Adsorption behaviour and mechanism of natural citric acid modified MOF-808 for Cr(III) and Cr(III)-EDTA in water and site energy analysis

Industrial wastewater often contains potentially toxic metals and it's chelates, posing serious threats to human health and aquatic ecosystems, and adsorption is frequently used for the minimization of potentially toxic metals from water. In this study, citric acid modified MOF-808 (MOF-808-CA) was prepared by using citric acid to modify MOF-808 for the removal of Cr(III) and Cr(III)-EDTA from wastewater. MOF-808-CA with the BET surface area of 653.59 m² g^-1 and the pore volumes of 0.467 cm³ g^-1 was successfully synthesized. The adsorption of Cr(III) and Cr(III)-EDTA by MOF-808-CA was 40.46 and 17.03 mg g^-1 at pH 4.0 and 25°C, respectively. The adsorption isotherms and adsorption kinetics of Cr(III) and Cr(III)-EDTA were summarized using Langmuir-Freundlich isothermal adsorption model and the pseudo-second-order model. Even in high salinity wastewater (35,000 mg L^-1), MOF-808-CA displayed a strong affinity for Cr(III) and Cr(III)-EDTA. The site energy (E*) values reduced with the increasing of adsorption capacities, and Cr(III) and Cr(III)-EDTA firstly dominated the high-energy adsorption sites before low-energy adsorption sites. The average site energies for the adsorption of Cr(III) and Cr(III)-EDTA by MOF-808-CA were 26.7 and 24 kJ mol^-1, respectively, and the differences in the average site energies further illustrated the essential differences in their adsorption mechanisms. The adsorption by electrostatic adsorption and surface complexation were the main adsorption mechanisms for Cr(III) on MOF-808-CA, whereas hydrogen bonding and complexation were the main adsorption mechanisms for Cr(III)-EDTA on MOF-808-CA. The results showed that the MOF-808-CA adsorbent has a great potential for the removal of both Cr(III) and Cr(III)-EDTA from aqueous solutions.