1. Introduction
Currently the polyaluminum chloride (PAC) has been widely used in tap water treatment [1,2]. With the application of this chemical, it is generally inevitable to produce secondary pollution resulted from Al3+ [3], which brings threats and harms to human health [4]. Thus, there is a demand for an eco-friendly alternative to ensure treatment effect and human health.
In the study, a novel tap water flocculant was discovered based on lower concentration of Al3+ in exit water. The novel flocculant was made by polyaluminum chloride (PAC), Chitosan (CTS), and montmorillonite (MM). Owing to the decreased dosage of PAC, the concentration of Al3+ in exit water was significantly reduced. Besides, a large number of amino (NH2) and hydroxyl (OH) groups on the molecular chain of CTS could form stable cheated compounds with Al3+ so as to remove part of metal ions from water. MM mainly play an adsorption role in tap water treatment to reduce the SS in exit water.
2. Experiments
2.1. Main Apparatus
Magnetism msier (78-1, Ronghua Equipment Manufacture Co., Ltd, Jiangsu, China); Scattering-type optoelectronic SSmeter (WGZ-100, Jinziguang Apparatus Company, Beijing, China); Digital electronic scale (BA210, Ohaus, Berlin, Germany) accurate to 0.0001 g; Electrical inductive coupling plasma mass spectrometer (ELAN6000, Sigma, Boston, USA); High-speed disperser (GFJO4A, Coating Industry Factory, Shanghai, China); Digital PH meter (pHS-25, Lida Apparatus Company, Shanghai, China); Air dry oven (FN101-3A, Apparatus Company, Changsha, China); Quartz automatic triple water distiller (1810-C, Kanghua Electronic Apparatus Factory, Jiangsu, China).
2.2. Main Reagents
Chitosan (CTS) with a viscosity of about 30 - 3000 mPa·S at 25 degrees Celsius and a degree of deacetylation of about 85% - 98%; Poly (aluminum chloride) with an Al2O3 content of more than 32%; Polymerized ferrous sulfate (PFS) with an Fe content of more than 22%; Natural montmorillonite (MM) with its content more than 70%, fineness less than 0.043 μm and specific surface of 260 m2·g–1; Cationic polyacrylamide (CPAM) with molecular weight of about 3 - 15 million and degree of cationic of about 5% - 80%; Acetic acid with an HAc content of more than 99%.
2.3. Raw Water
The raw water was obtained from The Yangtze River of Wuhan in China (SSvalue = 85.6 NTU, water temperature of about 21 - 25 degrees Celsius, pH = 7.2).
2.4. Preparation of the Composite Flocculant
There were 5 steps in the process of single-component flocculant preparation: 1) CTS was first dissolved in acetic acid. This formed suspension was diluted with water and stirred for 3.5 h at 25 degrees Celsius to prepare CTS working solution of 50 mg·L–1. 2) Similarly PAC was mixed with water to form working solution of 1 g·L–1. It took about 5 min to dissolve completely under stirring at 25 degrees Celsius. 3) MM was mixed with water to form working solution of 3 g·L–1. It took about 6 h under stirring at 25 degrees Celsius. 4) CPAM was diluted with water and oscillated for 4 h at 25 degrees Celsius to form CPAM solution of 1 g·L–1. 5) PFS was mixed with water and stirred for 5 min at 25 degrees Celsius to prepare PFS solution of 1 g·L–1.
2.5. Experimental Methods
Eight samples of 200 mL raw water were placed into eight 250 mL beakers, and various different categories and dosages of flocculants were added under stirring. The solution was quickly stirred for 4 min at a speed of 260 r·min–1 and then slowly stirred for 8 min at the speed of 65 r·min–1. The liquid was transferred to a separating funnel, where the floc was allowed to settle for 30min. A small volume of the upper layer was removed from the funnel, and the concentration of Al3+ and turbidity in exit water in this liquid were measured. In this way, a set of data were obtained.
The liquid was stirred by magnetism msier. The turbidity in exit water was measured by Scattering-type optoelectronic turbidity meter. The concentration of Al3+ was measured by electrical inductive coupling plasma mass spectrometer.
3. Results and Discussion
3.1. Confirming the Optimal Prescription
Based on reaching lower cost than that of traditional flocculant, three single-component flocculant were selected as a group and mixed in the proportion of 1:1:1. Eight specimens were designed and tested to determine the optimal prescription in terms of lower cost and better removal rate of SS.
Taking the higher accuracy and lower cost into account, we used 0.1 mL as the volume unit. The flocculant category and experimental data could be seen in Table 1.
The SS of 8 treatments in exit water was all higher than 16 NTU (Table 1). The treatment effect of the composite flocculant which contained CPAM was not satisfactory. The specimens containing both CPAM and MM were worse than other specimens. On the whole, the specimen 4 had the best effect and it was the optimal combination to be used. Its turbidity in exit water was only 16.2 NUT and removal rate of SS reached 81.07%. So the optimal prescription was made by 1 g·L–1 PAC, 50 mg·L–1 CTS, and 3 g·L–1 MM.
3.2. Confirming the Optimum Dosage
The optimum dosage of single-component flocculant was determined based on lower cost and better treatment effect. Thus this flocculant was superior to traditional one for the improved performance-price ratio and strong market competitiveness.
3.2.1. Confirming the Optimum Dosage of PAC
With the content of CTS and MM maintaining to 0.1 mL, the dosage of PAC was gradually changed in order to determine its optimum dosage.
Figure 1 showed that the larger the dosage of PAC; the lower the turbidity in exit water. When the dosage of PAC was less than 0.6 mL, with the dosage of PAC increasing, the turbidity in exit water was significantly decreased. When the dosage of PAC was more than 0.6 mL, the treatment effect of tap water was not very satisfactory with the dosage of PAC increasing. When the dosage of PAC was 1.0 mL, the turbidity in exit water was 3.12 NTU and the removal rate of SS was 96.3%. However, the cost of composite flocculant was higher than that of traditional flocculant.