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Effect of dose methods of a synthetic organic polymer and PFC on floc properties in dyeing wastewater coagulation process
Chemical Engineering Journal 243 (2014) 169–175
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Effect of dose methods of a synthetic organic polymer and PFC on floc properties in dyeing wastewater coagulation process Hongyan Rong, Baoyu Gao ⇑, Ruihua Li, Yan Wang, Qinyan Yue, Qian Li Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, No. 27 Shanda South Road, Jinan 250100, Shandong, People’s Republic of China
h i g h l i g h t s Papermaking sludge was recycled to synthetize a flocculant. Polyferric chloride (PFC) was prepared using spent pickling liquor. The products from industrial wastes were used in treating dyeing wastewaters. Different dose methods of coagulant/flocculant were studied. Floc properties were comparatively investigated.
a r t i c l e
i n f o
Article history: Received 5 September 2013 Received in revised form 29 November 2013 Accepted 2 December 2013 Available online 10 December 2013 Keywords: Papermaking sludge Spent pickling liquor Second addition Dual coagulant Floc properties
a b s t r a c t Papermaking sludge and spent pickling liquor were recycled to synthetize an organic polymer flocculant (LA) and polyferric chloride (PFC), respectively. They were used in synthetic dyeing wastewater treatment – disperse yellow (DY) and reactive blue (RB). Effect of PFC, dual-coagulant (PFC + LA) and second dose of LA after breakage period (PFC LA) on color removal, floc regrowth and floc structure were comparatively investigated. The result showed that PFC could achieve excellent color removal, around 90% for the two dyes at optimal dosages. LA addition after PFC gave a little higher color removal efficiency but much larger flocs. Specifically, ratio values of PFC–flocs for DY and RB were approximately 1.43 and 0.59 respectively, compared with 1.75 and 0.72 for flocs formed by PFC + LA. Floc recoverability was also much higher at the usage of LA when there was a breakage period, and they were in the order of PFC LA > PFC + LA > PFC. Additionally, comparing with those of PFC, roughly opener flocs were reformed by PFC LA and PFC + LA after breakage because of LA bridging. For instance, TWV values (%) were 10.24, 13.24 and 16.16 respectively for PFC, PFC + LA and PFC LA in RB treatment. LA favored charge neutralization, adsorption and bridging in the coagulation process. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Papermaking industry is one of the significant industries in many countries. Vast wastewater is produced during the production process [1]. Because of high concentration and low biodegradability of organic matters in the suspension, pretreatment processes, such as sedimentation and coagulation, are requested. After coagulation, part of the matters precipitate, which contribute to the papermaking sludge. Traditionally, papermaking sludge has been disposed of through land filling. However, landfill is currently unacceptable due to environmental deterioration (e.g. polluting underground water) and competition for land [2]. Over the past decades, researchers have been concentrating on recycling papermaking sludge because of the rich content of organic matters ⇑ Corresponding author. Tel.: +86 531 88366771; fax: +86 531 88364513. E-mail address: [email protected] (B. Gao).
[3,4]. Lignin is a kind of natural organic matter with a great variability of highly active functional groups, such as carboxylic groups, alcoholic hydroxyl groups and phenolic groups. So it is an attractive macromonomer for polymer synthesis [5]. Fang et al. [6] have reported a kind of cationic flocculant by grafting acetone and formaldehyde onto hydroxymethylated lignin. According to the above discussion, papermaking sludge can be recycled to prepare a flocculant by utilizing the contained lignin. Spent pickling liquor is the industrial byproduct from removing oxides and scale of metal surface using acid in steel plants. It is characterized by extremely acidic and highly containing heavy metals. This kind of liquor would cause serious harms to environment and human in terms of corroding pipes, destroying the healthy conditions of soil and harming people’s health, if it is unreasonably disposed of. Traditional methods of disposal include electrodialysis, membrane distillation, selective precipitation and anion exchange/membrane electrowinning [7], through which iron
1385-8947/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.12.005
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products and clean water could be obtained. However, high cost would be expended during these procedures. Therefore, other methods for fully reusing the spent pickling liquor should be promoted. Lan et al. [8] provided a method using the spent pickling liquor to prepare a composite inorganic polymer coagulant-PFAC, through which the liquor was totally utilized. Dyeing wastewater, which comes from textile industry, can cause serious environmental pollution in terms of toxicity, aesthetics, carcinogenicity and teratogenicity due to high chroma and COD, thereby being listed as one of the industrial wastewaters that are difficult to be treated [9]. Biochemical treatment method is considered to be efficient in BOD and color removal but unsatisfied due to dyes’ recalcitrance [10]. So pretreatment is needed. Coagulation is proved to be a method of low cost and considerably efficient in decoloration and COD removal [11,12], and it is widely employed as pretreatment process in dyeing wastewater treatment [13,14]. Inorganic metal salts, such as iron and aluminum salts, are the most common coagulants to promote the aggregation of particles and thus have the impurities removed [15]. Recently, a growing attention is being paid on natural/synthetic organic polymers in the aim of better floc properties and lower dosage. They are usually used as flocculant or coagulant aid for inorganic coagulant, and have been proved to achieve better flocs characteristics in terms of size, strength and settling ability as well as better resistance to pH changes, which can improve coagulation efficiency and thus lower the coagulation cost [16–18]. Additionally, the operation parameters of coagulation, for instance, stir time, hydraulic condition and coagulant/flocculant dosing method, also could influence coagulation efficiency. There are researches that have drawn conclusions that two-stage addition of aluminum could give better floc recoverability [19,20]. However, second addition of an organic polymer on the basis of metal salts has not been studied. In this study, papermaking sludge and spent pickling liquor were recycled to prepare an organic polymer flocculant (LA) and polyferric chloride (PFC) respectively that used in dyeing wastewater treatment. In this way, we achieve a goal of using industrial waste to control water pollution. The aim of the study is to investigate the effect of different dosing methods of LA and PFC on coagulation performance and floc properties. The three dose methods were single use of PFC, LA used as aid for PFC (PFC + LA) and second dose of LA after breakage stage on the basis of PFC (PFC LA). Floc properties related to floc size, growth rate, strength, recoverability, regrowth rate and structure. Coagulation mechanisms were also studied.
