Aluminum sulphate (AS) as a common coagulant has been widely used in water treatment. Pre-hydrolysed AS (PAS) has higher molecular weight, presenting stronger water treatment performance; however, its poor stability has greatly limited its application. Therefore, there is still a need to obtain a stable PAS. In this study, a method for stabilizing PAS product was proposed and a new coagulant, phosphate-aluminum sulphate (PPAS), was prepared. The parameters affecting PPAS performance, such as n(P)/n(Al) and n(OH)/n(Al) molar ratios, and organic stabilizer dose containing a mixture of tartaric acid and sodium potassium tartrate, were examined. The coagulation–flocculation performance of PPAS was assessed by measuring source water treatment efficiency. The results showed that the PPAS at n(P)/n(Al) molar ratio of 0.15, n(OH)/n(Al) molar ratio of 0.2 and the stabilizer dose of 7.5% presented good performance and stability. PPAS was significantly distinct from AS in the morphology and species distribution, which indicated that the distribution of the Alb and Alc in PPAS largely affected the coagulation–flocculation performance, especially the Alb. The coagulation–flocculation examined at an appropriate pH was effective for source water turbidity, ultraviolet adsorption at 254 nm and total dissolved organic nitrogen removal in the investigated range.
The coagulation–flocculation process is a key environmental protection technology that finds a wide range of applications in water and wastewater treatment facilities (Zhu et al. 2011, 2012; Zheng et al. 2011b; Tolkou & Zouboulis 2015). It is regarded as a viable option for the production of drinking water through removing suspended solids, colloidal particles and natural organic matter from surface waters, as well as the treatment of heavy loaded, toxic or hazardous wastewater (Meriç et al. 2005; Assaad et al. 2007; Matilainen et al. 2010; Zhu et al. 2014, 2016). The most common coagulants used for the coagulation–flocculation process are hydrolysing metals of aluminum or iron salts such as AlCl3, Al2(SO4)3, FeCl3, Fe2(SO4)3, etc. (Duan & Gregory 2003; Moussas et al. 2011). Their major drawback is that during dilution, their rapid formation of coagulant species could result in uncontrollable formation of hydrolysis species (Tshukudu et al. 2013), which would decrease their stability. To overcome this problem, a relatively new type of coagulant commonly known as inorganic polymer coagulant, such as polymeric forms of ferric chloride and ferric sulphate, polymeric aluminum sulphate (PAS) and aluminum chloride (PAC), have been developed worldwide (Lei et al. 2009; Zhu et al. 2011; Luo et al. 2015). These coagulants include hydrolysis and polymeric species, which are able to slow the speed of hydroxide precipitation upon dilution allowing polymeric species to be maintained for a longer period.
Although the PAS as a pre-hydrolysed coagulant has a similar property to PAC, its stability during dilution is relatively weak compared with other hydrolysed coagulants (Lin Wang et al. 2009). During preparation of pre-hydrolysed coagulants, controlling their species distribution is significantly important because their species distribution and transformation laws, characterization, and the influence of the species on the stability and performance of one coagulant closely relates to the coagulant's physico-chemical properties (Fu et al. 2007). There are two viable ways of changing the physico-chemical properties of the inorganic coagulant. One is to increase the proportion of polymeric species in the original composition and the other is to add extra components to produce new composite coagulants. Zouboulis et al. (Moussas & Zouboulis 2008; Zouboulis & Moussas 2008) suggested three types of compounds as modifiers: inorganic anions, such as poly-silicates and phosphates; organic anions, such as various organic acids; and organic polyelectrolytes, which are cationic, non-anionic or anionic. For example, a modified PAS with the sodium silicate as modifier indicates that its stability is stronger than without modification (Liu et al. 1999; Wei & Gu 2005). Introducing an appropriate amount of phosphate in PAS can produce a modified PAS, polymeric phosphate aluminum sulphate (PPAS), which is a new class of high charge with multi-core phosphate intermediate complex similar to those coagulants modified by phosphate (Voegelin et al. 2010; Zheng et al. 2011b; Razack et al. 2015).
