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

Environmental Sciences Europe

Table 5 Speciation, dissolution, and complexation reactions and reaction constants involved in the sorption of gluconic acid to gibbsite within the pH range of 3.4–12.2

Reaction	Reaction constant lg K
Dissociation of gluconic acid
HGLU \(\Leftrightarrow\) H⁺ + GLU⁻	− 3.86
Surface speciation of gibbsite (constant capacitance model)
≡SOH + H⁺ \(\Leftrightarrow\) ≡SOH₂⁺	5.66^a
≡SOH \(\Leftrightarrow\) ≡SO⁻ + H⁺	− 10.41^a
Outer-sphere complexation
≡SOH₂⁺ + GLU⁻ \(\Leftrightarrow\) ≡SOH₂⁺ GLU⁻	13.93^a
Inner-sphere complexation
≡SOH + GLU⁻ \(\Leftrightarrow\) ≡SGLU + OH⁻	6.92^a
Dissolution of gibbsite
Al(OH)₃(s) + 3 H⁺ \(\Leftrightarrow\) Al³⁺ 3 H₂O	8.50^b
Al³⁺ + OH⁻ \(\Leftrightarrow\) AlOH²⁺	3.53^b
Al³⁺ + 2 OH⁻ \(\Leftrightarrow\) Al(OH)₂⁺	− 0.80^b
Al³⁺ + 3 OH⁻ \(\Leftrightarrow\) Al(OH)₃	− 6.50^b
Al³⁺ + 4 OH⁻ \(\Leftrightarrow\) Al(OH)₄⁻	− 14.50^b
Formation of Al–gluconate complexes
Al³⁺ + GLU⁻ \(\Leftrightarrow\) [AlGLU]²⁺	1.98^c
Al³⁺ + GLU⁻ \(\Leftrightarrow\) [Al_H − 1GLU]⁺ + H⁺	− 0.89^c
Al³⁺ + GLU⁻ \(\Leftrightarrow\) [Al_H − 3GLU]⁻ + 3 H⁺	− 10.18^c
Formation of mixed Al–hydroxo-gluconate complexes (k: 1 or 2)
k Al(OH)₄⁻ + GLU⁻ \(\Leftrightarrow\) [Al_k(OH)_{[3k+(k − 1) − n]}H_−nGLU]^k− + OH⁻ + n H₂O	9.80^d
Sorption of the Al–hydroxo-gluconate complexes by inner-sphere complexation
≡SOH + [Al_k(OH)_{[3k+(k − 1) − n]}H_−nGLU]^k−\(\Leftrightarrow\) ≡S[Al_k(OH)_{[3k+(k − 1) − n]}H_−nGLU]^{(k − 1)−} + OH⁻	7.09^d

Further model parameters: capacity of the Helmholtz layer: 5 F m⁻²; solid-to-liquid ratio: 200 g L⁻¹; specific surface area: 11 m² g⁻¹; initial gluconate concentration: 1 mmol L⁻¹: ionic strength: 100 mmol L⁻¹, density of surface sites: 5.5 sites nm⁻²
^aResults of surface speciation modelling (FITEQL, constant capacitance model)
^b[32]
^cI₀ 0.1 M KNO₃, T 25 °C [9]
^dResults of gluconate sorption edge modelling (FITEQL, constant capacitance model)