2. Materials and methods 2.1. Preparation of PFC and LA PFC with basicity value 0.5 (mole ratio of [OH]/[Fe]) was prepared by spent pickling liquor. The spent pickling liquor was obtained from a steel plant of Shandong Province (China). The content of Fe was determined by 1,10-phenanthroline spectrophotometry and it was 170 g/L. The concentration of hydrochloric acid was about 1.69 mol/L. Firstly, Fe2+ in the spent pickling liquor was oxidized by sodium chlorate. Then, a certain amount of anhydrous sodium carbonate solution was titrated to adjust the basicity value. After that, solid disodium hydrogen phosphate as stabilizer was added and the mole ratio of phosphorus to Fe was 0.08. The mixture was stirred for further 3 h after the stabilizer dissolved. The result of Fe (III) species distribution in PFC is shown in Table 1. The dosage of PFC was calculated as mg/L of Fe in all experiments. A kind of air-dried papermaking sludge was supplied by a paper mill in Shandong Province (China). The polymer LA was
Table 1 Distribution of Fe (III) species in PFC (Fea: monomeric species; Feb: polymeric species; Fec: precipitated species). PFC Fea (%) 40.74
Feb (%) 46.88
Fec (%) 12.38
synthetized by grafting acrylamide onto lignin that contained in the papermaking sludge. The detailed synthesis process can be found in our previous research [21]. 2.2. Zeta potentials of PFC and LA Variation of zeta potential of PFC and LA as a function of pH was measured using a Zetasizer 3000HSa (Malvern Instruments, UK) [22]. Determination experiments of PFC were conducted at dosage of 5 mg/L as Fe. HCl and NaOH solutions (both 0.1 mol/L) were used to adjust pH. 2.3. Dyeing wastewater preparation The disperse yellow dye (RGFL) and reactive blue dye (K-GL) that used in this study represented hydrophobic and water-soluble dyes respectively and they were obtained from Jinan No. 2 Textile Dyeing Mill, China. The name, type, maximum adsorbance wavelength (kmax) and molecular structure are shown in Table 2. The synthetic dyeing wastewater was prepared by dissolving 0.1 g of dye into 1.0 L of tap water. The concentration of the solution was similar to that of actual dyeing wastewater. The pHs of DY solution and RB solution were both 7.72 ± 0.20. The initial maximum adsorbance of DY wastewater was 0.850 ± 0.003 and that of RB wastewater was 1.000 ± 0.010. 2.4. Coagulation jar test Coagulation jar test experiments were performed on a ZR4-6 apparatus (Zhongrun Water Industry Technology Development Co. Ltd., China) at room temperature (about 15 °C). 1 L of dyeing wastewater was used. Coagulant/flocculant was added at rapid stir (200 rpm). After PFC/LA evenly dispersed, slow stir (40 rpm) for 15 min was introduced to make flocs aggregate, followed by sedimentation for 20 min [23]. The upper sample was drawn to measure the absorbance value when coagulation process was finished. In the case of the dual-coagulant PFC + LA, PFC and LA were added in sequence at rapid stir stage. The relationship between adsorbance value at kmax and dye concentration was linear; therefore, color removal efficiency was calculated by comparing the adsorbance value of the treated sample to that of initial water according to the following equation:
Color removal efficiency ð%Þ ¼
A0 A 100% A0
ð1Þ
where A0 and A were absorbance values of raw water sample and treated water sample, respectively. To investigate coagulation kinetic, online monitor was conducted on an optical unit, Photometric Dispersion Analyzer (PDA 2000, Rank Brothers Ltd.). The suspension was continuously drawn through PDA 2000 by a peristaltic pump (LEAD-1, Baoding Longer Precision Pump Co., Ltd., China) at a rate of 1.2 L/h [24]. The inflow/ outflow tube with inner diameter of 3 mm was positioned at a depth just above the paddle in the holding ports. In order to study floc strength and recoverability, rapid stir for 5 min and slow stir for 15 min were successively employed after the floc aggregation [23]. For second addition of LA on the basis of PFC, while PFC
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H. Rong et al. / Chemical Engineering Journal 243 (2014) 169–175 Table 2 Dye characteristics. Name
Type
Molecular structure
Disperse yellow (RGFL)
Azo
445
Reactive blue (KG-L)
Phthalocyanine
598
was dosed at the initial rapid stir, LA was added at the end of breakage period to investigate effect of second addition of LA on floc recoverability. The PDA measures the average transmitted light intensity (DC value) and the root mean square (RMS value). Ratio, the ratio of RMS and DC (RMS/DC), can be read from the computer that connected with the PDA. The Ratio value was strongly related to particle size, providing a useful indication of floc dynamic changes [25]. Monitor result model of PDA 2000 during coagulation is shown in Fig. 1. In the chart, floc growth rate is the slope of growth region and can be calculated as follows:
Growth rate ¼
Dratio Dtime
i¼1 ðratioi timei Þ PN i¼1 timei
ð3Þ
To investigate floc strength and recoverability when exposed to strong shear force, strength factor and recovery factor are adopted [26]:
Ratio2 100 Ratio1
Recovery factor ðRf Þ ¼
ð4Þ
Ratio3 Ratio2 100 Ratio1 Ratio2
ð5Þ
where Ratio1, Ratio2 and Ratio3 are the ratio values in the growthsteady region, after breakage and regrowth-steady state, respectively. 0.8 Steady-state stage Steady-state stage
Ratio
0.6
0.4 Growth stage 0.2
Lag stage 100
Time-weighted variance ðTWVÞ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P .P ffi 2 N N ½ðratio RatioÞ time time i i i i¼1 i¼1 ¼ 100% Ratio
ð6Þ
It is thought that a smaller TWV value signifies tighter and more homogeneous flocs, while a bigger one illustrates flocs with opener and more porous structure. 3. Results and discussion
PN
Strength factor ðSf Þ ¼
The time-weighted ratio variance of the steady-state ratio value, donated as TWV, can be computed as:
ð2Þ
In this system, time-weighted average steady-state ratio value, donated as Ratio, is a very important parameter. It indicates the floc size in the steady-state region and can be calculated as:
Ratio ¼
Wavelength (nm)
200
Regrowth stage Breakage 300
400
500
Coagulation time (s) Fig. 1. The curve of variation of flocs monitored on-line.