In this study, PPAS was prepared. Fourier transform infrared spectroscopy (FT-IR) and a scanning electron microscope (SEM) were utilized to characterize the structure and morphology of PPAS. The Al-Ferron method was adopted to measure Al(III) species distribution in PPAS. The parameters affecting PPAS performance, such as n(P)/n(Al) molar ratio, n(OH)/n(Al) molar ratio, and organic stabilizer dose were examined. The coagulation performance of PPAS was examined by measuring its treatment effect on source water turbidity, ultraviolet adsorption at 254 nm (UV254) and total dissolved organic nitrogen (TDN).
MATERIAL AND METHODS
The reagents used for preparation of PPAS mainly consisted of aluminum sulphate (Al2(SO4)3), sodium carbonate (Na2CO3), aurintricarboxylic acid triammonium salt, potassium dihydrogen phosphate (KH2PO4), tartaric acid and sodium potassium tartrate, which were analytical grade. For analysis of PPAS species, the analytical grade Ferron reagent (8-hydroxy-7-iodoquinoline-5-sulfonic acid) was used. Aluminum potassium sulphate (KAl(SO4)2) (guaranteed reagent) and aluminum powder were applied in measuring the content of Al(III). All other reagents were analytical grade. All aqueous solutions were prepared with deionized water.
Water samples were collected from the Xiangjiang River with sampling located in Xiangtan, China. This source water has suffered from serious pollution by agricultural, industrial and domestic pollutants, which has led to water quality deterioration. The quality analysis of the raw water was investigated in this study. The results are shown in Table 1. The results showed that UV254, pH, turbidity, ammonia, nitrate, nitrite and TDN were between 0.05 and 0.1, 7.0 and 8.0, 8 and 30 NTU, 0.5 and 1 mg/L, 1 and 2 mg/L, 0.1 and 0.3 mg/L, and 5.0 and 7.0 mg/L, respectively. The water temperature tested in this study ranged from 18 to 22 °C.
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Preparation of PPAS
The PPAS investigated in this study was a composite coagulant and the steps for its preparation were as follows.
Firstly, a given mass of Al2(SO4)3 and 150 mL deionized water were mixed slowly in a water bath at 100 °C for 1 hour. The reaction temperature was controlled using a thermostatic water bath. Then, the temperature was dropped to 60 °C, accompanied by rigorous stirring using a magnetic stirrer. A certain amount of Na2CO3 was added to the liquid system gently according to the predetermined n(OH)/n(Fe) molar ratio. The n(OH)/n(Fe) molar ratio was determined following the procedure reported by Tang (Tang 2006). The reaction time was kept constant for 10 min. After the addition of Na2CO3, KH2PO4 was added to the reaction system with a slow agitation speed for 30 min to increase the degree of polymerization. The synthesized product at this stage was PPAS. Finally, the mixture of tartaric acid and sodium potassium tartrate as a stabilizer at a specific molar ratio was added to stabilize the PPAS. After aging for 24 hours at room temperature, a liquid PPAS was produced.
Characterization of PPAS
The samples of PPAS were dried in an oven at 50 °C and ground into powders. The powders were mixed with potassium bromide (KBr) and the respective pellets suitable for FT-IR analysis were prepared. The FT-IR spectrum in the range of 4,000–400 cm−1 was recorded on a Perkin-Elmer Spectrum One B spectrometer. Then, the powders were characterized by SEM to examine their surface morphology, which was obtained on a JEOL JSM-6380LV electron microscope instrument.
Al(III) speciation distribution analysis
The Al(III) hydrolysis species distribution in PPAS solution was measured by timed complexation spectroscopy involving the reaction between Al and the Ferron reagent, following the detailed operational process reported by Hu et al. (2001). Visible light absorbance was measured as a function of time at a wavelength of 365 nm to quantify the amount of metal complexes. The Al(III) hydrolysis species reacting with the Ferron reagent in 1 min is considered to be the Al(III) monomeric species (denoted as Ala), and the Al(III) species reacting with the Ferron reagent over the next 2 hours is considered to be the Al(III) medium polymeric species (denoted as Alb). Any Al(III) that remained unreacted after 2 hours is considered to be precipitated Al(III) (denoted as Alc).
