The simple, accurate and precise method based on fluorescence properties of aluminium (III)–quercetin complex, for the determination of quercetin has been developed and validated. The complex has strong emission at pH 3.30, lem = 480 nm, with lex = 420 nm. Linearity range of quercetin determination was 1.5 - 60.5 ng mL-1 with LOD 0.09 ng mL-1 and LOQ 0.27 ng mL-1. Recovery values in the range of 99.9 – 100.2 % indicate a good accuracy of the method. The established method was applied for the determination of quercetin in capsules, with Recovery value 98.3 %, standard deviation 0.22 % and coefficient of variation 0.09 %. The reliability of the method was checked by RP-HPLC/UV method for capsules with direct determination of quercetin after separation. The good agreement between two methods indicates the applicability usability of the proposed spectroflurometric method for quercetin determination in pharmaceutical dosage forms, with high reproducibility, and enables direct and simple determination without its prior extraction from samples.The proposed spectrofluorimetric method has much better sensitivity and about 1000 times lower LOD and LOQ values compared to data reported in literature.

This study is aimed at extending the soybean hulls’ lifetime by their utilization as an adsorbent for metal ions (Cd²⁺ and Cu²⁺) and dyes (Reactive Yellow 39 (RY 39) and Acid Blue 225 (AB 225)). ATR-FTIR spectroscopy, FE-SEM microscopy, and zeta potential measurements were used for adsorbent characterization. The effect of the solution’s pH, peroxidase extraction, adsorbent particle size, contact time, the pollutant’s initial concentration, and temperature on the soybean hulls’ adsorption potential was studied. Before peroxidase extraction, soybean hulls were capable of removing 72% Cd²⁺, 71% Cu²⁺ (at a pH of 5.00) or 81% RY 39, and 73% AB 225 (at a pH of 3.00). For further experiments, soybean hulls without peroxidase were used for several reasons: (1) due to their observed higher metal ion removal, (2) in order to reduce the waste disposal cost after the peroxidase (usually used for wastewater decolorization) extraction, and (3) since the soybean hulls without peroxidase possessed significantly lower secondary pollution than those with peroxidase. Cd²⁺ and Cu²⁺ removal was slightly increased when the smaller adsorbent fraction (710-1000 μm) was used, while the adsorbent particle size did not have an impact on dye removal. After 30 min of contact time, 92% and 88% of RY 39 and AB 225 were removed, respectively, while after the same contact time, 80% and 69% of Cd²⁺ and Cu²⁺ were removed, respectively. Adsorption of all tested pollutants follows a pseudo-second-order reaction through the fast adsorption, intraparticle diffusion, and final equilibrium stage. The maximal adsorption capacities determined by the Langmuir model were 21.10, 20.54, 16.54, and 17.23 mg/g for Cd²⁺, Cu²⁺, RY 39, and AB 225, respectively. Calculated thermodynamic parameters suggested that the adsorption of all pollutants is spontaneous and of endothermic character. Moreover, different binary mixtures were prepared, and the competitive adsorptions revealed that the soybean hulls are the most efficient adsorbent for the mixture of AB 225 and Cu²⁺. The findings of this study contribute to the soybean hulls’ recovery after the peroxidase extraction and bring them into the circular economy concept.