600
3.1. Zeta potentials of PFC and LA Zeta potentials of PFC and LA as a function of pH are shown in Fig. 2. Zeta potential of PFC decreased dramatically with pH, and PFC attained its isoelectric point at about pH 7.5. Meanwhile, LA was positively charged, for which LA can play charge neutralization in precipitating pollutants. It can be concluded that charge neutralization of PFC in dyeing water treatment (pH 7.70 ± 0.20) was minor, while sweep and enmeshment dominated. 3.2. Optimizing dosages of PFC and LA Fig. 3 shows that color removal efficiencies for both DY and RB dramatically increased at first and then reached to a plateau, respectively. PFC gave perfect color removals, approximately 90% for DY and 94% for RB, at dosages of 40 mg/L and 50 mg/L. When LA was dosed after PFC, color removal efficiency for DY showed slight increase, less than 20%. For RB, there was almost no improvement in color removal. Zeta potential of the water sample after coagulation was also investigated and the result is presented in Fig. 3. It soared with PFC dosage. This illustrated that positively charged ferric species were growing as PFC dosage increased although most of ferric hydrolyzed under the raw water pH condition, and thus charge neutralization between ferric and negatively charged phenolic hydroxyl group on DY or –SO3- on RB enhanced with the increase of PFC dosage. When LA was dosed after PFC, zeta potential augmented sharply due to the positive charge on LA surface (1.1 mV). In the case of DY treatment, there was no significant difference in zeta potential among different LA dosages. However, it should be observed that a little better color removal was achieved at lower LA dosage (Fig. 3). Therefore, it can be concluded that almost all the negative charges on DY molecules were neutralized by PFC and LA at low LA dosage and charge neutralization was fully functioning at this condition [27]. When LA dosage increased, there was a little excess LA on the surface of DY–Fe microflocs, which could lead to restabilization of colloids, thus decreasing color
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2.2 35 PFC
30
2.0 1.8
Zeta potential (mV)
25
Zeta potential (mV)
LA
20 15 10 5
1.6 1.4 1.2 1.0
0 0.8
-5
0.6
-10 5.0
5.5
6.0
6.5
7.0
7.5
8.0
5
6
7
8
9
pH
pH Fig. 2. Zeta potentials of PFC and LA as a function of pH.
9.0
120
-14
60
-16 8.6 8.4
Color removal
40
8.2
20 8.0
pH
0
7.8
-20
Zeta potential (mV)
Color removal (%)
80
8.8
pH
100
LA dosage 0 mg/L 0.5 mg/L 1.0 mg/L 1.5 mg/L
-18 -20 -22 -24 -26
-40
7.6
-28
-60
7.4
-30
25
30
35
40
45
50
25
PFC dosage (mg/L) 11
LA dosage 0 mg/L 0.5 mg/L 1.0 mg/L 1.5 mg/L
40
45
50
-2 -4
10 Color removal
9 80 pH
8 70
Zeta potential (mV)
90
35
PFC dosage (mg/L)
pH
Color removal (%)
100
30
-6 -8 -10 -12 -14
7 -16
60 35
40
45
50
55
60
PFC dosage (mg/L)
35
40
45
50
55
60
PFC dosage (mg/L)
Fig. 3. Color removal efficiencies, final pH and zeta potentials for synthetic dyeing wastewaters.
removal efficiency. For the treatment of RB, zeta potential for dosing 1.5 mg/L of LA was the highest while best color removal was got by 1.0 mg/L of LA, which illustrated that complete charge neutralization was given by 1.0 mg/L of LA and a higher dosage of LA could cause excessive charge on microflocs. It also can be seen from Fig. 3 that pH slightly decreased as PFC dosage increased. However, LA addition rarely had any effect on final pH values. According to the above result, the optimal dosages of coagulant and flocculant for the subsequent experiments were as follows: 45 mg/L of PFC and 0.5 mg/L of LA for DY water treatment; 55 mg/L of PFC and 1.0 mg/L of LA for RB water treatment.
3.3. Color removal efficiencies with/without breakage process In order to investigate the effect of breakage process and second addition of LA on coagulation performance, a period of rapid stir followed by slow stir and sedimentation was introduced after flocs’ formation. Color removal efficiencies with/without breakage period are presented in Table 3. It was evident that a period of breakage for PFC and PFC + LA reduced dyes removal efficiencies. The reason was that flocs were usually irrecoverable and the smaller reformed flocs had lower removal efficiencies [28,29]. However, dye removal efficiencies for PFC + LA were a little higher than
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single dose of PFC after breakage and regrowth. Additionally, when LA was dosed at the end of breakage period, color removal efficiencies for DY and RB were both over 90%, about 5% higher than only PFC dosed. The result strongly attributed to different floc properties formed by the three coagulants, which would be discussed in detail later. 3.4. Dynamic change of flocs during coagulation process In order to investigate effect of strong shear force on floc strength and recoverability, four kinds of velocity gradients, 40.3, 70.7, 102.5 and 137.2 s1, were adopted at the breakage stage. Dynamic changes of flocs are presented in Fig. 4. From the profiles, it was evident that different coagulants and coagulation conditions gave different floc sizes, growth rates and regrowth size. For the three dose methods, flocs grew rapidly to plateaus after coagulant/flocculant was dosed. It could be observed that LA addition on the basis of PFC gave larger flocs than PFC dosed alone. In the case of DY, Ratios were about 1.43 and 1.75 respectively for PFC and PFC + LA. For RB treatment, Ratios of PFC and PFC + LA were 0.59 and 0.72, respectively. In addition, growth rate of PFC + LA flocs was higher than that of PFC flocs for both DY and RB (Table 4). There is a large body of researches suggesting that polymer used as coagulant aid could gave larger floc size and rapider growth rate than single use of metal salt [30,31]. This can be explained by the adsorption and bridging ability of LA: dye aggregated into Table 3 Color removal efficiencies (%) with/without breakage period.
