Efficiency of hexavalent chromium adsorption by polymeric membranes with graphene oxide nanoparticles determined using thermal lens spectroscopy

Thermal lens spectroscopy (TLS) is a highly sensitive, non-destructive photothermal method for detecting and quantifying light-absorbing species. We implemented a dual-beam configuration (focused pump, collimated probe, both propagating coaxially) operated in the transient regime. Quantification of Cr (VI) was performed via the Cr (VI)-1,5-diphenylcarbazide complex measured by Thermal lens spectroscopy at 532 nm. The calibration (0-5 mg L^-1) curve showed excellent linearity (R² > 0.997) with a limit of detection of 293 μg L^-1. We then assessed the performance of polymeric membranes containing graphene-oxide nanoparticles under continuous-flow filtration (initial concentration C₀ = 4 mg L^-1; total volume 35 mL), acquiring time-resolved adsorption curves from microvolume aliquots (200 μL) with negligible perturbation of the experiment. Two membrane types were compared: (i) graphene oxide synthesized from obtaining graphite oxide following the modified Hummers' method and (ii) Commercial graphene oxide modified with surfactant to improve its interlayer spacing. Both membranes achieved rapid adsorption, reaching equilibrium within 20-35 min, with the functionalized membrane equilibrating faster and exhibiting enhanced adsorption performance. The adsorption kinetics were analyzed using both pseudo-first-order (PFO) and pseudo-second-order (PSO) models, allowing a comparative evaluation of the kinetic descriptions while yielding consistent performance metrics for both membranes. These results demonstrate that Thermal lens spectroscopy provides a sensitive, resource-efficient readout to resolve adsorption kinetics and quantify removal efficiency and adsorption capacity using small membrane specimens and microvolumes, offering a practical route to characterize and optimize graphene-oxide-based polymer membranes for water remediation under realistic flow conditions.