Adsorption of dye from aqueous solution using Sr-doped lanthanum, calcium and barium manganites

Industrial effluents containing dyes are a serious threat to the ecosystems and the removal of these dyes is most often conducted by adsorption. In this study, Sr-doped lanthanum (LSMO), calcium (CSMO) and barium (BSMO) manganites were prepared by the autocombustion and co-precipitation synthesis, and tested as the adsorbents for the removal of methyl orange (MO) as a model pollutant from the aqueous solution. The highest adsorption of the MO solution obtained by CSMO adsorbents was 83.5%, while the highest adsorption of 95% was achieved using LSMO and BSMO due to their large specific surface area and average pore diameter promoting the diffusion of the MO to the adsorbent surface. The oxygen nonstoichiometry of these materials also contributed to their adsorption properties due to the presence of unpaired electrons, which affected their electronic structure and lowered the adsorption energy. Kinetic studies showed that Ho’s pseudo-second-order model exhibited the best fit with the experimental data, indicating that chemisorption is the rate-limiting step. The rate constants were the highest for BSMO adsorbents ranging from 3.35 to 3.50 × 10–2 g mg–1·min–1. The adsorbents can be fully regenerated by calcining at 700°C for 2 h with no loss of their activity.