Disperse yellow Reactive blue
Without breakage
With breakage
PFC
PFC + LA
PFC
PFC + LA
PFC LA
89.12 93.79
97.12 94.82
85.62 86.43
89.96 88.97
92.90 90.66
(a) 2.5
3.5 PFC 100 rpm 200 rpm 300 rpm 400 rpm
2.0
1.0
1.5 1.0
0.5
0.0 0
500
1000
1500
2000
-0.5
2500
0
500
1000
1500
2000
2500
0
500
Time (s)
1000
1500
2000
2500
2000
2500
Time (s) 1.0
1.0
PFC
Ratio
0.8
0.4 0.2 0.0 0
500
1000
1500
Time (s)
2000
PFC-LA
0.8
100 rpm 200 rpm 300 rpm 400 rpm
0.6
PFC+LA
2500
0.6
0.6
Ratio
0.8
Ratio
1.0
0.0
Time (s)
(b)
1.5
0.5
0.5
0.0
PFC-LA
2.0
2.5
Ratio
1.5
2.5
PFC+LA
3.0
Ratio
2.0
Ratio
microflocs under sweep and complexation after PFC was dosed. Once LA was added, it rapidly adsorbed onto the surface of microflocs formed by PFC and dye. The long-chain LA chains may extend from the particles and link other particles together. Thus, flocs size was enlarged and growth rate was accelerated by LA. Fig. 5 shows the TWV values as a function of coagulation time. Velocity gradient of 70.7 s1 in the breakage period was selected. For DY and RB, TWV of PFC + LA flocs at growth stage was larger than that of PFC. This meant that flocs’ structure became opener when additional LA was dosed. There were strong charge neutralization and adsorption among microflocs and LA, and interior repulsion of flocs was significantly reduced, which would have contributed to compact flocs [32]. There was also research that concluded that bridging flocculation gave the most open structure of flocs among charge neutralization and sweep [33]. Thus, the larger TWV of PFC + LA flocs mainly attributed to the bridging of LA rather than electrostatic force between LA and microflocs and adsorption of LA. When strong shear force was applied, Ratio values dropped and it varied according to different velocity gradients, the larger the velocity gradient was, the smaller the floc ratio was. Sf of PFC and PFC + LA flocs under different shear forces are shown in Table 5. It could be found that PFC–DY flocs were a little stronger than PFC + LA-DY flocs, while flocs formed by PFC were much weaker than those formed by PFC + LA for RB. The result closely related to the different floc sizes formed by PFC and PFC + LA and coagulation mechanisms: For PFC, ferric hydrolyzed into Fe(OH)3 at pH about 7.7, and sweep entrapment and complexation were the dominant coagulation mechanisms [34]. It was reported that flocs with larger size and looser structure were prone to erode [35]. However, it contradicts the finding that that flocs formed under bridging were stronger than sweep and charge neutralization [33]. So according to discussion above, that dual-coagulant flocs for DY were a little weaker than PFC flocs primarily resulted from
0.4
0.4
0.2
0.2
0.0
0.0 0
500
1000 1500 2000
2500
Time (s) Fig. 4. Dynamic changes of flocs during coagulation process.
0
500
1000
1500
Time (s)
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H. Rong et al. / Chemical Engineering Journal 243 (2014) 169–175
Table 4 Growth rates and regrowth rates of flocs for different dose methods. Disperse yellow Velocity gradient (S1) 40.3
Reactive blue Velocity gradient (S1)
70.7
102.5
Growth rate (min1) PFC PFC + LA
137.2
40.3
70.7
0.36 0.44
Regrowth rate (min1) PFC 0.12 PFC + LA 0.23 PFC LA 0.35
0.10 0.19 0.29
0.070 0.16 0.25
Disperse yellow
15 PFC PFC+LA PFC-LA
10 5 0 500
1000
1500
2000
2500
TWV (%)
TWV (%)
20
0
137.2
0.040 0.053 0.19
0.040 0.040 0.18
0.15 0.18
0.11 0.19 0.32
25
102.5
0.036 0.085 0.23
24 22 20 18 16 14 12 10 8 6 4 2 0 -2
0.040 0.075 0.19
Reactive blue
PFC PFC+LA PFC-LA
0
500
Time (s)
1000
1500
2000
2500
Time (s)
Fig. 5. Time-weighted variance as a function of coagulation time.
Table 5 Floc strength and recoverability. DY Velocity gradient (S1) 40.3 Sf PFC PFC + LA
39.86 38.29
Rf PFC PFC + LA PFC LA
34.88 53.70 143.84
70.7
RB Velocity gradient (S1)
102.5
137.2
40.3
70.7
102.5
137.2
29.37 29.71
28.67 25.71
26.57 25.14
32.20 65.28
27.12 55.56
23.73 45.83
20.34 36.11
31.68 39.02 107.87
30.39 36.15 87.13
19.05 30.53 84.87
22.50 68.00 87.50
23.26 46.88 84.09
22.22 41.03 80.43
21.28 26.09 78.26
combination of the porosity of large flocs and LA bridging. For RB treatment, although TWV of PFC + LA flocs was higher than that of PFC flocs because of bridging of LA, the binding bonds of LA chains were strong and there was additional attractive forces between LA and microflocs caused by electrostatic patch, which contributed to much higher Sf value of PFC + LA flocs. Besides, Sf decreased with increase of velocity gradient. It can be inferred that weak bonds in large flocs were broken under high hydraulic shear force, and then strong bonds in small parts ruptured when shear force further enforced. Thus, lower value of Sf at higher velocity gradient was got. Once velocities slowed to the initial value, flocs’ recoverability at different rates was observed. From Fig. 4, flocs did not regrow to their previous sizes except for second addition of LA. Take velocity gradient of 70.7 s1 as an example, recovered values for PFC, PFC + LA and PFC LA in DY treatment were 0.74, 1.00 and 1.50, respectively, and those in RB treatment were 0.26, 0.55 and 0.53.
The regrowth rate and Rf of flocs after breakage are presented in Tables 4 and 5. The floc recoverability gave some indication of internal bonding structure in flocs. Previous researches have shown that charge neutralization could give full recoverability [36]. Therefore, the irreversible breakage indicated that the flocs for the three coagulants were not dominated by single charge neutralization. It was the result of combination of chemical and physical bonds between dye and coagulant. From Table 5, PFC gave the weakest floc recoverability, meanwhile, LA addition greatly enhanced the recoverability, especially for the second addition of LA. Zeta potential of PFC flocs (Fig. 3), the indicator of charge neutralization, was the lowest. So it could be concluded that weak charge neutralization was one of the most important reasons that caused the poor recoverability of PFC flocs. Sweep and entrapment triggered by ferric hydrolysates were the dominant coagulation mechanisms. For PFC + LA, when high shear force was applied, some of the bonds among large flocs formed by LA bridging, and then ones formed by charge neutralization were broken. After shear force slowed, part of LA chains played primary roles in bridging and charge neutralization in binding small fragments together again, while sweep of iron hydrolysates was the secondary reason for floc recoverability. For PFC LA, all the LA molecules worked, in which the role of LA was stronger than that in PFC + LA. Thus, PFC LA flocs had better recoverability than PFC + LA flocs. 4. Conclusions Polyferric chloride (PFC) and an organic polymer (LA) were synthetized by spent pickling liquor and papermaking sludge, respectively. PFC, dual-coagulant and second addition of LA were comparatively investigated in dyeing wastewater treatment in terms of color removal and floc characteristics. While
H. Rong et al. / Chemical Engineering Journal 243 (2014) 169–175
PFC showed perfect color removal for disperse yellow and reactive blue dyes, LA addition after PFC gave slight increases in dye removal efficiencies. Compared with single dose of PFC, much larger flocs were formed by the dual-coagulant at a rapider growth rate. However, the dual-coagulant flocs had more open structure because of bridging of LA chains. And dual-coagulant flocs were a little weaker than the PFC flocs when exposed to high velocity gradient for DY, while they were much stronger than those formed by PFC for RB. Sf decreased as hydraulic shear force increased. Additional dose of LA drastically enhanced floc recoverability and regrowth rate, and they were in the order of PFC LA > PFC + LA > PFC. Recovered flocs had relatively open structure for the three coagulants/flocculants. LA played significant roles in charge neutralization, adsorption and bridging in floc regrowth.
