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Effects of pH on coagulation behavior and floc properties in YellowRiver water treatment using ferric based coagulants
CAO BaiChuan1, GAO BaoYu1*, XU ChunHua1, FU Ying2 & LIU Xin1
1
Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University,
Jinan 250100, China;
2
School of Civil and Architecture, Jinan University, Jinan 250022, China
Received June 13, 2009; accepted October 19, 2009
Enhanced coagulation is one of the major methods to control disinfection by-products
(DBPs) in water treatment process. Coagulation pH is an important factor that affects the enhanced coagulation. Recently, many studies focus on the coagulation effects andmechanisms, and few researchers studied the properties of flocs formed under different coagulation pH. Two inorganic polymercoagulants, polyferric silicate sulphate (PFSS) and polyferric sulphate (PFS), were used in Yellow River water treatment. The influence of pH on coagulation effect was investigated under the optimum dosage, and the results show that both coagulants gave excellent organism removal efficiency when pH was 5.50. According to the variation of zeta potential in coagulation process, coagulation mechanisms of the coagulants were analyzed. An on-line laser scatter instrument was used to record the development of floc sizes during the coagulation period. For PFSS, pH exerted great influence on floc growth rates but little influence on formed floc sizes. In PFS coagulation process, when pH was 4.00, PFS flocs did not reach the steady-state during the whole coagulation period, while little difference was observed in floc formation when pH was 5.50 and above. The preformed flocs were exposed to strong shear force, and the variation of floc sizes was determined to evaluate the influence of pH on floc strength and
re-growth capability. In comparison of the two coagulants, PFS flocs had higher floc strength and better recovery capability when pH was 4.00, while PFSS flocs had higher floc strength but weaker recovery capability when pH was 5.50 and above.
ferric based inorganic polymer coagulants, pH, coagulation effect, floc formation, floc breakage and re-growth
behavior and floc properties in Yellow River water treatment using ferric based coagulants.
Chinese Sci Bull, 2010, 55: 1382?1387, doi: 10.1007/s11434-010-0087-5
Enhanced coagulation is one of the processes recommended by USEPA to remove natural organic matters (NOM) in surface water and to control disinfection by-products (DBPs) [1]. The development of new types of coagulants with high coagulation effect is one of the hot topics in the field of enhanced coagulation [2]. In the field of drinking water treatment, many studies focus on ferric based inorganic polymer coagulants, because of their advantages in removing dissolved
organic compounds, such as high removal efficiency, low bio-toxicity, high coagulation rate and large flocs, excellent sedimentation and dewatering abilities [3]. Polyferric silicate sulphate (PFSS) and polyferric sulphate (PFS) are two kinds of typical coagulants [4,5], which can achieve good removal of natural organic compounds. As the ferric coagulants are dosed, the organic compounds in surface
water are removed in the following ways [6]. The coagulants may react with the functional groups of organic compounds to form complexes or chelates, and part of the organic compounds could be captured by absorption and sweep flocculation. The coagulation pH is one of the main factors that influence the removal effect of enhanced coagulation, and for ferric coagulants the optimum pH range is 5.5?6.5 [7].
Floc breakage is accompanied with the growth and transmission of flocs in coagulation process, and the fragCAOBaiChuan, et al. Chinese Sci Bull May (2010) Vol.55 No.14 1383 mentation of flocs influences the efficiency of solid/liquid separation and follow-up treatment processes [8]. The degree of floc breakage is related to the applied shear force, shear period and floc strength. Under smaller hydrodynamic conditions, the broken flocs have certain capability for re-growth.The extent of re-growth depends on the mechanism of floc formation [9,10].Recently, many studies focused on the breakage and re-growth capability of flocs formed by traditional inorganic coagulants [11,12], and there have been reports about researches
on the floc properties of organic flocculants and polyaluminum chloride [10,13]. For ferric based inorganic polymer coagulants, Wei et al. [14] studied the size distribution
and fractal of flocs of polyferric chloride and different polyferric-cationic polymer dual-coagulant in humic acid solution. Fu et al. [15] quantitatively studied the size
distribution of residual particles and the influence of turbulent shear on the flocs in the poly-silicic-ferric coagulation process. There has been no report about the influence of pH on floc formation, floc breakage and re-growth capability for ferric polymer coagulants. Two inorganic polymer coagulants, polyferric silicate sulphate (PFSS) and polyferric sulphate (PFS), were selected in this study to treat the Yellow River water, and the influence of pH on coagulation effects was investigated. A laser diffraction particle size analyzer was used to determine the particle size of flocs on-line, and the floc formation process, floc breakage and re-growth capability were evaluated under different pH conditions.
