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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

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

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+ 5.66a
 ≡SOH \(\Leftrightarrow\) ≡SO + H+ − 10.41a
Outer-sphere complexation
 ≡SOH2+ + GLU \(\Leftrightarrow\) ≡SOH2+ GLU 13.93a
Inner-sphere complexation
 ≡SOH + GLU \(\Leftrightarrow\) ≡SGLU + OH 6.92a
Dissolution of gibbsite
 Al(OH)3(s) + 3 H+ \(\Leftrightarrow\) Al3+ 3 H2O 8.50b
 Al3+ + OH \(\Leftrightarrow\) AlOH2+ 3.53b
 Al3+ + 2 OH \(\Leftrightarrow\) Al(OH)2+ − 0.80b
 Al3+ + 3 OH \(\Leftrightarrow\) Al(OH)3 − 6.50b
 Al3+ + 4 OH \(\Leftrightarrow\) Al(OH)4 − 14.50b
Formation of Al–gluconate complexes  
 Al3+ + GLU \(\Leftrightarrow\) [AlGLU]2+ 1.98c
 Al3+ + GLU \(\Leftrightarrow\) [AlH − 1GLU]+ + H+ − 0.89c
 Al3+ + GLU \(\Leftrightarrow\) [AlH − 3GLU] + 3 H+ − 10.18c
Formation of mixed Al–hydroxo-gluconate complexes (k: 1 or 2)
 k Al(OH)4  + GLU \(\Leftrightarrow\) [Al k (OH)[3k+(k − 1) − n]HnGLU]k + OH + n H2O 9.80d
Sorption of the Al–hydroxo-gluconate complexes by inner-sphere complexation
 ≡SOH + [Al k (OH)[3k+(k − 1) − n]HnGLU]k\(\Leftrightarrow\)  ≡S[Al k (OH)[3k+(k − 1) − n]HnGLU](k − 1)− + OH 7.09d
  1. 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
  2. aResults of surface speciation modelling (FITEQL, constant capacitance model)
  3. b[32]
  4. cI0 0.1 M KNO3, T 25 °C [9]
  5. dResults of gluconate sorption edge modelling (FITEQL, constant capacitance model)