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Effect of dose methods of a synthetic organic polymer and PFC on floc properties in dyeing wastewater coagulation process Hongyan Rong, Baoyu Gao ⇑, Ruihua Li, Yan Wang, Qinyan Yue, Qian Li Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, No. 27 Shanda South Road, Jinan 250100, Shandong, People’s Republic of China
h i g h l i g h t s Papermaking sludge was recycled to synthetize a flocculant. Polyferric chloride (PFC) was prepared using spent pickling liquor. The products from industrial wastes were used in treating dyeing wastewaters. Different dose methods of coagulant/flocculant were studied. Floc properties were comparatively investigated.
a r t i c l e
i n f o
Article history: Received 5 September 2013 Received in revised form 29 November 2013 Accepted 2 December 2013 Available online 10 December 2013 Keywords: Papermaking sludge Spent pickling liquor Second addition Dual coagulant Floc properties
a b s t r a c t Papermaking sludge and spent pickling liquor were recycled to synthetize an organic polymer flocculant (LA) and polyferric chloride (PFC), respectively. They were used in synthetic dyeing wastewater treatment – disperse yellow (DY) and reactive blue (RB). Effect of PFC, dual-coagulant (PFC + LA) and second dose of LA after breakage period (PFC LA) on color removal, floc regrowth and floc structure were comparatively investigated. The result showed that PFC could achieve excellent color removal, around 90% for the two dyes at optimal dosages. LA addition after PFC gave a little higher color removal efficiency but much larger flocs. Specifically, ratio values of PFC–flocs for DY and RB were approximately 1.43 and 0.59 respectively, compared with 1.75 and 0.72 for flocs formed by PFC + LA. Floc recoverability was also much higher at the usage of LA when there was a breakage period, and they were in the order of PFC LA > PFC + LA > PFC. Additionally, comparing with those of PFC, roughly opener flocs were reformed by PFC LA and PFC + LA after breakage because of LA bridging. For instance, TWV values (%) were 10.24, 13.24 and 16.16 respectively for PFC, PFC + LA and PFC LA in RB treatment. LA favored charge neutralization, adsorption and bridging in the coagulation process. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Papermaking industry is one of the significant industries in many countries. Vast wastewater is produced during the production process [1]. Because of high concentration and low biodegradability of organic matters in the suspension, pretreatment processes, such as sedimentation and coagulation, are requested. After coagulation, part of the matters precipitate, which contribute to the papermaking sludge. Traditionally, papermaking sludge has been disposed of through land filling. However, landfill is currently unacceptable due to environmental deterioration (e.g. polluting underground water) and competition for land [2]. Over the past decades, researchers have been concentrating on recycling papermaking sludge because of the rich content of organic matters ⇑ Corresponding author. Tel.: +86 531 88366771; fax: +86 531 88364513. E-mail address: [email protected] (B. Gao).
[3,4]. Lignin is a kind of natural organic matter with a great variability of highly active functional groups, such as carboxylic groups, alcoholic hydroxyl groups and phenolic groups. So it is an attractive macromonomer for polymer synthesis [5]. Fang et al. [6] have reported a kind of cationic flocculant by grafting acetone and formaldehyde onto hydroxymethylated lignin. According to the above discussion, papermaking sludge can be recycled to prepare a flocculant by utilizing the contained lignin. Spent pickling liquor is the industrial byproduct from removing oxides and scale of metal surface using acid in steel plants. It is characterized by extremely acidic and highly containing heavy metals. This kind of liquor would cause serious harms to environment and human in terms of corroding pipes, destroying the healthy conditions of soil and harming people’s health, if it is unreasonably disposed of. Traditional methods of disposal include electrodialysis, membrane distillation, selective precipitation and anion exchange/membrane electrowinning [7], through which iron
1385-8947/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.12.005
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products and clean water could be obtained. However, high cost would be expended during these procedures. Therefore, other methods for fully reusing the spent pickling liquor should be promoted. Lan et al. [8] provided a method using the spent pickling liquor to prepare a composite inorganic polymer coagulant-PFAC, through which the liquor was totally utilized. Dyeing wastewater, which comes from textile industry, can cause serious environmental pollution in terms of toxicity, aesthetics, carcinogenicity and teratogenicity due to high chroma and COD, thereby being listed as one of the industrial wastewaters that are difficult to be treated [9]. Biochemical treatment method is considered to be efficient in BOD and color removal but unsatisfied due to dyes’ recalcitrance [10]. So pretreatment is needed. Coagulation is proved to be a method of low cost and considerably efficient in decoloration and COD removal [11,12], and it is widely employed as pretreatment process in dyeing wastewater treatment [13,14]. Inorganic metal salts, such as iron and aluminum salts, are the most common coagulants to promote the aggregation of particles and thus have the impurities removed [15]. Recently, a growing attention is being paid on natural/synthetic organic polymers in the aim of better floc properties and lower dosage. They are usually used as flocculant or coagulant aid for inorganic coagulant, and have been proved to achieve better flocs characteristics in terms of size, strength and settling ability as well as better resistance to pH changes, which can improve coagulation efficiency and thus lower the coagulation cost [16–18]. Additionally, the operation parameters of coagulation, for instance, stir time, hydraulic condition and coagulant/flocculant dosing method, also could influence coagulation efficiency. There are researches that have drawn conclusions that two-stage addition of aluminum could give better floc recoverability [19,20]. However, second addition of an organic polymer on the basis of metal salts has not been studied. In this study, papermaking sludge and spent pickling liquor were recycled to prepare an organic polymer flocculant (LA) and polyferric chloride (PFC) respectively that used in dyeing wastewater treatment. In this way, we achieve a goal of using industrial waste to control water pollution. The aim of the study is to investigate the effect of different dosing methods of LA and PFC on coagulation performance and floc properties. The three dose methods were single use of PFC, LA used as aid for PFC (PFC + LA) and second dose of LA after breakage stage on the basis of PFC (PFC LA). Floc properties related to floc size, growth rate, strength, recoverability, regrowth rate and structure. Coagulation mechanisms were also studied.
