Styrene removal by surfactant-modified smectite group minerals: Efficiency and factors affecting adsorption/desorption

Abstract

In this study the styrene adsorption/desorption efficiency by surfactant-modified smectites was evaluated. The materials showed a remarkable styrene uptake at relative humidity (RH) in the range of 40–50% ± 5%. In most cases the uptake was much higher than observed for commonly used activated carbons. The research indicated the importance of specific surface area (SBET) and cation exchange capacity (CEC) of the host minerals. A high SBET was crucial for the montmorillonite material (STx) which also had a high CEC (~68.9 meq/100 g). However, effective adsorbents were also based on beidellite (BId) which had a lower CEC (~37.8 meq/100 g). In this case a lower charge resulted in lower density of intercalated surfactants. This provided more space for styrene and limited the steric effects. The surfactant to CEC ratio close to 1.0 was optimal to achieve the highest adsorption. This was connected to sufficient hydrophobization of mineral surfaces and expansion of the interlayer space. The highest styrene uptake was observed for smectites intercalated with surfactants having a long alkyl chain. A bulky benzyl head group lowered the adsorption efficiency due to steric hindrance. Raman spectroscopy confirmed styrene polymerization after subsequent adsorption/desorption cycles. This led to a significant swelling of the organosmectites attested by XRD and resulted in an increasing styrene uptake in subsequent cycles. However it also inhibited the styrene desorption and the adsorbent regeneration. The research also indicated the negative impact of high humidity (80% ± 5%) which significantly lowered the adsorption by organosmectites and activated carbon. Thus, the application of adsorbents for styrene removal must be accompanied with strict humidity control.