface Complexation during Mn(II) Removal by Biochar via Experiments and Atomistic Simulations

Audrey Ngambia, Anastasiia Gavrilova+5 more|arXiv (Cornell University)|Mar 23, 2026

Adsorption and biosorption for pollutant removal0 citations

TL;DR

The study combines experiments and atomistic simulations to decouple Mn(II) removal by biochar into precipitation-driven and surface-complexation-driven pathways, revealing how feedstock, pyrolysis temperature, and surface deprotonation control mechanisms.

ABSTRACT

Manganese(II) mobilised by mining activity poses a persistent water-quality challenge, yet the mechanisms by which low-cost sorbents, such as biochar, sequester Mn(II) remain poorly resolved. This study identifies the specific chemical drivers of Mn(II) sequestration by combining fixed-bed column and batch experiments with atomistic molecular dynamics simulations. Oilseed rape straw biochars, produced at 350 extdegree C, 550 extdegree C, and 700 extdegree C, removed 20-50% of dissolved Mn from acidic influent (pH 4, 5 ppm). High-temperature biochar achieved the greatest removal ($\sim$50%) and rapidly increased effluent pH to 9, triggering alkaline precipitation. Conversely, lower-temperature biochars removed 20-30% of Mn while maintaining a near-neutral pH (7-7.5). Enhanced \ce{K+} release in these systems indicates significant cation exchange and non-precipitative pathways. Molecular simulations confirmed that while neutral surfaces show weak Mn(II) association, deprotonated sites drive strong adsorption through inner-sphere complexation ($\sim$50% removal) and outer-sphere association ($\sim$10%). These results establish a mechanistic framework to distinguish between precipitation-led and surface-complexation-led removal. By providing specific chemical criteria for Mn-targeted sequestration, this work enables the rational design of engineered biochars for sustainable water remediation.

Motivation & Objective

• Identify how biochar pyrolysis temperature and feedstock influence Mn(II) sequestration mechanisms.
• Decouple Mn removal into precipitation, cation exchange, and surface complexation pathways.
• Use atomistic simulations to resolve surface interactions without ash contributions.
• Link experimental observations with molecular-level adsorption motifs to guide biochar design.

Proposed method

• Combine fixed-bed column and batch experiments with oilseed rape straw biochars produced at 350°C, 550°C, and 700°C.
• Perform atomistic molecular dynamics simulations of four ash-free biochar models (W400, W800, S400, S800) and deprotonated variants to study Mn adsorption mechanisms.
• Define inner-sphere coordination with Mn–O/N ≤ 0.30 nm and outer-sphere association with Mn within 0.60 nm of surface heteroatoms.
• Quantify Mn uptake and partitioning into inner- and outer-sphere contributions across models.
• Analyse FTIR to infer changes in surface functionalities post Mn exposure.

Figure 1 : Manganese uptake over time from 5 ppm solution by OSR350 (purple), OSR550 (cyan) and OSR700 (yellow), and the changes in pH of the solution.

Experimental results

Research questions

• RQ1 What are the dominant mechanisms for Mn(II) removal by biochar under different pyrolysis temperatures and feedstocks?
• RQ2 To what extent do cation exchange, surface complexation, and precipitation contribute to Mn removal, and how can these be distinguished experimentally and computationally?
• RQ3 How does surface deprotonation influence Mn adsorption motifs on biochar?
• RQ4 Can atomistic simulations resolve ash-free, surface-specific Mn interactions that bulk experiments cannot?
• RQ5 What design rules emerge for engineering biochars with targeted Mn sequestration?

Key findings

• High-temperature biochar (OSR700) achieved ~50% Mn removal with rapid pH rise to ~9, suggesting precipitation pathways dominate under these conditions.
• Lower-temperature biochars (OSR350/OSR550) removed 20–30% Mn with near-neutral pH, indicating more substantial non-precipitative pathways including cation exchange and surface complexation.
• Cation exchange contributes to Mn uptake, especially for OSR350/OSR550, evidenced by release of K+ and other cations, though not stoichiometrically accounting for all Mn uptake in high-temperature chars.
• Molecular dynamics show Mn forms inner-sphere complexes with deprotonated surface groups (dominant in W400-DP ~47.7–57.3% of Mn uptake) and outer-sphere associations across all models; protonated low-temperature models show minimal Mn-surface association.
• Ash-free, deprotonated biochar surfaces enable strong Mn adsorption via inner-sphere coordination to phenolic/anisole oxygens, while high-porosity high-temperature models rely more on pore diffusion for outer-sphere Mn uptake.
• FTIR indicates Mn–O interactions with OSR350/OSR550 surfaces, while OSR700 shows limited spectral evolution, consistent with mineral-controlled Mn retention via precipitation.

Figure 2 : Representative Mn adsorption motifs on biochar, illustrated for S400-DP and S800. In the deprotonated systems, the dominant motif is an inner-sphere complex (ISC) formed by monodentate coordination to ${-}\text{O}{\vphantom{\text{X}}}^{\vphantom{\smash[t]{\text{2}}}\hphantom{\text{}}\text