A comparative study on defluoridation capabilities of biosorbents: isotherm, kinetics, thermodynamics, cost estimation, and eco-toxicological study.

The present study aims towards fluoride remediation from synthetic water using steam-activated carbon of Aegle marmelos (bael shell/wood apple) (BAC) and Parthenium hysterophorus (PHAC) according to batch sorption techniques. The impact of different parametric conditions viz. initial fluoride concentration (4-12 mg/L), time (0-5 h), temperature (293.15-333.15 K), adsorbent dosage (4-14 g/L), pH (4-9), and RPM (150-350) were considered for both the adsorbents. Maximum defluoridation of 89% was achieved by BAC at a concentration of 10 mg/L, adsorbent dose 6 g/L, pH 5, temperature 313.15 K, agitation speed 250 rpm, and contact time 9 h, whereas PHAC attained maximum removal of 78% at an initial concentration of 8 mg/L, adsorbent dose 10 g/L, pH 4, temperature 313.15 K, and contact time 12 h. Instrumental analysis by SEM, EDX, and FTIR confirmed about the fluoride binding ability of the adsorbents. The Langmuir isotherm model provided the best fit (R2  = 0.9962 and 0.9945) to the removal process with maximum adsorptive uptake of 16.85 and 6.22 mg/g by BAC and PHAC respectively. The adsorption phenomenon was found to obey pseudo-second-order kinetics. The endothermic, spontaneous, and feasible nature of the sorption process was confirmed by the thermodynamic study. The total costs of 1 kg adsorbent preparation were calculated as 1.122 USD and 1.0615 USD which helped us in determining the economic feasibility of the adsorbents in large-scale applications. The growth of Chlorella sorokiniana BTA 9031 was also observed to be affected by the fluoride solution. Comparing the removal efficiencies of both the adsorbents, it can be concluded that BAC shell proved to be an efficient adsorbent over PHAC for fluoride elimination from aqueous solution. Graphical abstract Defluoridation of aqueous solution using biochar derived from Aegle marmelos shell and Parthenium hysterophorus.