The coagulation experiments were carried out using a program-controlled jar test apparatus (ZR4-6, Zhongrun Water Industry Technology Development Co. Ltd, China) at room temperature. An amount of 1,000 mL of water sample was transferred into a beaker; under rapid stirring of 300 r/min, the predetermined amount of coagulant was dosed. After 1.5 mins, the speed was changed to a slow stirring speed of 70 r/min for 15 mins; after quiescent settling of 30 mins, samples were collected from 2 cm below the surface for water quality analysis.
The dosage of coagulant was calculated in this study by using an effective component of Al(III). The Al(III) was measured using aurintricarboxylic acid triammonium salt by spectrophotometric determination on a TU-1901 spectrophotometer (Puxi Analytic Instrument Ltd of Beijing, China). The turbidity of the supernatant was measured with a turbidity meter (HACH 2100Q, HACH Company, USA). Ultraviolet–visible adsorption at 254 nm (UV254) was also measured. TDN was digested by alkaline potassium persulphate and determined by an ultraviolet spectrophotometric method (Chinese Standard GB 11894-89). For analysis of TDN and UV254, all water samples were filtered through a membrane (0.45 μm).
RESULTS AND DISCUSSION
Characterization of PPAS
FT-IR and SEM instruments were used to analyze and characterize the structure and morphology of PPAS, respectively. In addition, Al-Ferron analysis was employed to examine the Al(III) hydrolysis species distribution.
FT-IR spectra analysis
The possible chemical bonds present in PPAS and AS were examined using FT-IR in the range of 4,000–400 cm−1 with KBr as dispersant.
Figure 1 shows absorption peaks at 2,072 cm−1 for PPAS and 2,065 cm−1 for AS, which were assigned to the HSO4− group. FT-IR spectral studies have revealed that the characteristic absorption peak for sulphate is evident in the range of 900–1,200 cm−1 (Frost et al. 2003). The sulphate structure is tetrahedral and it exhibits symmetrical and anti-symmetrical stretching vibration with IR activity. The characteristic absorption peaks at 914 cm−1 for PPAS and 940 cm−1 for AS were attributed to SO42− symmetrical stretching vibrations. There were other absorption peaks at around 1,110 cm−1 for PPAS and around 1,098 cm−1 for AS, which are characteristic frequencies for SO42−. Figure 1 shows that there was a significant difference in the spectrum of PPAS at wave numbers of 1,244 cm−1 and 690 cm−1. The absorptions at these frequencies were assigned to the –P = O, PO43− stretching and bending vibrations, which were only present in PPAS. In summary, the IR spectrum indicates that PPAS may generate a number of surface-active substances such as hydroxyl-sulphate. On the other hand, for PPAS the presence of PO43− generated a new class of highly charged poly-nuclear poly-hydroxyl complexes.
The surface morphologies of PPAS and AS were analyzed using SEM. The PPAS used in this experiment was a product with fixed component ratios at n(P)/n(Al) ratio of 0.15, n(OH)/n(Al) ratio of 0.2 and stabilizer dose of 7.5%.
PPAS coagulation for source water
Effect of n(OH)/n(Al)
During preparation of PPAS, the hydrolysis of Al(III) and hydroxyl resulted in a series of multi-hydroxyl polymeric compounds. The hydrolysis and polymeric reactions were both affected by Al(III) concentration and pH value. During the hydrolysis, the increase of pH could promote the hydrolysis and polymeric reaction. However, if hydroxyl content was over-dosed, it would lead to a hydroxyl aluminum precipitation. Thus an appropriate amount of n(OH)/n(Al) molar ratio is conductive to the preparation, generating a stable product.
The effect of n(OH)/n(Al) on Al(III) species distribution in PPAS was investigated with the results shown in Table 2. According to the previous report about Al(III) species in AS (Tang 2006), it was mainly composed of Ala with lesser amounts of Alb and Alc. Similar results were found in this study, which indicated that the Al(III) species in AS consists of 90.2% Ala, 8% Alb, and 1.8% Alc, with the increase of n(OH)/n(Al) molar ratio. As a result, most Ala of the AS was transferred to the Alb and the Alc of the PPAS, which showed that a higher n(OH)/n(Al) molar ratio could increase the polymeric degree content of intermediate complex and high molecular compounds.
