Phosphorylated cellulose for water purification: a promising material with outstanding adsorption capacity towards methylene blue

Enhancing the sorption properties of cellulose is a prerequisite for its efficient use in water purification as an alternative to costly activated carbon. Here, solvent-free phosphorylation of cellulose using environmentally benign and non-toxic chemicals was pursued resulting in a negatively charged material that was used to remove methylene blue (MB) from aqueous solution. Three different cellulose sources were selected, i.e., locally abundant Alfa grass, wood, and microcrystalline cellulose, with the aim to investigate the effect of the cellulose source on the functionalization degree and the removal efficiency of methylene blue. The poor MB adsorption capacity of native cellulose (12–40 mg g⁻¹) reached exceptionally high values after phosphorylation (446–705 mg g⁻¹) resulting in one of the most promising bio-based sorbents reported up-to-date. The highest phosphorylation degree was registered on cellulose from wood conferring it with the maximum adsorption properties. Curve-fitting experimental results revealed that the adsorption data were well described by the Langmuir equation and that the pseudo-second-order kinetic represents well the interactions between cellulose and MB molecules. Finally, we show the possibility to release MB from a used sorbent when it is successively washed with phosphate ions leading to a quasi-total (97%) regeneration.