1 Materials and methods
1.1 Materials
All reagents used were analytical grade chemicals. Deionized water was used to prepare all solutions.The raw water used in the study was sampled from Luokou section of the Yellow River in November 2008, and it is characterized by high turbidity, high organic carbon and high alkalinity. The raw water was allowed to settle for 24 h, and the supernatant was then withdrawn by siphon from about 20 cm below the water surface and stored in refrigerator for subsequent test. The properties of the supernatant are as follows: temperature=12.4?13.0°C, pH = 8.07?8.23, turbidity=
11.3?13.0 NTU, UV254=0.049?0.057 cm?1, hardness=260.3?291.4 mg CaCO3/L.
1.2 Preparation of coagulants
PFSS was prepared using polysilicic acid (PS), FeSO4·7H2O and NaClO3 as raw materials. First, PS solution with a SiO2 concentration of 5% (w/w) was prepared as follows:14.20 g Na2SiO3·9H2O was dissolved in 70 mL deionized water and then introduced slowly to H2SO4 solution (1:3) under magnetic stirring at room temperature to obtain a polysilicic acid solution (PS). The pH of the system was controlled at 1.50 and the PS solution was aged for 2 h.Then, 13.90 g FeSO4·7H2O was added to the PS solution to yield the Si/Fe ratio 1:1, and mixed rapidly at 40°C. Subsequently, 0.89 g NaClO3 was added as an oxidant. PFSS samples were taken out after 3 h’ aging, and then were diluted to 10 g Fe/L.Commercial solid polyferric sulfate (PFS, Fe concentration: 21% (w/w), B=12%) was purchased from Zibo Tianshui Chemical Industrial Co., Ltd, China.
1.3 Coagulation test
Coagulation tests were conducted on a jar tester (ZR4-6, Zhongrun Water Industry Technology Development Co.Ltd., China). One liter of water was transferred into each beaker. The jar test involved 1 min rapid mix at 200 r/min, a 15 min 40 r/min coagulation stage, and a 15-min settlement period. The samples was collected from 2 cm below the surface and a portion was filtered through a 0.45 μm membrane to measure the absorbance at 254 nm wavelength (UV254) using a JH-752 UV/VIS spectrophotometer (Jinghua Scientific Instrument Co. Ltd., Shanghai, China). The residual turbidity was measured via a 2100P turbidimeter (Hach Co. US), and a Zetasizer 3000HSa (Malvern Instruments, UK) was used to measure the zeta potential.
1.4 Floc breakage and re-growth
A 15 min stirring phase at 200 r/min was introduced following the flocculation phase, and the preformed flocs were broken. After the breakage period, a slow stir at 40 r/min was reintroduced for a further 15 min to allow flocs reaggregation. The variation of floc sizes was recorded on-line via a laser diffraction instrument Mastersizer 2000 (Malvern).
