Comparison of Cr(VI) Adsorption Using Synthetic Schwertmannite Obtained by Fe³⁺ Hydrolysis and Fe²⁺ Oxidation: Kinetics, Isotherms and Adsorption Mechanism

• Main Authors: Justyna Ulatowska, Łukasz Stala, Izabela Polowczyk
• Format: Article
• Language: English
• Published: MDPI AG 2021-07-01
• Series: International Journal of Molecular Sciences
• Subjects: schwertmannite; Cr(VI); adsorption kinetics; isotherm models; pollution; iron-based sorbents
• Online Access: https://www.mdpi.com/1422-0067/22/15/8175

Good sorption properties and simple synthesis route make schwertmannite an increasingly popular adsorbent. In this work, the adsorption properties of synthetic schwertmannite towards Cr(VI) were investigated. This study aimed to compare the properties and sorption performance of adsorbents obtained by two methods: Fe³⁺ hydrolysis (SCH_A) and Fe²⁺ oxidation (SCH_B). To characterise the sorbents before and after Cr(VI) adsorption, specific surface area, particle size distribution, density, and zeta potential were determined. Additionally, optical micrographs, SEM, and FTIR analyses were performed. Adsorption experiments were performed in varying process conditions: pH, adsorbent dosage, contact time, and initial concentration. Adsorption isotherms were fitted by Freundlich, Langmuir, and Temkin models. Pseudo-first-order, pseudo-second-order, intraparticle diffusion, and liquid film diffusion models were used to fit the kinetics data. Linear regression was used to estimate the parameters of isotherm and kinetic models. The maximum adsorption capacity resulting from the fitted Langmuir isotherm is 42.97 and 17.54 mg·g⁻¹ for SCH_A and SCH_B. Results show that the adsorption kinetics follows the pseudo-second-order kinetic model. Both iron-based adsorbents are suitable for removing Cr(VI) ions from aqueous solutions. Characterisation of the adsorbents after adsorption suggests that Cr(VI) adsorption can be mainly attributed to ion exchange with SO₄²⁻ groups.