Circular waste valorization: humic acid adsorption onto ZnS nanoparticle-loaded biomass and conversion into nutrient-rich alginate beads

Abstract

This study aims to investigate the adsorption of humic acid onto zinc sulphide (ZnS) nanoparticle-loaded acid-activated biomass (ZnS-AAB) and its conversion to alginate beads with the addition of K⁺ and NO₃²⁻ for agricultural applications. The ZnS nanoparticles were synthesized using Manilkara zapota seed extract and exhibited particle sizes of less than 2 nm and a band gap of 3.16 eV. The successful incorporation of ZnS into biomass was evidenced by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The successful encapsulation of nutrients and ZnS nanoparticle-loaded biomass into the alginate beads was confirmed by EDX analysis. The adsorption of humic acid was optimized using the Box-Behnken design and was found to be significant, with a p-value of <0.0001 and a high F-value of 408.25. Error analysis of the isotherm and kinetic data validated the applicability of the Langmuir and pseudo second-order kinetic models with low root mean square error (RMSE) and average absolute relative error (AARE) values. The release of nutrients suggested that K⁺ and NO₃²⁻ released faster than humic acid and achieved 100% release within two weeks. The release kinetics of nutrients followed zero-order and Korsmeyer–Peppas (K–P) models, which were supported by correlation coefficients and low error values. The structural stability of the beads was confirmed through a swelling index study and using FTIR and SEM techniques. The overall results suggest the sustainable and circular approach of the present study with no environmental impact.