2 Results and discussion
2.1 Influence of pH on coagulation effects
Initially, coagulation optimization tests were carried out to ascertain the optimum dosage for turbidity and UV254 removal under the initial pH. For both of the coagulants used in this study, the results show that excellent turbidity removal effects were obtained at lower coagulants dosages. PFS gave optimum UV254 removal in the dosage region of 8?12 mg/L, and the highest UV254 removal efficiency of PFSS occurred when the dosage was 9?12 mg/L. Therefore, the 10 mg/L coagulant dosage was chosen for all the subsequent experiments. HCl and NaOH solutions with concentrations of 0.1 and 0.01 mol/L were used to adjust the sample pH, and the
1384 CAO BaiChuan, et al. Chinese Sci Bull May (2010) Vol.55 No.14 coagulation experiments were carried out on the jar tester with the coagulant dosage of 10 mg/L. Figure 1 shows the variation of UV254 and turbidity removal efficiencies under
the influence of pH. It was found that, for both of the coagulants, the residual turbidity of the effluence decreased with the increasing pH. When the coagulation pH was higher than 5.50, the residual turbidity dropped below 1 NTU. For UV254 removal, when the coagulation pH was less than 5.50, the removal efficiency of each coagulants increased as pH increased, and declines followed with pH increasing from 5.50 to 9.00. Excellent UV254 removal effect was obtained for PFSS in the 5.50?6.00 coagulation pH range, and the highest removal efficiency was 57.69% when pH 5.50. PFS had the optimum UV254 removal effect when pH was 5.00?5.50, and the highest efficiency was obtained at pH 5.50. Comparing PFSS and PFS, the former had better turbidity removal effect under the same coagulation pH. In the pH range of 5.50?7.50, PFSS showed higher UV254 removal efficiency, while PFS had better performance under acidic and alkaline conditions.The variations of zeta potential of flocs formed by PFSS and PFS under different pH conditions are shown in Figure 2. It can be seen that the zeta potential of flocs formed by PFSS and PFS under acidic condition was positive, and the positive charges decreased with the pH increasing. When pH was higher than 6.00, the surface electrical property became negative, which proved the weaker charge neutralization capabilities of the coagulants under alkaline condition.
Compared with PFSS, PFS had stronger charge neutralization capability and the value of zeta potential was higher at the same pH. Both coagulants reached the isoelectric
points when pH was 5.50?6.00, and both of them had excellent performance in turbidity and UV254 removal.When the coagulation condition is acidic, the density of
positive charges in the surfaces of coagulants hydrolysates is quite high. Charge neutralization and complex reaction or chelate reaction between ferric pieces and organic compounds dominate the coagulation mechanism [6]. At the isoelectric point, the colloid surface is electroneutral and the combined action of charge neutralization and chelate reaction results in higher organic removal efficiency. This may be the main reason for that both of the coagulants achieved optimum UV254 removal effect when pH 5.50. However, both coagulants did not perform well in organic matter removal when pH 4.00. It may be explained by the fact that when the coagulation condition is acidic, the carboxyl groups of organic compounds are difficult to be hydrolyzed due to the high concentration of H+; furthermore, H+ could react with organic molecules which competed with the complex reaction between ferric pieces and organic compounds [16]. Under neutral and alkaline conditions, parts of the colloid particles and organic compounds are absorbed by the hydrolysis products of coagulants, and parts of them are swept and coprecipitated with the coagulants hydrolysates.In this case, the reaction between OH? and ferric pieces interferes with the formation of ferric-organism complexes [17], and thus organic compounds removal efficiencies were lower.
2.2 Influence of pH on floc formation
The particle diameters in the suspension were determined on-line via a laser diffraction instrument Mastersizer 2000 during the coagulation period. Figure 3 shows the variation of median particle diameters (d0.5) of the flocs formed by PFSS and PFS at different pH versus coagulation time (0?16 min).The floc formation process can be considered to be a rapid growth stage followed by a stable growth stage. The floc growth rate can be expressed by the steady-state floc sizes (d0) and the growth time needs to reach the steadystate (Table 1). The results show that pH had a great influence on floc growth rate in PFSS coagulation process.When pH was 4.00, after coagulation for 6.5 min, the d0.5 of PFSS flocs reached the steady size of 906.8 μm, and growth rate was 136.0 μm/min. Increasing coagulation pH, the growth time of PFSS flocs became shorter while the growth rate increased. Increasing pH to 9.00, the growth time of PFSS flocs was 3 min, with a growth rate up to 303.0μm/min. For PFS, the floc growth rate was lower when pH 4.00, and did not reach the steady-state during the 16 min coagulation period, while pH had little effect on floc formation
when the coagulation pH was 5.50 and above. Comparing the two coagulants, the floc sizes of PFSS were larger and the steady d0.5 was about 900 μm, while the floc
sizes of PFS were around 500 μm. Under the same coagulation pH conditions, the growth time of PFSS flocs was shorter than that of PFS flocs, and the growth rate was 3?5 times that of PFS flocs.Floc formation process is related to the coagulation mechanism. The particle sizes of colloids in raw water are distributed in the range of 0.2?20 μm, and the median particle diameter was 3.2±0.5 μm. At the moment when coagulants are dosed, the colloid particles in the raw water contact with the ferric pieces rapidly, and the colloids were destabilized because of the charge neutralization. During the rapid stirring period, the unstable colloids collide with each other and aggregate into large flocs. In the slow stirring period, flocs grow in the following ways: parts of the organic compounds and colloid particles are absorbed by the coagulants hydrolysates, and parts of them are swept with the coagulants hydrolysates. Take PFSS as an example, flocs grew slowly when pH 4.00 because of the stronger charge neutralization capability but weaker sweep capability of the coagulant hydrolysis products. After stirring at 40 r/min for 5 min, the particle diameters of the suspension are distributed in the range of 30?1200 μm, and d0.5 was 361.3 μm.
