Adsorption Of Cu²⁺ By Zn-Al-LDHs Intercalated With Chelating Agent EDDS

Heavy metals cations elimination from wastewater, specially mining effluents, is still an important issue in environmental science, copper is essential for plants and animals at low concentrations but it is potentially lethal for humans at high levels. The aim of this work is to study the uptake process of heavy metals cations by EDDS modified LDHs using the Cu2+ cation and a Zn-Al-LDH sample as model. First, the structure, composition and the interfacial properties of the sorbent LDH are analyzed. After that, the uptake process was followed as a function of time at different cation concentrations and solid amounts. The stability and affinity of the sorbent was also evaluated to establish the environmental application of the synthesized LDH. The sorbent is synthesized by co-precipitation. Then The polydentate ligand was introduced by an exchange method in a Zn-Al-LDH, which takes place with partial erosion of the layers, causing the intercalation of [Zn(EDDS)]2- complex instead of the ligand. [Cu(H2O)6]2+ cation was selected as a model cation to study the uptake mechanism, exploring the elimination kinetics from the first minutes up to the steady state.The chemical analysis of the three LDH samples is conducted. The precursor-LDH composition corresponds to the following formula: [Zno.65Al0.35(OH)2](N03)0.35·0.4H2O]. From the XRD patterns of the studied samples we can see that basal spacing values change from 0.88 nm (precursor-LDH) to 1.45 and 1.42 nm for the EDDS-LDH and Cu-LDH samples, respectively. Substracting LDH layer thickness 0.48 nm to the interlaminar distances, gallery heights of around 0.95 nm are estimated for both modified LDH. The FT-IR spectra of the three samples showed that the EDDS-LDH formula is [Zn0.50Al0.50(OH)2][Zn(EDDS)]0.17(A-)x·0.79H2O, and The maximum copper complexation capacity of EDDS-LDH is 1.06 mmol/g.Electrophoretic mobilities vs pH measurements for the precursor-LDH, EDDS-LDH, and Cu-LDH samples showed that the mobilities are positive between pH 6 and 11, at pH higher than 11 the negative surface sites overcompensate the structural charges resulting in negative electrophoretic mobilities.The scavenging kinetic curves were modeled using the Lagergren equation, the pseudo-second-order model best described the adsorption kinetics. The batch experimental data were analyzed by the Langmuir and Freundlich isotherm models, it shows that the isotherm data has been fitted to the Langmuir model, which provides the best results for these sorts of curves based on the correlation coefficients (>0.98).The [Cu(H2O)6]2+ elimination is produced by an exchange reaction with [Zn(EDDS)]2- anions placed either in the solid interlayer or in the aqueous solution, this last being released from the sorbent. Additional [Cu(H2O)6]2+ removal is produced by Cu(OH)2 precipitation at high copper concentrations due to the LDHs high pH buffering capacity. The sorbent removes [Cu(H2O)6]2+ with high affinity in a wide concentration range. The elimination process reaches equilibrium in less than 30 min and leaves metal cation concentrations lower than 0.05 ppm in the supernatants.Two experiments exploring the stability of the EDDS-LDH and the Cu-LDH samples, at pH higher than 5 the release is constant, representing around 20% of the uptake capacity. The release of [Zn(EDDS)]2- anions to the solution represents the main drawback of EDDS intercalated LDHs to be used as heavy metal scavengers. However, this problem may be solved by washing the sorbent with KNO3 and adding washing solution to a nitrate intercalated LDH.In comparison with other resins and clays used for copper elimination, EDDS-LDH presents a Cu2+ uptake capacity and an equilibration time of the same order. EDDS modified layered double hydroxides (LDHs) were investigated as potential sorbents to remediate heavy metals pollution. However, its copper affinity is higher, allowing lower equilibrium concentration in the supernatants.