The effect of salinity on the sorption of cadmium ions from aqueous medium on Fe(III)-sepiolite

Abstract

In this study, the sorption of cadmium ions onto sepiolite modified with hydrated iron(III) oxide, Fe(III)-sepiolite, has been investigated in natural seawater, artificial seawater, aqueous solution of NaCl of the same ionic strength as the seawater and distilled water. The sorption experiments were performed as a function of the initial solution pH value, initial metal concentration, and equilibration time, using the batch method. The equilibrium sorption data were analyzed by the Langmuir, Freundlich and Sips isotherm models and the kinetics of sorption was analyzed using the pseudo-first-order and the pseudosecond-order kinetic models. The maximum sorption capacity and the strength of the sorbate-sorbent bonds at initial pH 7 were found to decrease in the following order: distilled water > NaCl solution > artificial seawater > natural seawater. The values of parameter nS in the Sips model, which fitted the equilibrium sorption results best, showed that heterogeneity of the sorbent surface was the highest in distilled water and the lowest in natural seawater. The sorption kinetic data fitted well with the pseudo-second-order kinetic model, which suggests that the rate-limiting step in Cd2+ sorption onto Fe(III)-sepiolite could be chemisorption. The low desorption percentage in both distilled water and 0.001 M HNO3 indicated that sorption occurred mainly by chemisorption mechanisms.

U ovom radu je proučavana sorpcija jona kadmijuma iz prirodne morske vode, laboratorijski pripremljene morske vode, vodenog rastvora NaCl iste jonske jačine kao morska voda i destilovane vode, na sepiolitu modifikovanom hidratisanim gvožđe(III)-oksidom. Eksperimenti su izvedeni u šaržnim uslovima, pri različitim početnim pH vrednostima, početnim koncentracijama jona kadmijuma i vremenima uravnotežavanja. Rezultati sorpcije u ravnotežnim uslovima su analizirani primenom modela Langmira, Frojndliha i Sipsa, a kinetika sorpcije je analizirana kinetičkim modelima pseudo-prvog i pseudo-drugog reda. Utvrđeno je da maksimalni adsorpcioni kapacitet i jačina veza sorbat-sorbent pri početnoj pH 7 opadaju u nizu: destilovana voda > NaCl rastvor > laboratorijski pripremljena morska voda > prirodna morska voda. Vrednosti parametra ns u modelu Sipsa, koji najbolje opisuje eksperimentalne rezultate, pokazuju da je heterogenost površine sorbenta najveća u destilovanoj vodi i najmanja u prirodnoj morskoj vodi. Rezultati ispitivanja kinetike sorpcije se bolje fituju modelom pseudo-drugog reda, što sugeriše da stupanj koji određuje brzinu sorpcije može biti hemisorpcija. Mali stepen desorpcije i u destilovanoj vodi i u 0.001 M HNO3 je potvrdio da se joni kadmijuma vezuju za modifikovani sepiolit uglavnom hemisorpcijom.