Haematococcus pluvialis residue as a promising dye adsorbent: enhanced by choline chloride:p-toluenesulfonic acid and cellulase treatment

The cell wall of Haematococcus pluvialis ( H. pluvialis ) is rich in polysaccharides. After astaxanthin extraction, cellulose and hemicellulose remain abundantly present in the residue, which could form chemical interactions of hydroxyl (-OH), carbonyl (C=O) with dyes. In this study, H. pluvialis residue (HPR) was treated using acid deep eutectic solvent (DES) combined with cellulase at low temperature. The treatment parameters and adsorbent conditions were investigated to improve the removal rate of Congo red (CR) from water. The HPR-based biosorbent was characterized via SEM, FTIR, and elemental and BET analysis. After the treatment with choline chloride:p-toluenesulfonic acid (ChCl:pTSA) at 60 °C for 3   h and cellulase at 60 °C for 16   h, the specific surface area of the HPR was increased from 1.44 m 2 /g to 7.45 m 2 /g, while the pore volume increased from 0.0031 cm 3 /g to 0.0599 cm 3 /g, and the maximum removal rate of CR reached 95% with a dosage of 0.67   g/L. The adsorption kinetic was better described by pseudo-second-order model. The Langmuir model provided the best fit and the maximum sorption capacity of 443.42   mg/g was recorded. The findings demonstrate that HPR treated with acid DES and cellulase can serve as an eco-friendly bioadsorbent for anionic dye removal.