Adsorption of aliphatic polyhydroxy carboxylic acids on gibbsite: pH dependency and importance of adsorbate structure

Abstract

Background: Aliphatic (poly)hydroxy carboxylic acids [(P)HCA] occur in natural, e.g. soils, and in technical (waste disposal sites, nuclear waste repositories) compartments . Their distribution, mobility and chemical reactivity, e.g. complex formation with metal ions and radionuclides, depend, among others, on their adsorption onto mineral surfaces. Aluminium hydroxides, e.g. gibbsite [α-Al(OH)₃], are common constituents of related solid materials and mimic the molecular surface properties of clay minerals. Thus, the study was pursued to characterize the adsorption of glycolic, threonic, tartaric, gluconic, and glucaric acids onto gibbsite over a wide pH and (P)HCA concentration range. To consider specific conditions occurring in radioactive wastes, adsorption applying an artificial cement pore water (pH 13.3) as solution phase was investigated additionally.

Results: The sorption of gluconic acid at pH 4, 7, 9, and 12 was best described by the "two-site" Langmuir isotherm, combining "high affinity" sorption sites (adsorption affinity constants [Formula: see text] > 1 L mmol^-1, adsorption capacities < 6.5 mmol kg^-1) with "low affinity" sites ([Formula: see text] < 0.1 L mmol^-1, adsorption capacities ≥ 19 mmol kg^-1). The total adsorption capacities at pH 9 and 12 were roughly tenfold of that at pH 4 and 7. The S-shaped pH sorption edge of gluconic acid was modelled applying a constant capacitance model, considering electrostatic interactions, hydrogen bonding, surface complex formation, and formation of solved polynuclear complexes between Al³⁺ ions and gluconic acid. A Pearson and Spearman rank correlation between (P)HCA molecular properties and adsorption parameters revealed the high importance of the size and the charge of the adsorbates.

Conclusions: The adsorption behaviour of (P)HCAs is best described by a combination of adsorption properties of carboxylic acids at acidic pH and of polyols at alkaline pH. Depending on the molecular properties of the adsorbates and on pH, electrostatic interactions, hydrogen bonding, and ternary surface complexation contribute in varying degrees to the adsorption process. Linear distribution coefficients K_d between 8.7 and 60.5 L kg^-1 (1 mmol L^-1 initial PHCA concentration) indicate a considerable mineral surface affinity at very high pH, thus lowering the PHCA fraction available for the complexation of metal ions including radionuclides in solution and their subsequent mobilization.

Keywords: Adsorption; Artificial cement pore water; Gibbsite; Gluconic acid; Nuclear waste repository; Polyhydroxy carboxylic acids; Sorption edge; Sorption isotherm; Surface complexation; Tartaric acid.

Fig. 1

Kinetics of d-gluconic acid adsorption onto gibbsite at pH 9 in 5 mmol L⁻¹ NaClO₄ solution. Data points: experimental values, solid line: two-stage kinetic model (TSKM), model fit: r²: 0.931, X²/DOF: 0.041

Fig. 2

Sorption isotherms of d-gluconic acid at four pH values (a pH 4, b pH 7, c pH 9, d pH 12). Isotherms were recorded in 5 mM NaClO₄ solution. Best fits were obtained by the two-site Langmuir isotherm. The data correspond to equilibrium concentrations

Fig. 3

pH sorption edge of d-gluconic acid and sorption modelling. Initial d-gluconic acid concentration in the aqueous phase 1 mmol L⁻¹, background electrolyte 0.1 M NaClO₄, gibbsite suspension concentration 20 g L⁻¹; circles: experimental data

Fig. 4

Sorption isotherms of glycolate (a), threonate (b), tartrate (c), gluconate (d), and glucarate (e) determined in the gibbsite/ACPW system at pH 13.3. Fitted isotherm functions (lines): glycolate and gluconate: composite Langmuir–Freundlich (parameters: Table 8), threonate: Langmuir (parameters: Table 7), tartrate and glucarate: two-site Langmuir. The data correspond to equilibrium concentrations. Isotherm parameters for tartrate: X²/DOF: 1.39, r²: 0.96, K_L1: 6.6 ± 2.2 L mmol⁻¹, C_sat1: 16.1 ± 2.7 mmol kg⁻¹, K_L2: 0.007 ± 0.002 L mmol⁻¹, C_sat2: 228 ± 33 mmol kg⁻¹. Isotherm parameters for glucarate: X²/DOF: 1.66, r²: 0.97, K_L1: 1.0 ± 0.5 L mmol⁻¹, C_sat1: 36 ± 13 mmol kg⁻¹, K_L2: 0.03 ± 0.01 L mmol⁻¹, C_sat2: 154 ± 14 mmol kg⁻¹