Effect of Acid Modification on Porous Structure and Adsorption Properties of Different Type Ukrainian Clays for Water Purification Technologies

Clay minerals, as adsorbents, are widely available, have large specific surface area, and the ability for cation exchange, which makes them applicable in wastewater treatment from heavy metal ions. This study investigated the effect of acid modification on the porous structure and adsorption properties of montmorillonite (MMT) and palygorskite (PAL) clays of the Cherkasy deposit (Ukraine). Acid modification of clay samples was carried out with chloride acid after preliminary refining. Clay particles were analyzed using optical polarization microscopy with recording of digital images of the study. The area and perimeter of individual particles of clay samples were determined. The equivalent diameter and shape factor were calculated. The porosity characteristics of the samples were determined. Pore surface area was calculated by the Brunauer-Emmette-Teller (BET) method. Total volume, mean diameters, and pore size distribution were obtained using the Density Functional Theory (BFT) method. Acid modification improved the porous structure of clay samples. BET surface area of the pores of the initial sample of palygorskite clay is significantly higher compared to the montmorillonite type clay (140.66 and 83.61 m2/g, respectively). Acid modification of the PAL sample contributes to an increase in the BET surface area by approximately 1.7 times. Acid modification increases the surface area and total pore volume by approximately 2.3 times for MMT samples and 1.76 times for PAL samples. Dimensional characteristics of the pores of the MMT sample were to shift after activation, while for the PAL sample, their average size increases sharply by approximately 3.2 times. This suggested the additional formation of pores in the interfibrillar regions of the PAL sample during its acid activation. The effect of acid modification on the sorption capacity was investigated by means of methylene blue exhaustion. Sorption efficiency increases by 6-14%, compared to unmodified samples and is determined by the contact time and the type of clay. Increasing the contact time from 24 to 72 hours leads to an increase in the sorption efficiency of activated MMT and PAL samples by 36 and 30%, respectively. Acid-activated clay minerals, in particular montmorillonite, can be used as part of sorption elements to increase the efficiency of wastewater treatment technologies from heavy metal ions.