Preparation of High Concentration Polyaluminum Chloride with High Alc Content by Membrane Distillation*

ZHAO Changwei (趙長偉), WANG Jun (王軍) and LUAN Zhaokun (欒兆坤)

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Preparation of High Concentration Polyaluminum Chloride with High AlcContent by Membrane Distillation*

ZHAO Changwei (趙長偉)**, WANG Jun (王軍) and LUAN Zhaokun (欒兆坤)

State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

A direct contact membrane distillation (DCMD) process was applied to prepare high concentration polyaluminum chloride (PACl) with high Alccontent. The changes in total Al concentration (AlT) and Al species distribution were investigated. The results showed that AlTincreased but the flux decreased with operating time during the DCMD process. The Alccontent increased from 65% to 81% while the Albcontent decreased from 34% to 18%, and the Alacontent was almost 1% in the process. 2.1 mol·L-1PACl with 81% Alcwas successfully prepared by DCMD method. Thus the DCMD is an effective method for preparing high concentration PACl with high Alccontent.

polyaluminum chloride, high concentration, membrane distillation, preparation

1 INTRODUCTION

Membrane distillation (MD) is a membrane separation process with less dependence on the initial salinity of the feed as well as a higher salt rejection ratio [20, 21]. Direct contact membrane distillation (DCMD) is a thermally driven membrane separation process, in which two solutions at different temperatures are separated by a microporous hydrophobic membrane. The driving force of DCMD is the vapor pressure difference across the membrane resulting from the temperature gradient in the boundary layers adjacent to the membrane surface. Thus the solution can be further concentrated even over the saturation concentration. DCMD has been applied in water desalination [22-25], wastewater reuse [26-28] and other concentration processes [29, 30]. In our previous study, DCMD method was applied successfully to prepare high concentration PACl with high Albcontent [31, 32]. A higher concentrated PACl solution with stable Al species distribution can be achieved at normal pressure and at lower temperature compared to evaporation.

As a continuation of the previous work, high concentration PACl is prepared using DCMD method based on low concentration PACl with high Alccontent, to explore the feasibility of the DCMD method.

2 EXPERIMENTAL

2.1 Materials

AlCl3and NaOH were purchased from Beijing Chemical Company (analytical grade, Beijing, China). All reagents were of analytical grade and used without any pretreatment.

2.2 Membrane distillation experiments

A schematic DCMD apparatus is shown in Fig. 1. It consists of two thermostatic cycles, with the feed cycle and the permeate one connected to the membrane module. The membrane module is equipped with a flat-sheet membrane with the effective area of 70 cm2. The membrane material is a hydrophobic PVDF (IPVH00010, Millipore, USA) with 0.45 μm pore size and is placed between the two identical chambers.

The DCMD flux was measured by increasing the mass of pure water on the permeate side. The value of flux is calculated by the following equation

Figure 1 The schematic diagram of the experimental DCMD set-up

where1and2are the mass of water to the permeate side from the pure water tank at the beginning and timein the process, respectively,is the effective membrane area, andis the time for collection of pure water in the process.

2.3 Preparation of high concentration PAC with high Alc content

0.2 mol·L-1PACl containing 67% Alcwas first obtained by chemical synthesis process: a certain amount of 1 mol·L-1AlCl3solution was added into a 1000 ml glass reactor equipped with a Teflon anchor stirrer and a reflux condenser. The solution was kept at 65°C using a thermostatic apparatus, a certain amount of 0.6 mol·L-1NaOH solution was pumped into the reactor through peristaltic pump with rapid stirring. The solution was reacted for 12 h with stirring at 90°C with a thermostatic apparatus. Then DCMD method was applied to concentrate 0.2 mol·L-1PACl containing 67% Alc, with the feed temperature and the permeate temperature controlled at 55°C and 20°C, respectively. The initial feeding of 0.2 mol·L-1PACl solution containing 67% Alcin the stirred tank was pumped into the feed side of membrane module, then returned to the stirred tank for circulation.

2.4 Analytical methods

Al species was measured by Ferron assay with a timed colorimetric reaction with Ferron reagent to provide speciation based on chemical reactivity on UV-Vis spectrophotometer (DR/4000U, HACH, USA). Based on the difference in the dissociation and reaction rate between Ferron reagent and Al species, the Al species can be divided into three types: monomeric species (Ala) (reacting with Ferron within 1 min), planar oligomeric and medium polymeric species (Alb) (reacting with Ferron from 1 min to 120 min), and three dimensional species or sol-gels (Alc) (reacting with Ferron after 120 min or non-reacting with Ferron). The pH value was measured with a pH meter (FE20, Mettler Toledo).

3 RESULTS AND DISCUSSION

3.1 Effect of feed temperature on water flux

To investigate the water vapor permeability of the membrane, a set of experiments were carried out using pure water as the feed. The feed temperature varied from 40 to 90°C while the permeate temperature was maintained at 20°C. The flow rate was controlled at 4 cm·s-1. The effect of feed temperature on water flux is shown in Fig. 2. The flux increases significantly with feed temperature. This can be attributed to that the vapor pressure of water, which is the driving force for water flux, increases exponentially with temperature (.. Antoine relation for vapor pressure).

Figure 2 Effect of feed temperature on water flux for pure water

3.2 Flux change during DCMD process

The 0.2 mol·L-1PACl containing 67% Alcobtained in our lab was used as the initial feed to concentrate by DCMD process. The feed temperature and the permeate temperature were controlled at 55°C and 20°C, respectively. The flow rate was 4 cm·s-1. The effect of operating time on water flux and AlTis shown in Fig. 3. Water flux declines slowly and is maintained at 15 kg·m-2·h-1before 25 h. As operating time increases, flux decreases and AlTincreases. The increase of AlTdecreases the saturated steam and increases the viscosity. The pressure drop of steam results in the steam pressure difference and decreases the driving force, which consequently decreases the flux. Higher viscosity increases the thickness of diffusion layer beside the membrane, increasing resistances of mass transfer and heat transfer. It is found that the flux can be maintained at 11 kg·m-2·h-1even though the experiment stopped owing to crystallization phenomenon. Finally, the 2.1 mol·L-1PACl with 81% Alcwas successfully prepared. The result indicates that DCMD concentration method is a feasible technology to prepare high concentration PACl with high Alccontent.

Figure 3 Flux and AlTchange with operating time during DCMD process

3.3 Al species distribution in DCMD process

To investigate the changes of Al species distribution in a DCMD process, Al species was measured by Ferron assay and corresponding results are shown in Fig. 4. As AlTincrease, the Alccontent increases from 65% to 81% while the content of Albdecreases from 34% to 18%. The content of Alais almost maintained at 1% during whole DCMD process. The pH value changes from 4.7 to 3.4.

Figure 4 Al species distribution at different AlTduring DCMD process

In this study, Direct contact membrane distillation method was successfully applied to prepare 2.1 mol·L-1polyaluminum chloride with 81% Alc. During the concentration of 0.2 mol·L-1PACl with high Alccontent, the Alccontent increased from 65% to 81% while the content of Albdecreased from 34% to 18%, and the Alacontent was almost 1%. The results indicated that DCMD concentration method was a feasible technology to prepare high concentration PACl with high Alccontent.

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** To whom correspondence should be addressed. E-mail: zhaocw@rcees.ac.cn

2010-05-13,

2010-09-18.

National Natural Science Foundation of China (50708109, 21076219) and the National High Technology Research and Development Key Program of China (2007AA06Z339, 2009AA062901).