Phycoremediation of lithium ions from aqueous solutions using free and immobilized freshwater green alga Oocystis solitaria: mathematical modeling for bioprocess optimization

Lithium is registered as a serious pollutant that causes environmental damage to an irrigation water supply. Freshwater green alga (Oocystis solitaria) was studied for its potential to remove lithium ions from aqueous solutions. The Plackett–Burman design was applied for initial screening of six factors for their significances for the removal of lithium from aqueous solutions using Oocystis solitaria cells. Among the variables screened, pH, lithium concentration, and temperature were the most significant factors affecting lithium removal. Hence, the levels of these significant variables were further investigated for their interaction effects on lithium removal using the Box–Behnken statistical design. The optimum conditions for maximum lithium removal from aqueous solutions by Oocystis solitaria were the initial lithium concentration of 200 mg/L, contact time of 60 min, temperature of 30 °C, pH 5, and biomass of Oocystis solitaria cells of 1 g/L with agitation condition. Under the optimized conditions, the percentage of maximum lithium removal was 99.95% which is larger than the percentage of lithium removal recorded before applying the Plackett–Burman design (40.07%) by 2.49 times. The different properties of Oocystis solitaria, as an adsorbent, were explored with SEM and via FTIR analysis. The spectrum of FTIR analysis for samples of Oocystis solitaria cells before lithium biosorption showed different absorption peaks at 3394 cm⁻¹, 2068 cm⁻¹, 1638 cm⁻¹, 1398 cm⁻¹, 1071 cm⁻¹, and 649 cm⁻¹ which has been shifted to 3446 cm⁻¹, 2924 cm⁻¹, 1638 cm⁻¹, 1384 cm⁻¹, 1032 cm⁻¹, and 613 cm⁻¹, respectively, after lithium biosorption by the alga. The treatment of aqueous solution containing lithium with Oocystis solitaria cells immobilized in alginate beads removed 98.71% of lithium at an initial concentration of 200 mg/L after 5 h. Therefore, Oocystis solitaria may be considered as an alternative for sorption and removal of lithium ions from wastewaters.