Influence of selected co-solutes on copper (ii) removal from aqueous solutions using a micaceous mineral of Kenyan origin

Abstract

In this study, a micaceous mineral (Micalvlx) (a local natural resource that occurs abundantly in Kenya), whose efficiency has been demonstrated for Cu2+ removal from water, was employed as an affordable adsorbent for Cu2+ ions in aqueous solutions containing Ethylenediaminetetracetic acid (EDTA), Nitrilotriacetic acid (NTA), Trisodium citrate (TSC) and Sodium Dodecyl Sulphate (SDS) as co-solutes. A UVNis spectrophotometer was used to monitor the adsorption of Cu2+ ions in the presence of the various co-solutes. The amount of Cu2+ ions re-mobilized by various co-solutes from the Cu2+-loaded mineral was quantified using AAS. Various factors affecting adsorption of Cu2+ ions from aqueous media such as contact time, adsorbate concentration, adsorbent dose and its particle size, pH, and ionic strength were studied in the presence of the various co-solutes at room temperature (25 ± 0.5°C). The presence of the three chelating agents of EDTA, NTA and TSC was found to suppress the adsorption of Cu2+ ions by the micaceous mineral while the presence of SDS afforded a negligible effect on the adsorption of Cu2+ ions by Micalvls; EDTA provided the highest inhibitory effect on the adsorption of Cu2+ ions by Micalvl., followed by NTA, and finally TSC. A 20 minutes contact time with constant shaking on an orbital shaker at a constant rate of 400 rpm yielded about 100% adsorption of 2.5 mM Cu2+ ions and was considered . as optimum contact time of adsorption, even in the presence of the co-solutes. The Cu2+: Chelating agent concentration ratio was found to be an important factor influencing the uptake of Cu2+ ions by the micaceous mineral. Maximum removal of 2.5 mM Cu2+ ions occurred with the highest Cu2+: chelating agent concentration ratio of 2500: 1. With the highest Cu2+:EDTA concentration ratio of2500:1, about 97% of2.5 mM Cu2+ ions were adsorbed, while with 2.5:1 concentratiop ratio, only 7.5% of Cu2+ ions were adsorbed. When the concentration of EDTA exceeded that of Cu2+ ions, adsorption of Cu2+ ions was completely inhibited . Adsorption of 2.5 mM Cu2+ ions occurred when the concentrations of NTA and TSC were 2.5 mM while the presence of SDS at concentrations 2.5 mM did not have any significant influence on 2.5 mM Cu2+ ions uptake. The adsorption trends of Cu2+ ions in the presence of the three chelating agents was found to correlate well with the stability constants of the Cu2+-co-solute complexes. The adsorption of Cu2+ ions in the presence of all the co-solutes separately increased with an increase in the ratio of mineral dosage to initial Cu2+ ion concentration. This was attributed to increase in the surface energy of the adsorbent which might have assisted in the decomplexation of the various Cu2+-complexes, thus availing free Cu2+ ions for adsorption. The pH adsorption edge for co-solute-free Cu2+ ions was found to be 4.0-7.0. The presence of 0.08 roM of EDTA, NTA and TSC lowered the % adsorption of 2.0 mM Cu2+ ions over the pH adsorption edge. The presence of higher concentrations of KCI decreased the uptake Cu2+ ions signifying the effect of ionic strength on adsorption. Pre-treatment of the micaceous mineral with the various co-solutes resulted in a decreased uptake of 2.5 rrtM Cu2+ ions. The amount of Cu2+ ions desorbed from a . previously Cu2+-loaded mineral using the co-solutes as extactants was highest with EDTA, with the re-mobilization efficiency taking the order: EDTA> NTA > TSC >SDS The adsorption of Cu2+ ions onto the micaceous mineral both in the presence and absence of co-solutes fitted well to both the Langmuir and Freundlich adsorption isotherm models. With the Langmuir isotherm model, a Qo of 14.493 mg/g was determined for co-solute- free Cu2+ ions. Presence of 0.1 mM EDTA, NTA and TSC decreased the level of adsorption of Cu2+ ion with the reduction in the adsorption appearing to be dependent on the stability constants of Cu2+-EDTA, Cu2+ -NTA and Cu2+_ citrate complexes. The obtained adsorption results have demonstrated that the micaceous mineral can be used in removing Cu2+ ions from aqueous solutions containing chelating agents at lower concentrations than that of Cu2+ ion without prior pre-treatment.