Adsorption and quantitative fluorescence-based measurement of ammonium ions using a chitosan-based hydrogel combined with p-hydroxybenzoic acid

Abstract

Ammonium removal in high-density fish cultures is highly reliant on time-consuming and costly biological treatment methods. Therefore, the development of cheap and rapid chemical methods has recently garnered increasing attention. Our study developed a dissolved ammonium removal method using a chitosan-based hydrogel (Cht-HG) modified with 0.3 mM of p-hydroxybenzoic acid (Cht-0.3PHBA-HG). Our proposed method enabled the adsorption and measurement of dissolved ammonium via fluorescence detection. In the ammonium adsorption test, the maximum adsorption capacities (qm) of Cht-HG and Cht-0.3PHBA-HG were 11.6 mg g−1 and 19.9 mg g−1, respectively. Cht-0.3PHBA-HG exhibited a relatively elevated adsorption capacity (i.e., 1.71-fold higher adsorption efficiency) due to the morphological and chemical modifications attributed to p-hydroxybenzoic acid (PHBA). These findings were confirmed by our chemical characterization and adsorption test results. Particularly, the adsorbed ammonium atomic percentages (at.%) of Cht-HG-NH4 and Cht-0.3PHBA-HG-NH4 were 6.33 % and 15.2 %, respectively. Compared to Cht-HG, the adsorption properties of Cht-0.3PHBA-HG were more stable in a pH range of 2–8. Moreover, we quantitatively analyzed the oxidation of the combined PHBA at different dissolved ammonium concentrations (0.1–5.0 mM) using fluorescence detection. The fluorescence sensitivity of ammonium was maintained, even in high concentrations of common electrolytes (Ca2+, K+, Mg2+, Na+, and PO43-). The ammonium adsorption mechanisms of Cht-0.3PHBA-HG were strongly consistent with the pseudo-first order (PFO), pseudo-second order (PSO), and Langmuir models. In addition to its role as an adsorbent, Cht-0.3PHBA-HG could also be used to detect ammonium in real time.