Toward Rational Design of Biomass-Derived Hydrochars for Water Purification: A Research Perspective

The conversion of agricultural residues into functional carbon-based adsorbents represents a promising approach for sustainable water purification. However, adsorption performance is governed not only by surface area, but also by the ability to tailor surface chemistry, textural properties, and active sites for specific pollutant interactions. Rather than comparing individual hydrochar systems, this research perspective synthesizes findings from several previously published studies to establish a unified framework linking modification strategies with adsorption mechanisms and pollutant-specific removal performance.

Hydrochars and pyro-hydrochars derived from lignocellulosic biomass, including grape pomace, Miscanthus × giganteus, corn cob, and Paulownia leaves, were engineered through surface activation, metal incorporation, and thermochemical transformation strategies. The resulting materials were investigated for the removal of chemically diverse pollutants, including dyes, organophosphorus pesticides, and heavy metal ions, enabling the identification of relationships between material properties and adsorption pathways.

Across the investigated systems, adsorption performance was governed by the interplay between surface chemistry, textural properties, and active adsorption sites rather than by surface area alone. Alkaline activation enhanced oxygen-containing functional groups, promoting hydrogen bonding, π-π interactions, and surface complexation during organic pollutant removal. Pyrolysis altered textural properties and surface characteristics, influencing the affinity of hydrochars toward hydrophobic pesticides. Furthermore, metal incorporation introduced additional active sites, enabling efficient Pb²⁺ removal through ion exchange and surface complexation mechanisms. Maximum adsorption capacities in different hydrochar systems reached 751.0 mg/g for Rhodamine B, 30.10 mg/g for chlorpyrifos, and 157.24 mg/g for Pb²⁺, while thermochemical activation generated biomass-derived carbon materials with specific surface areas up to 1290 m²/g.

Collectively, these findings demonstrate that next-generation biomass-derived adsorbents should be designed according to pollutant chemistry through rational control of surface functionality, textural properties, and active adsorption sites. This perspective provides a framework for the development of advanced hydrochar-based materials for sustainable water purification.