Synthesis of adsorption materials based on volcanic glass and kaolin
Authors: Kholodko Yurii , Bondarieva Antonina , Tobilko Viktoriia 
Published: September 2022

Obtaining adsorbent materials based on cheap and available raw materials for water purification from toxic cadmium ions is an important environmental task. It is economically feasible and promising to use natural clays and silicates modified with iron-containing compounds, for example, zero-valent iron.

Adsorbents based on kaolin, volcanic glass (perlite), and nanoscale zero-valent iron (Fe0) were studied using the methods of scanning electron microscopy, IR-spectroscopy, X-ray phase analysis, and the method of low-temperature nitrogen adsorption-desorption. Adsorption experiments were performed to evaluate the efficiency of Cd2+ removal from water. The equilibrium metal concentration was determined by inductively coupled plasma atomic emission spectroscopy. The successful completion of modifying the surface of kaolin and perlite with nanoscale zero-valent iron is confirmed by the obtained IR-spectra of the samples, which show characteristic bands at 447 and 682 cm-1, which correspond to Fe-O valence vibrations. On morphology photos of the pure perlite show many artificially formed open and closed pores of different diameters. A typical layered structure is observed for kaolin. After modification of silicates with zero-valent Fe0, clusters of nanosized iron particles appear in the images. The diffractograms of the modified adsorbents show the formation of a crystalline phase of zero-valent iron (α-Fe0), its oxides (FeO), and oxyhydroxides (FeOOH). After applying a layer of iron-containing compounds, the specific surface of the obtained samples increases by 20 - 25%. For modified kaolin and perlite, it reaches values ​​of 12 cm2/g and 10 cm2/g, respectively. At the same time, the total volume of pores increases by 1.5-3 times, but their radius decreases. Thus, the pore radius of the modified kaolin is 2.36 nm, and that of the pure one is 4.3 nm. Coincident, for the modified samples of perlite, there is an insignificant decrease in the size of the pores compared to the unmodified sample - 2.05 nm and 2.36 nm, respectively. This is because, in the modification process, a porous reaction layer of nanosized iron is formed on the surface of silicates, which by its properties differs from the inorganic matrix. The main physicochemical features of removing cadmium ions from water were determined to determine the effectiveness of the obtained adsorbents. During research of the optimal conditions were studied the influence of the concentration of adsorbents, the pH of the aqueous medium, the kinetics of the cadmium removal process and the construct of adsorption isotherms. Under the given research conditions, the optimal dose of adsorbents for the maximum removal of cadmium ions from water is 2 g/l.  The study of the dependence of the contact time of modified materials and model solutions on the efficiency of cadmium removal showed that its removal from water occurs relatively quickly. In 20 minutes of interaction, the samples adsorbed about 96% of Cd2+. Experiments to study the effect of pH on adsorption processes established that the degree of Cd2+ removal does not depend on the pH of the aqueous medium in the range of 3.2 - 7.5. The maximum value of cadmium adsorption is 7.8 mg/g for the perlite-based composite and 8.0 mg/g for the kaolin-based material, which is significantly higher than that for the natural silicates - 0.16 mg/g and 0.35 mg/ g respectively. Adsorption isotherms were calculated using the empirical Freundlich and Langmuir equations. The calculated parameters of the equations indicate that the Langmuir equation better describes the adsorption isotherms on the pure kaolin and perlite (correlation coefficients R2 are 0.978 and 0.946, respectively). In order, the Freundlich equation better describes the isotherms for the modified samples of kaolin and perlite (R2 is 0.990 and 0.980, respectively).

Applying a layer of nanosized zero-valent iron on the surface of natural silicates significantly increases their adsorption capacity to cadmium ions. The resulting composite materials are promising for deep purifying polluted waters with heavy metals in concentrations close to the maximum permissible.