|n(OH)/n(Al) .||Al(III) species percentage (%) .|
|n(OH)/n(Al) .||Al(III) species percentage (%) .|
Figure 3 shows that as the n(OH)/n(Al) molar ratio increased in the range of 0.05–0.10, the residual turbidity at PPAS dosage of 2 mg/L increased and then decreased as the PPAS dosage increased further. The residual turbidity reached the minimum at n(OH)/n(Al) molar ratio of 0.20. At PPAS dosages of 1 mg/L and 4 mg/L, the residual turbidity decreased as the n(OH)/n(Al) molar ratio increased. Therefore, the appropriate n(OH)/n(Al) molar ratio was 0.20, at which the residual turbidity achieved 0.42 NTU. The coagulation–flocculation was affected by the Al(III) species in PPAS. It showed that PPAS at n(OH)/n(Al) molar ratio of 0.20 has the highest Alc and Alb values and it could present a better removal of turbidity than others. Alb is known as the most active coagulant component in aluminum salt coagulants, and is responsible for coagulation performance. Alc is a mixture of Al(III) high molecular weight polymers and Al(III) colloids, which can absorb more contaminants, and plays an important function in the adsorption-bridging role between the colloids and coagulant molecules, thus improving flocculation efficiency. As shown in Table 2, for the PPAS with n(OH)/n(Al) molar ratio of 0.05, although the Alb content was relatively higher, the Alc content was very small, and its coagulation–flocculation performance was not superior to others. Therefore, Alc played a significant role in the coagulation–flocculation process for turbidity removal.
Effect of n(P)/n(Al)
This reaction mechanism indicates that the phosphate is one of the important species components of PPAS by replacing the partial hydroxyl during preparation of PPAS. The presence of phosphate changes the Al(III) species distribution, especially slowing the hydrolysis of Al(III) and thus improving the stability of PPAS. The Al(III) species distribution is shown in Table 3.
|n(P)/n(Al) .||Al(III) species percentage (%) .|
|n(P)/n(Al) .||Al(III) species percentage (%) .|
Table 3 shows that phosphate could enhance the Alb content in PPAS, which shows that the Al(III) species distribution in PPAS was changed due to the presence of phosphate, which produced a new class of multi-core phosphate intermediate complex with high charge.
An appropriate amount of phosphate introduced into PPAS to replace partial hydroxyl to attend the polymeric reaction could improve the coagulation–flocculation performance of PPAS and its stability. For example, the presence of phosphate in polymeric aluminum chloride (PAC) could significantly improve PAC performance (Zheng et al. 2011a; Zheng et al. 2011b), whereas there is a need to add an appropriate amount of phosphate. Compared with the PAC, PAS was more unstable. The presence of phosphate would be helpful in improving the stability of PAS.
Figure 5 shows that as the n(P)/n(Al) molar ratio increased from 0.05 to 0.15, the residual turbidity at PPAS dosages of both 1 mg/L and 16 mg/L decreased and then increased as the dosage increased further. The residual turbidity reached the minimum at an n(P)/n(Al) molar ratio of 0.15. Therefore, the appropriate amount of phosphate was at this n(P)/n(Al) molar ratio. Coagulation–flocculation was significantly affected by the PPAS species. Al(III) hydrolysis species distribution in PPAS (see Table 3) showed that Ala was the majority in PPAS with the n(P)/n(Al) molar ratio ranging from 0.05 to 0.15. As the n(P)/n(Al) molar ratio increased, Alb increased significantly with the highest value of 40.39% at an n(P)/n(Al) molar ratio of 0.15. Table 3 shows that the highest Alc value in PPAS was at an n(P)/n(Al) molar ratio of 0.1. However, the minimum residual turbidity was achieved at an n(P)/n(Al) molar ratio of 0.15. Although the PPAS had a smaller Alc value (11.04%), its Alb was relatively high (40.39%) at an n(P)/n(Al) molar ratio of 0.15. This implies that coagulation–flocculation for turbidity removal mainly relied on Alb. This result combined with the study of the n(OH)/n(Al) molar ratio on turbidity removal indicated that the Alb and Alc were both the decisive factors of coagulation–flocculation and Alb showed a more significant effect than Alc.
