On the acid–base stability of Keggin Al<Subscript>13</Subscript> and Al<Subscript>30</Subscript> polymers in polyaluminum coagulants

Zhaoyang Chen; Zhaokun Luan; Zhiping Jia; Xiaosen Li

doi:10.1007/s10853-009-3412-0

Journal of Materials Science

June 2009, Volume 44, Issue 12, pp 3098–3111 | Cite as

On the acid–base stability of Keggin Al₁₃ and Al₃₀ polymers in polyaluminum coagulants

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Zhaoyang Chen
Zhaokun Luan
Zhiping Jia
Xiaosen Li

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First Online: 01 June 2009

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Abstract

The acid–base stabilities of Al₁₃ and Al₃₀ in polyaluminum coagulants during aging and after dosing into water were studied systematically using batch and flow-through acid–base titration experiments. The acid decomposition rates of both Al₁₃ and Al₃₀ increase rapidly with the decrease in solution pH. The acid decompositions of Al₁₃ and Al₃₀ with respect to H⁺ concentration are composed of two parallel first-order and second-order reactions, and the reaction orders are 1.169 and 1.005, respectively. The acid decomposition rates of Al₁₃ and Al₃₀ increase slightly when the temperature increases from 20 to ca. 35 °C, but decrease when the temperature increases further. Al₃₀ is more stable than Al₁₃ in acidic solution, and the stability difference increases as the pH decreases. Al₃₀ is more possible to become the dominant species in polyaluminum coagulants than Al₁₃. The acid catalyzed decomposition and followed by recrystallization to form bayerite is one of the main processes that are responsible for the decrease of Al₁₃ and Al₃₀ in polyaluminum coagulants during storage. The deprotonation and polymerization of Al₁₃ and Al₃₀ depend on solution pH. The hydrolysis products are positively charged, and consist mainly of repeated Al₁₃ and Al₃₀ units rather than amorphous Al(OH)₃ precipitates. Al₃₀ is less stable than Al₁₃ upon alkaline hydrolysis. Al₁₃ is stable at pH < 5.9, while Al₃₀ lose one proton at the pH 4.6–5.75. Al₁₃ and Al₃₀ lose respective 5 and 10 protons and form [Al₁₃]_n and [Al₃₀]_n clusters within the pH region of 5.9–6.25 and 5.75–6.65, respectively. This indicates that Al₃₀ is easier to aggregate than Al₁₃ at the acidic side, but [Al₁₃]_n is much easier to convert to Al_sol–gel than [Al₃₀]_n. Al₃₀ possesses better characteristics than Al₁₃ when used as coagulant because the hydrolysis products of Al₃₀ possess higher charges than that of Al₁₃, and [Al₃₀]_n clusters exist within a wider pH range.

Keywords

Gibbsite Apparent Rate Constant Base Titration Bayerite Acid Decomposition

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Notes

Acknowledgements

The work was financially supported by the National High Technology Research and Development Key Program of China (863 Program) (No. 2002AA601290) and the National Natural Science Foundation of China (No. 50874098 and No. 40673003).

Appendix

Al₃₀ and Al₁₃ are decomposed by H⁺ in the continuous flow reactor. It is assumed that Al₃₀ species added into the reactor exists in two extreme states: keeping Al₃₀ species or decomposing completely into Al³⁺ by H⁺ according to the following stoichiometry:

$$ [{\text{Al}}_{2} {\text{O}}_{8} {\text{Al}}_{28} ({\text{OH}})_{56} ({\text{H}}_{2} {\text{O}})_{24} ]^{18 + } + 72{\text{H}}^{ + } \to 30{\text{Al}}^{3 + } + 88{\text{H}}_{2} {\text{O}} . $$

(4)

It is assumed that the above acid decomposition reaction is a pseudo-first-order reaction with respect to Al₃₀, and the decomposition rate described as:

$$ r = k_{{\text{Al}}_{30}} \cdot [{\text{Al}}_{30} ]. $$

(5)

$ k_{\text{Al}_{30}}$ is the apparent rate coefficient of the pseudo-first-order reaction calculated by Al₃₀ concentration (min⁻¹); [Al₃₀] is the concentration of Al₃₀ in reactor (M).

It is assumed that the solution in the reactor is fully mixed and homogeneous, and the concentrations of Al₃₀ and H⁺ in the outflow are equal to that in the reactor. The mass balance equations of Al₃₀ and H⁺ in continuous flow reactor are:

$$ \frac{{{\text{d}}[{\text{Al}}_{30} ]}}{{{\text{d}}t}} = \frac{{V_{{{\text{Al}}_{30} }} \cdot [{\text{Al}}_{30} ]_{\text{in}} - (V_{{{\text{Al}}_{30} }} + V_{\text{HCl}} )\cdot [{\text{Al}}_{30} ]}}{{V_{\text{r}} }} - k_{{{\text{Al}}_{{_{30} }} }} \cdot [{\text{Al}}_{30} ] $$

(6)

and

$$ \frac{{{\text{d}}[{\text{H}}^{ + } ]}}{{{\text{d}}t}} = \frac{{V_{\text{HCl}} \cdot [{\text{HCl}}]_{\text{in}} - (V_{{{\text{Al}}_{30} }} + V_{\text{HCl}} ) \cdot [{\text{H}}^{ + } ]}}{{V_{\text{r}} }} - 72k_{{{\text{Al}}_{30} }} \cdot [{\text{Al}}_{30} ]. $$

(7)

$ V_{{{\text{Al}}_{ 30} }} $and V_HCl are the volume flow rates of PAC_Al30 and hydrochloric acid that added into the reactor, respectively (L/min); V_r denotes the volume of solution in the reactor (L); [Al₃₀]_in = Al_T/30 and [HCl]_in are the concentration of Al₃₀ and hydrochloric acid that are added into the reactor, respectively (M).

