Charge-dependent removal in water treatment using NaOH-modified olive stone: Batch and fixed-bed evaluation of Pb(II), Cd(II), and As(V)

Abstract

Low-cost biosorbents from agricultural residues are promising for water treatment but are usually evaluated for single contaminants under batch conditions only. We assessed the charge-dependent removal of cationic Pb(II) and Cd(II) and anionic As(V) by NaOH-modified olive stone (Olea europaea L.) in batch and fixed-bed systems, using a common biosorbent preparation and parallel protocols. NaOH treatment shifted the pHPZC from 4.47 to 6.83 and produced FTIR changes consistent with ester-bond saponification and greater exposure of carboxylate and hydroxyl groups. Under optimized batch conditions, Pb(II) removal reached 99.57% and remained above 98.9% across independent series, although removal at pH 6.8–8.0 should be interpreted as an upper bound because phosphate buffering may contribute to lead phosphate precipitation. Cd(II) removal varied from 5.5% to 99.7% depending on operational conditions, with the upper value (99.72%) obtained at 15 g L⁻¹ in the dose × size factorial; the variability is largely attributable to operational differences between experimental series (dose, contact time, replication regime); a single biosorbent preparation was used throughout, so batch-to-batch variability could not be evaluated. As(V) removal remained below 8% under all conditions. Fixed-bed breakthrough curves were adequately described by Thomas and Yoon-Nelson models (R² > 0.97), with column capacities of 4.01 mg g⁻¹ for Cd(II) (extrapolated) and 2.27 for Pb(II); the 1.77 value for As(V) reflects transient column retention rather than true uptake. NaOH-modified olive stone is robust for Pb(II), conditional for Cd(II), and unsuitable for As(V), supporting contaminant-specific biosorbent selection.