Investigating the sustainable and efficient mechanism for adsorptive removal of riboflavin by CaCe-layered double hydroxide functionalized spent tea biochar

Abstract

Riboflavin (RF) is chiefly produced through fermentation technology. However, its separation and purification upon successful fermentation are quite complex and expensive due to presence of impurities in fermentation broth. Adsorption is considered as a cost-effective method for RF recovery during which an efficient and reusable adsorbent is crucial. Herein, the adsorptive removal of RF from aqueous phase by spent tea biochar supported CaCe-layered double hydroxide (STB/CaCe-LDH) composite was investigated. The STB/CaCe-LDH samples which were prepared at different Ca: Ce molar ratios via a co-precipitation-impregnation-pyrolysis route, were analyzed to gain insight into their physicochemical and structural properties using BET, FTIR, XRD, SEM and pH_pzc techniques. STB/CaCe-LDH sample synthesized at Ca: Ce molar ratio of 2:1 showed good uptake capacity for RF removal. The adsorption experimental conditions were optimized using Taguchi design approach, and the optimization results revealed that STB/CaCe-LDH2:1 dosage had the most significant influence on RF removal efficiency. The adsorption optimum conditions in relation to STB/CaCe-LDH2:1 dosage, RF concentration, adsorption duration and pH as operating parameters were obtained to be 50 mg, 20 mg L⁻¹, 90 min and 6.0, respectively. These optimum experimental conditions resulted in 98.41$\pm 0.39$% RF removal efficiency, signifying the potential of the STB/CaCe-LDH2:1 in adsorbing RF. The RF adsorption data best conformed to Langmuir isotherm model with monolayer uptake capacity of 695.2 mg g-¹, while the pseudo-second-order model adequately fitted the adsorption kinetics data than pseudo-first-order model. Moreover, the STB/CaCe-LDH2:1 showed relative stability after seven adsorption-desorption cycles. A remarkable reduction of 72.6 % in STB/CaCe-LDH2:1 consumption was predicted for two-stage batch adsorber as compared to a single-stage process. These findings demonstrate that STB/CaCe-LDH2:1 is a promising adsorbent for removing RF from aqueous solution.