highly selective separation of zn and cd utilizing the superparamagnetic nanoadsorbents functionalized with d2ehpa and tbp dual extractants

The organic extractant-functionalized superparamagnetic nanoparticles are a novel generation of adsorbent materials used for metal ions separation/removal in hydrometallurgy or wastewater treatment. they merge the advantages of solvent extraction and conventional adsorption processes while eliminating the relevant shortcomings, such as lengthy phase disengagements or limited loading capacities. thanks to the superparamagnetic nature of the employed nanoadsorbents that arise for sizes typically smaller than 30 nm, they can repeatedly disperse through the aqueous solution and subsequently undergo an immediate magnetic separation. due to similar physicochemical properties of zn and cd, their aqueous separation has always been considered a critical challenge from both hydrometallurgical and environmental viewpoints. this research investigates the selective separation of zn and cd in a sulfate solution through adsorption utilizing magnetite (fe3o4)-based superparamagnetic nanoadsorbents bi-functionalized with di-(2-ethylhexyl) phosphoric acid (d2ehpa) and tributyl phosphate (tbp) dual extractants. for this purpose and aiming to explore the synergistic effect of tbp on improving the separation selectivity, different nanoadsorbents containing 300 mg.g-1 d2ehpa with varying tbp contents (0, 10, 20, and 30 wt.%) are utilized. the batchwise adsorption experiments are conducted by contacting 500 mg of various nanoadsorbents with 100 ml of feed solution containing 100 mg.l-1 of each zn and cd. except for the kinetics study, all colloidal systems are mechanically stirred for 10 minutes after ph adjustment by adding sulfuric acid or ammonium hydroxide solution. at last, the loaded nanoadsorbents are collected using a magnet tool and the raffinate is sent for the remaining metals determination. according to the results, increasing the nanoadsorbents’ tbp content causes the extraction curve of cd to shift toward higher ph values. this finding implies that both d2ehpa and tbp take part in forming the organometallic complex through direct interactions with cd. on the other hand, the tbp available on the surface of nanoadsorbents does not remarkably affect the extraction behavior of zn. the reason is that tbp can only extract zn in the form of anionic complexes that are dominant at alkaline conditions. nevertheless, the corresponding extraction curve experiences a slight shift toward higher ph values for the ‘d2ehpa + 30 wt.% tbp’ nanoadsorbent. as d2ehpa is the leading extracting agent for zn, this curve shift can be attributed to the interaction between tbp and d2ehpa molecules rather than participating tbp in the extraction process. indeed, tbp and d2ehpa interaction through hydrogen bonding decreases the free hydroxyl (–oh) group of d2ehpa required for zn extraction through the cationic exchange mechanism, which subsequently causes drops in the extraction percentage at a constant ph value. thus, tbp acts more as a modifier, facilitating the selective separation of zn and cd by making the corresponding extraction curves far apart. the nanoadsorbent containing 60 mg.g-1 tbp (‘d2ehpa + 20 wt.% tbp’) represents the best composition for selective separation of zn over cd, which results in the largest δph0.5 = ph0.5cd – ph0.5zn value of 1.6. accordingly, the maximum separation factor of 683 is obtained at the optimal ph = 1.9. in these circumstances, 18 mg.g-1 (≡ 90%) of zn is adsorbed from the solution, whereas the co-adsorbed cd value is negligible (< 0.3 mg.g-1 or 1.3%). kinetics study reveals the fast rate of zn adsorption, with only about 5 minutes necessary for reaching equilibrium. moreover, the pseudo-second-order kinetic model best describes the intended adsorption mechanism, indicating that the chemical reaction might be the rate-controlling step.