Adsorption of charged synthetic organic dyes by pine nut husk biochars modified with hydroxyapatite: Adsorption mechanisms and reusability

This study examined the effect of surface modification with hydroxyapatites (HAPs) on the adsorption of positively or negatively charged synthetic dyes [i.e., methylene blue (MB) or Congo red (CR)] by pine nut husk biochars. Compared with that of pristine pine nut husk biochars (PNHB), the O/C ratio of pine nut husk biochars modified with HAPs (PNHB-H) was much larger (O/C ratio: PNHB = 0.08; PNHB-H = 0.11). This result likely indicates that HAP modification enhanced the hydrophilic functional groups on the surface of PNHB. The Q_max value of PNHB-H was slightly less than that of PNHB for MB (Q_max: PNHB = 6.09 mg/g, PNHB-H = 4.10 mg/g), and approximately 50-fold higher than that of PNHB for CR (Q_max of CR: PNHB = 1.11 mg/g, PNHB-H = 53.42 mg/g). Such findings indicate that chemisorption plays an important role in the adsorption of MB and CR by PNHB and PNHB-H. PNHB effectively removed MB, a cationic dye, due to its negatively charged surface. In contrast, PNHB-H was able to remove both MB and CR through the phosphate and Ca components of HAPs. Thus, these results suggest that PNHB-H produced by HAP modification is effectively applicable for removing charged synthetic dyes in actual water treatment.