Bio-based surfactants modified phosphogypsum for cationic dye wastewater treatment

In this study, phosphogypsum (PG) was modified by two cardanol-based surfactants, namely single-chain sodium cardanol sulfonate (CNS) and its Gemini-type analogue (GCNS), to produce a series of adsorbent materials for the removal of cationic dye wastewater, which are abbreviated as CPG and GPG, respectively. Compared with bare PG or the PG modified with sodium dodecylbenzene sulfonate (SDBS), CPG and GPG exhibited significantly higher removal efficiency for cationic dyes (Rhodamine B (RhB) and Astrazon Pink FG (FG)). The dye removal efficiency of CPG and GPG could be obtained over 99%, while the PG modified by SDBS only had a dye removal efficiency of 10%. The modification mechanism of negatively-charged PG particles with anionic cardanol surfactants was investigated using Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), contact angle, and zeta potential measurements. The XPS analysis revealed a decrease in surface O and Ca contents and an increase in C content of modified PG, with Ca 2p binding energy decreasing by 1.25 eV (CPG) and 1.15 eV (GPG), indicating the formation of chemical interactions between the surfactants and PG. Adsorption of RhB and FG fitted pseudo-second-order kinetics and Langmuir isotherm, proceeding as a spontaneous endothermic process, and the modified materials maintained stability over 5 cycles. The differences between single-head single-tail and Gemini type surfactants in the modification of PG particles were investigated, which elucidated the effects of molecular structure of the surfacatnts on the disparities in PG particle modification and dye adsorption. This work establishes a bio-based surfactant-modified PG system for high-efficiency cationic dye removal, providing insights for surfactant design, wastewater treatment and PG resource utilization.