Competitive Sorption and Retention of Chromium, Copper, Lead, and Zinc in Brewery Biosolid-Amended Ferralsol (Oxisol): Selectivity, Mechanisms, and Environmental Implications

Abstract

This study investigated the competitive sorption and desorption behaviour of potentially toxic elements (PTEs) or trace metals (chromium, copper, lead, and zinc) from brewery biosolid-amended Ferralsol (Oxisol), addressing critical gaps in understanding metal mobility and retention in tropical agricultural soils receiving industrial organic amendments. Batch equilibrium experiments were conducted using Ferralsol amended with brewery biosolid at application rates of 0, 2.5, 5.0, and 7.5 tons∙ha−1, with single superphosphate (SSP) at 0, 25, 50, and 75 kg∙ha−1. Multi-metal nitrate solutions (25 - 500 mg∙L−1) containing equal concentrations of Cr3+, Cu2+, Pb2+, and Zn2+ were equilibrated with sorbents for 1 day (24 hours) at pH 4.5 (buffered with 0.02 M acetic acid/sodium acetate). Metal concentrations were determined by atomic absorption spectrophotometry. Sorption and desorption data were fitted to Langmuir and Freundlich isotherms, and distribution coefficients (Kd) were calculated to establish selectivity sequences. Chromium (Cr3+) exhibited the highest sorption and retention capacity across all treatment rates, with Kd values 2 - 3 orders of magnitude greater than those of other metals. The selectivity sequence for adsorption followed Cr > Zn > Pb > Cu at 100 mg∙L−1, shifting to Zn > Cr > Pb > Cu at 7.5 metric tons∙ha−1 brewery biosolid application. Desorption studies revealed near-irreversible binding for chromium (retention Kd = 14.2 - 33.4 L∙g−1), while zinc demonstrated the greatest reversibility (retention Kd = 0.09 - 0.27 L∙g−1). Langmuir and Freundlich models showed limited applicability (only 43.6% of potential isotherms fitted r2 > 0.75), with Freundlich providing superior fits for most metal-sorbent combinations. The high charge-to-radius ratio of Cr3+ (49.2 against 16.8 - 27.4 for other metals) drives its preferential retention through inner-sphere complexation and possible surface precipitation. Brewery biosolid addition increased organic matter content (from 2.51% to 4.5%) and cation exchange capacity (from 8.7 to 17.6 cmol∙kg−1), enhancing overall metal retention capacity. The S-type isotherms observed indicate cooperative adsorption mechanisms. These findings demonstrate that brewery biosolid-amended Ferralsol (Oxisol) effectively immobilizes chromium, reducing its bioavailability and leaching potential, while zinc remains comparatively mobile. Application rates above 5.0 tons∙ha−1 optimize metal retention without compromising soil quality.