2. Materials and methods 2.1. Preparation of PFC and LA PFC with basicity value 0.5 (mole ratio of [OH]/[Fe]) was prepared by spent pickling liquor. The spent pickling liquor was obtained from a steel plant of Shandong Province (China). The content of Fe was determined by 1,10-phenanthroline spectrophotometry and it was 170 g/L. The concentration of hydrochloric acid was about 1.69 mol/L. Firstly, Fe2+ in the spent pickling liquor was oxidized by sodium chlorate. Then, a certain amount of anhydrous sodium carbonate solution was titrated to adjust the basicity value. After that, solid disodium hydrogen phosphate as stabilizer was added and the mole ratio of phosphorus to Fe was 0.08. The mixture was stirred for further 3 h after the stabilizer dissolved. The result of Fe (III) species distribution in PFC is shown in Table 1. The dosage of PFC was calculated as mg/L of Fe in all experiments. A kind of air-dried papermaking sludge was supplied by a paper mill in Shandong Province (China). The polymer LA was
Table 1 Distribution of Fe (III) species in PFC (Fea: monomeric species; Feb: polymeric species; Fec: precipitated species). PFC Fea (%) 40.74
Feb (%) 46.88
Fec (%) 12.38
synthetized by grafting acrylamide onto lignin that contained in the papermaking sludge. The detailed synthesis process can be found in our previous research [21]. 2.2. Zeta potentials of PFC and LA Variation of zeta potential of PFC and LA as a function of pH was measured using a Zetasizer 3000HSa (Malvern Instruments, UK) [22]. Determination experiments of PFC were conducted at dosage of 5 mg/L as Fe. HCl and NaOH solutions (both 0.1 mol/L) were used to adjust pH. 2.3. Dyeing wastewater preparation The disperse yellow dye (RGFL) and reactive blue dye (K-GL) that used in this study represented hydrophobic and water-soluble dyes respectively and they were obtained from Jinan No. 2 Textile Dyeing Mill, China. The name, type, maximum adsorbance wavelength (kmax) and molecular structure are shown in Table 2. The synthetic dyeing wastewater was prepared by dissolving 0.1 g of dye into 1.0 L of tap water. The concentration of the solution was similar to that of actual dyeing wastewater. The pHs of DY solution and RB solution were both 7.72 ± 0.20. The initial maximum adsorbance of DY wastewater was 0.850 ± 0.003 and that of RB wastewater was 1.000 ± 0.010. 2.4. Coagulation jar test Coagulation jar test experiments were performed on a ZR4-6 apparatus (Zhongrun Water Industry Technology Development Co. Ltd., China) at room temperature (about 15 °C). 1 L of dyeing wastewater was used. Coagulant/flocculant was added at rapid stir (200 rpm). After PFC/LA evenly dispersed, slow stir (40 rpm) for 15 min was introduced to make flocs aggregate, followed by sedimentation for 20 min [23]. The upper sample was drawn to measure the absorbance value when coagulation process was finished. In the case of the dual-coagulant PFC + LA, PFC and LA were added in sequence at rapid stir stage. The relationship between adsorbance value at kmax and dye concentration was linear; therefore, color removal efficiency was calculated by comparing the adsorbance value of the treated sample to that of initial water according to the following equation:
Color removal efficiency ð%Þ ¼
A0 A 100% A0
ð1Þ
where A0 and A were absorbance values of raw water sample and treated water sample, respectively. To investigate coagulation kinetic, online monitor was conducted on an optical unit, Photometric Dispersion Analyzer (PDA 2000, Rank Brothers Ltd.). The suspension was continuously drawn through PDA 2000 by a peristaltic pump (LEAD-1, Baoding Longer Precision Pump Co., Ltd., China) at a rate of 1.2 L/h [24]. The inflow/ outflow tube with inner diameter of 3 mm was positioned at a depth just above the paddle in the holding ports. In order to study floc strength and recoverability, rapid stir for 5 min and slow stir for 15 min were successively employed after the floc aggregation [23]. For second addition of LA on the basis of PFC, while PFC
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H. Rong et al. / Chemical Engineering Journal 243 (2014) 169–175 Table 2 Dye characteristics. Name
Type
Molecular structure
Disperse yellow (RGFL)
Azo
445
Reactive blue (KG-L)
Phthalocyanine
598
was dosed at the initial rapid stir, LA was added at the end of breakage period to investigate effect of second addition of LA on floc recoverability. The PDA measures the average transmitted light intensity (DC value) and the root mean square (RMS value). Ratio, the ratio of RMS and DC (RMS/DC), can be read from the computer that connected with the PDA. The Ratio value was strongly related to particle size, providing a useful indication of floc dynamic changes [25]. Monitor result model of PDA 2000 during coagulation is shown in Fig. 1. In the chart, floc growth rate is the slope of growth region and can be calculated as follows:
Growth rate ¼
Dratio Dtime
i¼1 ðratioi timei Þ PN i¼1 timei
ð3Þ
To investigate floc strength and recoverability when exposed to strong shear force, strength factor and recovery factor are adopted [26]:
Ratio2 100 Ratio1
Recovery factor ðRf Þ ¼
ð4Þ
Ratio3 Ratio2 100 Ratio1 Ratio2
ð5Þ
where Ratio1, Ratio2 and Ratio3 are the ratio values in the growthsteady region, after breakage and regrowth-steady state, respectively. 0.8 Steady-state stage Steady-state stage
Ratio
0.6
0.4 Growth stage 0.2
Lag stage 100
Time-weighted variance ðTWVÞ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P .P ffi 2 N N ½ðratio RatioÞ time time i i i i¼1 i¼1 ¼ 100% Ratio
ð6Þ
It is thought that a smaller TWV value signifies tighter and more homogeneous flocs, while a bigger one illustrates flocs with opener and more porous structure. 3. Results and discussion
PN
Strength factor ðSf Þ ¼
The time-weighted ratio variance of the steady-state ratio value, donated as TWV, can be computed as:
ð2Þ
In this system, time-weighted average steady-state ratio value, donated as Ratio, is a very important parameter. It indicates the floc size in the steady-state region and can be calculated as:
Ratio ¼
Wavelength (nm)
200
Regrowth stage Breakage 300
400
500
Coagulation time (s) Fig. 1. The curve of variation of flocs monitored on-line.