Effect of stabilizer dosage
A higher n(OH)/n(Al) molar ratio could induce hydroxyl aluminum precipitation. The presence of some organic compounds would be able to make PPAS more stable. Compared with previous studies on the stability improvement of PAS, the introduction of phosphate and organic compounds are more convenient and simple. For example, sodium silicate and AS have been mixed to form a polymeric aluminum silicate sulphate under the condition of effective acid control (Wei & Gu 2005). The main drawback for this preparation is that the control of polysilicic acid formation as well as reaction with AS is hard work.
Effect of coagulant dosage
The effect of PPAS dosage on residual turbidity, UV254 and TDN removal efficiencies were investigated. The PPAS used in this experiment was a product with a fixed n(P)/n(Al) ratio of 0.15, n(OH)/n(Al) ratio of 0.2 and stabilizer dose of 7.5%. The quality of the raw source water indicated that the turbidity, pH, UV254 and TDN were 23.90 NTU, 7.29, 0.072 and 5.81 mg/L, respectively. The experimental result is shown in Figure 7.
Inorganic metal coagulant has long been considered as an important phosphorus removal agent. However, for removal of other contaminants such as dissolved organic carbon (DOC) and TDN, removal performance was largely dependent on the type of coagulant, dosage, pH and the specific environmental conditions (Matilainen et al. 2010). In this study, the effects of PPAS dosage on the removal of UV254 and TDN were investigated. UV254 was in general linear to the DOC, which was a potential indicator of DOC for rapidly assessing and controlling water quality (Sen Kavurmaci & Bekbolet 2014). TDN was a mixture of dissolved organic nitrogen (DON) and inorganic nitrogen (DIN). It has been known to affect water quality and be a potential threat to human health as a precursor of nitrogenous disinfection by-products, especially DON (Chuang & Tung 2015). The coagulation–flocculation process has been considered effective for carbon compound removal; however, for nitrogen organic compounds, it has not been as effective as the former is. The results showed that in the investigated PPAS dosage range, the removal efficiencies of UV254 and TDN increased significantly. At a PPAS dosage of 8 mg/L, the UV254 and TDN removal rates could achieve 38% and 35%, respectively.
Effect of pH
Figure 9 shows that as the pH increased from 5 to 7, the residual turbidity decreased and then increased as pH increased further. The UV254 removal efficiency achieved the maximum at pH 6; it decreased with either increasing or decreasing the pH from there. A better coagulation for UV254 and turbidity was obtained in the pH range of 6–8. For coagulation of TDN, a decrease in the removal efficiency was found as the pH increased from 5 to 9. At pH 9, coagulation was ineffective for TDN. This could be attributed to the depletion of the effective Al(III) species because of the formation of aluminum hydroxide precipitate. At pH 5, TDN removal efficiency achieved the maximum of 64%. However, the removal rates of UV254 and turbidity were not ideal at this pH point. At pH 6, UV254 removal efficiency reached the maximum and the lowest residual turbidity value was achieved. From the above results, it was concluded that the coagulation–flocculation effects on carbon and nitrogen compounds, respectively, were different. In this study, the pH value of 6 was considered as the optimum condition based on the removal efficiencies of turbidity, UV254 and TDN. A further study of the pH effect on TDN removal is needed in the future. Meanwhile, preparing a modified PPAS with its species favorable to the efficient removal of both the carbon and nitrogen compounds under the same environmental condition will be an interesting research direction.
In this study, a modified coagulant PAS, polymeric phosphate-aluminum sulphate, was prepared. Parameters affecting PPAS performance, such as n(P)/n(Al) molar ratio, n(OH)/n(Al) molar ratio, and stabilizer dose containing a mixture of tartaric acid and sodium potassium tartrate, were examined. The n(P)/n(Al) molar ratio of 0.15, n(OH)/n(Al) molar ratio of 0.2 and stabilizer dose of 7.5% were found to be optimum for stabilization of the PPAS. The PPAS had more morphology compression, which was distinct from the AS product. The influence of PPAS coagulation performance analyzed through the species distribution indicated that Alb and Alc in PPAS played a significant role, especially the Alb. At pH 6, the coagulation system with PPAS showed excellent removal rates of source water turbidity, UV254 and TDN.
This research was financially supported by the National Natural Science Foundation of China (nos 51408215 and 41502331) and supported by the Research Foundation of the Education Bureau of Hunan Province, China (grant no. 14B059) as well as Hunan Graduate Innovation Projects (nos CX2015B499 and CX2015B503).