Under the alkaline conditions, the hydrolysates of PFSS coagulants swept the colloid particles and micro-flocs, and aggregates with large sizes were formed rapidly. When the coagulation pH was 9.00, flocs with d0.5 of 912.2 μm were formed and most of the particle sizes ranged from 460?1500 μm.
2.3 Influence of pH on floc breakage and re-growth
After the slow stirring phase, the preformed flocs were exposed to a stronger shear force of 200 r/min for 15 min, and the flocs were broken. Then slow shear speed of 40r/min was reintroduced for a further 15 min, and the broken flocs began to reaggregate. The variation of d0.5 during the whole period is displayed in Figure 4. As can be seen, applying the shear force for 30 s, the d0.5 of flocs dropped drastically, and then a gentle decrease followed. When the shear speed was slowing down, the broken flocs reaggregated and reached a
new steady-state. The breakage and re-growth capability are usually evaluated by floc breakage factor (Bf) and recovery factor (Rf) bymany researchers, which are defined as follows ：
（1）
（2）
where d0 is the average floc size of the steady phase before breakage, d1 is the floc size after floc breakage, and d2 is the floc size after reaggregating to a new steady phase.The breakage and recovery factors of flocs preformed by PFSS and PFS under the experimental pH conditions are listed in Table 2. The results show that, with the same shear force and shear period, the breakage factors of flocs were related to the coagulation pH. The higher the pH value, the larger the floc breakage factors. The d0.5 of flocs formed by PFSS at pH 4.00 decreased from 906.8 μm to 294.5 μm
after exposure to 200 r/min shear speed for 15 min, and the breakage factor was 68.6%. When the coagulation pH was 9.00, the d0.5 was 214.4 μm and Bf was 76.0%. Compared with PFSS, PFS flocs had stronger floc strength when pH 4.00, and the Bf was 16.7%, while PFS flocs formed at higher pH gave higher breakage factors than PFSS.The broken flocs which reaggregated after slow stir were reintroduced, and the recovery degree was related to the floc formation conditions. The PFS flocs which were preformed at pH 4.00 had the strongest re-growth capability; the d0.5 of recovered flocs was 514.9 μm, which was larger than the 328.4 μm d0.5 of un-broken flocs, and the calculated recovery factor was 582.4%. With the coagulation pH increasing, the Rf of PFSS flocs increased at first and then decreased.When pH 4.00, the PFSS flocs had the largest Rf, i.e. 33.2%,while the recovery factor was 21.6% at the 7.00 coagulation pH. Compared with PFSS flocs, the flocs formed by PFS had larger recovery factors under the same pH, which indicated stronger re-growth capabilities.Floc breakage and re-growth are related to floc formation processes. As shown in Figure 2, under the same coagulation pH, the zeta potentials of PFS flocs were much higher than those of PFSS flocs, which indicates stronger charge neutralization abilities. Because of the larger molecular weight, special ramal sharp and larger surface areas of hydrolysis products [20,21], the absorption capability of PFSS is much stronger than that of PFS. Charge neutralization dominates the PFS coagulation mechanism, while the main coagulation mechanisms of PFSS are the absorption of organic compounds and sweep flocculation. This may be the main reason for that PFSS flocs and PFS flocs had different performance during the breakage and re-growth periods. When charge neutrality dominates the coagulation mechanism, the negative charges in the colloid surfaces are neutralized
by the positive charged coagulants and the destabilized colloids aggregate to form flocs.
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