600
3.1. Zeta potentials of PFC and LA Zeta potentials of PFC and LA as a function of pH are shown in Fig. 2. Zeta potential of PFC decreased dramatically with pH, and PFC attained its isoelectric point at about pH 7.5. Meanwhile, LA was positively charged, for which LA can play charge neutralization in precipitating pollutants. It can be concluded that charge neutralization of PFC in dyeing water treatment (pH 7.70 ± 0.20) was minor, while sweep and enmeshment dominated. 3.2. Optimizing dosages of PFC and LA Fig. 3 shows that color removal efficiencies for both DY and RB dramatically increased at first and then reached to a plateau, respectively. PFC gave perfect color removals, approximately 90% for DY and 94% for RB, at dosages of 40 mg/L and 50 mg/L. When LA was dosed after PFC, color removal efficiency for DY showed slight increase, less than 20%. For RB, there was almost no improvement in color removal. Zeta potential of the water sample after coagulation was also investigated and the result is presented in Fig. 3. It soared with PFC dosage. This illustrated that positively charged ferric species were growing as PFC dosage increased although most of ferric hydrolyzed under the raw water pH condition, and thus charge neutralization between ferric and negatively charged phenolic hydroxyl group on DY or –SO3- on RB enhanced with the increase of PFC dosage. When LA was dosed after PFC, zeta potential augmented sharply due to the positive charge on LA surface (1.1 mV). In the case of DY treatment, there was no significant difference in zeta potential among different LA dosages. However, it should be observed that a little better color removal was achieved at lower LA dosage (Fig. 3). Therefore, it can be concluded that almost all the negative charges on DY molecules were neutralized by PFC and LA at low LA dosage and charge neutralization was fully functioning at this condition [27]. When LA dosage increased, there was a little excess LA on the surface of DY–Fe microflocs, which could lead to restabilization of colloids, thus decreasing color
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2.2 35 PFC
30
2.0 1.8
Zeta potential (mV)
25
Zeta potential (mV)
LA
20 15 10 5
1.6 1.4 1.2 1.0
0 0.8
-5
0.6
-10 5.0
5.5
6.0
6.5
7.0
7.5
8.0
5
6
7
8
9
pH
pH Fig. 2. Zeta potentials of PFC and LA as a function of pH.
9.0
120
-14
60
-16 8.6 8.4
Color removal
40
8.2
20 8.0
pH
0
7.8
-20
Zeta potential (mV)
Color removal (%)
80
8.8
pH
100
LA dosage 0 mg/L 0.5 mg/L 1.0 mg/L 1.5 mg/L
-18 -20 -22 -24 -26
-40
7.6
-28
-60
7.4
-30
25
30
35
40
45
50
25
PFC dosage (mg/L) 11
LA dosage 0 mg/L 0.5 mg/L 1.0 mg/L 1.5 mg/L
40
45
50
-2 -4
10 Color removal
9 80 pH
8 70
Zeta potential (mV)
90
35
PFC dosage (mg/L)
pH
Color removal (%)
100
30
-6 -8 -10 -12 -14
7 -16
60 35
40
45
50
55
60
PFC dosage (mg/L)
35
40
45
50
55
60
PFC dosage (mg/L)
Fig. 3. Color removal efficiencies, final pH and zeta potentials for synthetic dyeing wastewaters.
removal efficiency. For the treatment of RB, zeta potential for dosing 1.5 mg/L of LA was the highest while best color removal was got by 1.0 mg/L of LA, which illustrated that complete charge neutralization was given by 1.0 mg/L of LA and a higher dosage of LA could cause excessive charge on microflocs. It also can be seen from Fig. 3 that pH slightly decreased as PFC dosage increased. However, LA addition rarely had any effect on final pH values. According to the above result, the optimal dosages of coagulant and flocculant for the subsequent experiments were as follows: 45 mg/L of PFC and 0.5 mg/L of LA for DY water treatment; 55 mg/L of PFC and 1.0 mg/L of LA for RB water treatment.
3.3. Color removal efficiencies with/without breakage process In order to investigate the effect of breakage process and second addition of LA on coagulation performance, a period of rapid stir followed by slow stir and sedimentation was introduced after flocs’ formation. Color removal efficiencies with/without breakage period are presented in Table 3. It was evident that a period of breakage for PFC and PFC + LA reduced dyes removal efficiencies. The reason was that flocs were usually irrecoverable and the smaller reformed flocs had lower removal efficiencies [28,29]. However, dye removal efficiencies for PFC + LA were a little higher than
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single dose of PFC after breakage and regrowth. Additionally, when LA was dosed at the end of breakage period, color removal efficiencies for DY and RB were both over 90%, about 5% higher than only PFC dosed. The result strongly attributed to different floc properties formed by the three coagulants, which would be discussed in detail later. 3.4. Dynamic change of flocs during coagulation process In order to investigate effect of strong shear force on floc strength and recoverability, four kinds of velocity gradients, 40.3, 70.7, 102.5 and 137.2 s1, were adopted at the breakage stage. Dynamic changes of flocs are presented in Fig. 4. From the profiles, it was evident that different coagulants and coagulation conditions gave different floc sizes, growth rates and regrowth size. For the three dose methods, flocs grew rapidly to plateaus after coagulant/flocculant was dosed. It could be observed that LA addition on the basis of PFC gave larger flocs than PFC dosed alone. In the case of DY, Ratios were about 1.43 and 1.75 respectively for PFC and PFC + LA. For RB treatment, Ratios of PFC and PFC + LA were 0.59 and 0.72, respectively. In addition, growth rate of PFC + LA flocs was higher than that of PFC flocs for both DY and RB (Table 4). There is a large body of researches suggesting that polymer used as coagulant aid could gave larger floc size and rapider growth rate than single use of metal salt [30,31]. This can be explained by the adsorption and bridging ability of LA: dye aggregated into Table 3 Color removal efficiencies (%) with/without breakage period.