The concentrations of Al₃₀ and H⁺ in the reactor remain constant at the stable state,

$$ \frac{{{\text{d}}[{\text{Al}}_{30} ]}}{{{\text{d}}t}} = 0 $$

(8)

$$ \frac{{{\text{d}}[H^{ + } ]}}{{{\text{d}}t}} = 0. $$

(9)

From Eqs. (6)–(9), the following expressions are derived:

$$ k_{{{\text{Al}}_{30} }} = \frac{{V_{\text{HCl}} \cdot (V_{{{\text{Al}}_{ 3 0} }} + V_{\text{HCl}} ) \cdot [{\text{HCl}}]_{\text{in}} - (V_{{{\text{Al}}_{ 30} }} + V_{\text{HCl}} )^{2} \cdot [H^{ + } ]_{\text{stat}} }}{{72V_{{{\text{Al}}_{ 3 0} }} \cdot V_{\text{r}} \cdot [{\text{Al}}_{30} ]_{\text{in}} + V_{\text{r}} \cdot (V_{{{\text{Al}}_{ 3 0} }} + V_{\text{HCl}} ) \cdot [{\text{H}}^{ + } ]_{\text{stat}} - V_{\text{r}} \cdot V_{\text{HCl}} \cdot [{\text{HCl}}]_{\text{in}} }}, $$

(10)

$$ [{\text{Al}}_{30}]_{\text{stat}} = \frac{{30V_{{{\text{Al}}_{ 3 0} }} \cdot [{\text{Al}}_{30} ]_{\text{in}}}}{{V_{\text{r}} \cdot k_{{{\text{Al}}_{30}}} + V_{{{\text{Al}}_{30} }} \cdot (V_{{{\text{Al}}_{30}}} + V_{\text{HCl}} )}}. $$

(11)

[Al₃₀]_stat and [H⁺]_stat denote the concentration of Al₃₀ (calculated by Al) and H⁺ in the reactor at stable state, respectively (M).

The half-life (t_1/2, min) of pseudo-first-order reaction of acid decomposition of Al₃₀ is calculated by the following equation:

$$ t_{1/2} = \frac{{{\text{LN}}(2)}}{{k_{{{\text{Al}}_{30} }} }}. $$

(12)

For Al₁₃, the acid decomposition stoichiometric equation is:

$$ [{\text{AlO}}_{4} {\text{Al}}_{12} ({\text{OH}})_{24} ({\text{H}}_{2} {\text{O}})_{12} ]^{7 + } + 32{\text{H}}^{ + } \to 13{\text{Al}}^{3 + } + 44{\text{H}}_{2} {\text{O}} . $$

(13)

The following equations are obtained according to the same calculation methods as described above:

$$ k_{{{\text{Al}}_{13} }} = \frac{{V_{\text{HCl}} \cdot (V_{{{\text{Al}}_{13} }} + V_{\text{HCl}} ) \cdot [{\text{HCl}}]_{\text{in}} - (V_{{{\text{Al}}_{13} }} + V_{\text{HCl}} )^{2} \cdot [H^{ + } ]_{\text{stat}} }}{{32V_{{{\text{Al}}_{13} }} \cdot V_{{\text{r}}} \cdot [{\text{Al}}_{13} ]_{\text{in}} + V_{{\text{r}}} \cdot (V_{{{\text{Al}}_{13} }} + V_{\text{HCl}} ) \cdot [H^{ + } ]_{\text{stat}} - V_{{\text{r}}} \cdot V_{\text{HCl}} \cdot [{\text{HCl}}]_{\text{in}} }}, $$

(14)

$$[{\text{Al}}_{{13}} ]_{{{\text{stat}}}} = \frac{{13V_{{{\text{Al}}_{{13}} }} \cdot [{\text{Al}}_{{13}} ]_{{{\text{in}}}} }}{{V_{r} \cdot k_{{{\text{Al}}_{{13}} }} + V_{{{\text{Al}}_{{13}} }} \cdot (V_{{{\text{Al}}_{{13}} }} + V_{{{\text{HCl}}}} )}},$$

(15)

$$ t_{1/2} = \frac{{{\text{LN}}(2)}}{{k_{{{\text{Al}}_{13} }} }}. $$

(16)

[Al₁₃]_stat is the concentration of Al₁₃ (calculated by Al) in the reactor at steady state (M). $ k_{{{\text{Al}_{13}}}} $ is the apparent rate coefficient of the pseudo-first-order reaction calculated by Al₁₃ concentration (min⁻¹); $ [{\text{Al}}_{13} ]_{\text{in}} = {{{\text{Al}}_{\text{T}} } \mathord{\left/ {\vphantom {{{\text{Al}}_{\text{T}} } {13}}} \right. \kern-\nulldelimiterspace} { 1 3}} $ and $ V_{{{\text{Al}}_{13} }} $ are the Al₁₃ concentration and volume flow rate of PAC_Al13 that are added into the reactor (M and L/min, respectively).

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Authors and Affiliations

Zhaoyang Chen
- 1
Email author
Zhaokun Luan
- 2
Zhiping Jia
- 2
Xiaosen Li
- 1
Email author

1.Key Laboratory of Renewable Energy and Gas Hydrate, CAS, Guangzhou Institute of Energy ConversionChinese Academy of SciencesGuangzhouChina
2.State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental SciencesChinese Academy of SciencesBeijingChina

On the acid–base stability of Keggin Al₁₃ and Al₃₀ polymers in polyaluminum coagulants

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On the acid–base stability of Keggin Al13 and Al30 polymers in polyaluminum coagulants

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On the acid–base stability of Keggin Al₁₃ and Al₃₀ polymers in polyaluminum coagulants