Disperse yellow Reactive blue
Without breakage
With breakage
PFC
PFC + LA
PFC
PFC + LA
PFC LA
89.12 93.79
97.12 94.82
85.62 86.43
89.96 88.97
92.90 90.66
(a) 2.5
3.5 PFC 100 rpm 200 rpm 300 rpm 400 rpm
2.0
1.0
1.5 1.0
0.5
0.0 0
500
1000
1500
2000
-0.5
2500
0
500
1000
1500
2000
2500
0
500
Time (s)
1000
1500
2000
2500
2000
2500
Time (s) 1.0
1.0
PFC
Ratio
0.8
0.4 0.2 0.0 0
500
1000
1500
Time (s)
2000
PFC-LA
0.8
100 rpm 200 rpm 300 rpm 400 rpm
0.6
PFC+LA
2500
0.6
0.6
Ratio
0.8
Ratio
1.0
0.0
Time (s)
(b)
1.5
0.5
0.5
0.0
PFC-LA
2.0
2.5
Ratio
1.5
2.5
PFC+LA
3.0
Ratio
2.0
Ratio
microflocs under sweep and complexation after PFC was dosed. Once LA was added, it rapidly adsorbed onto the surface of microflocs formed by PFC and dye. The long-chain LA chains may extend from the particles and link other particles together. Thus, flocs size was enlarged and growth rate was accelerated by LA. Fig. 5 shows the TWV values as a function of coagulation time. Velocity gradient of 70.7 s1 in the breakage period was selected. For DY and RB, TWV of PFC + LA flocs at growth stage was larger than that of PFC. This meant that flocs’ structure became opener when additional LA was dosed. There were strong charge neutralization and adsorption among microflocs and LA, and interior repulsion of flocs was significantly reduced, which would have contributed to compact flocs [32]. There was also research that concluded that bridging flocculation gave the most open structure of flocs among charge neutralization and sweep [33]. Thus, the larger TWV of PFC + LA flocs mainly attributed to the bridging of LA rather than electrostatic force between LA and microflocs and adsorption of LA. When strong shear force was applied, Ratio values dropped and it varied according to different velocity gradients, the larger the velocity gradient was, the smaller the floc ratio was. Sf of PFC and PFC + LA flocs under different shear forces are shown in Table 5. It could be found that PFC–DY flocs were a little stronger than PFC + LA-DY flocs, while flocs formed by PFC were much weaker than those formed by PFC + LA for RB. The result closely related to the different floc sizes formed by PFC and PFC + LA and coagulation mechanisms: For PFC, ferric hydrolyzed into Fe(OH)3 at pH about 7.7, and sweep entrapment and complexation were the dominant coagulation mechanisms [34]. It was reported that flocs with larger size and looser structure were prone to erode [35]. However, it contradicts the finding that that flocs formed under bridging were stronger than sweep and charge neutralization [33]. So according to discussion above, that dual-coagulant flocs for DY were a little weaker than PFC flocs primarily resulted from
0.4
0.4
0.2
0.2
0.0
0.0 0
500
1000 1500 2000
2500
Time (s) Fig. 4. Dynamic changes of flocs during coagulation process.
0
500
1000
1500
Time (s)
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H. Rong et al. / Chemical Engineering Journal 243 (2014) 169–175
Table 4 Growth rates and regrowth rates of flocs for different dose methods. Disperse yellow Velocity gradient (S1) 40.3
Reactive blue Velocity gradient (S1)
70.7
102.5
Growth rate (min1) PFC PFC + LA
137.2
40.3
70.7
0.36 0.44
Regrowth rate (min1) PFC 0.12 PFC + LA 0.23 PFC LA 0.35
0.10 0.19 0.29
0.070 0.16 0.25
Disperse yellow
15 PFC PFC+LA PFC-LA
10 5 0 500
1000
1500
2000
2500
TWV (%)
TWV (%)
20
0
137.2
0.040 0.053 0.19
0.040 0.040 0.18
0.15 0.18
0.11 0.19 0.32
25
102.5
0.036 0.085 0.23
24 22 20 18 16 14 12 10 8 6 4 2 0 -2
0.040 0.075 0.19
Reactive blue
PFC PFC+LA PFC-LA
0
500
Time (s)
1000
1500
2000
2500
Time (s)
Fig. 5. Time-weighted variance as a function of coagulation time.
Table 5 Floc strength and recoverability. DY Velocity gradient (S1) 40.3 Sf PFC PFC + LA
39.86 38.29
Rf PFC PFC + LA PFC LA
34.88 53.70 143.84
70.7
RB Velocity gradient (S1)
102.5
137.2
40.3
70.7
102.5
137.2
29.37 29.71
28.67 25.71
26.57 25.14
32.20 65.28
27.12 55.56
23.73 45.83
20.34 36.11
31.68 39.02 107.87
30.39 36.15 87.13
19.05 30.53 84.87
22.50 68.00 87.50
23.26 46.88 84.09
22.22 41.03 80.43
21.28 26.09 78.26
combination of the porosity of large flocs and LA bridging. For RB treatment, although TWV of PFC + LA flocs was higher than that of PFC flocs because of bridging of LA, the binding bonds of LA chains were strong and there was additional attractive forces between LA and microflocs caused by electrostatic patch, which contributed to much higher Sf value of PFC + LA flocs. Besides, Sf decreased with increase of velocity gradient. It can be inferred that weak bonds in large flocs were broken under high hydraulic shear force, and then strong bonds in small parts ruptured when shear force further enforced. Thus, lower value of Sf at higher velocity gradient was got. Once velocities slowed to the initial value, flocs’ recoverability at different rates was observed. From Fig. 4, flocs did not regrow to their previous sizes except for second addition of LA. Take velocity gradient of 70.7 s1 as an example, recovered values for PFC, PFC + LA and PFC LA in DY treatment were 0.74, 1.00 and 1.50, respectively, and those in RB treatment were 0.26, 0.55 and 0.53.
The regrowth rate and Rf of flocs after breakage are presented in Tables 4 and 5. The floc recoverability gave some indication of internal bonding structure in flocs. Previous researches have shown that charge neutralization could give full recoverability [36]. Therefore, the irreversible breakage indicated that the flocs for the three coagulants were not dominated by single charge neutralization. It was the result of combination of chemical and physical bonds between dye and coagulant. From Table 5, PFC gave the weakest floc recoverability, meanwhile, LA addition greatly enhanced the recoverability, especially for the second addition of LA. Zeta potential of PFC flocs (Fig. 3), the indicator of charge neutralization, was the lowest. So it could be concluded that weak charge neutralization was one of the most important reasons that caused the poor recoverability of PFC flocs. Sweep and entrapment triggered by ferric hydrolysates were the dominant coagulation mechanisms. For PFC + LA, when high shear force was applied, some of the bonds among large flocs formed by LA bridging, and then ones formed by charge neutralization were broken. After shear force slowed, part of LA chains played primary roles in bridging and charge neutralization in binding small fragments together again, while sweep of iron hydrolysates was the secondary reason for floc recoverability. For PFC LA, all the LA molecules worked, in which the role of LA was stronger than that in PFC + LA. Thus, PFC LA flocs had better recoverability than PFC + LA flocs. 4. Conclusions Polyferric chloride (PFC) and an organic polymer (LA) were synthetized by spent pickling liquor and papermaking sludge, respectively. PFC, dual-coagulant and second addition of LA were comparatively investigated in dyeing wastewater treatment in terms of color removal and floc characteristics. While
H. Rong et al. / Chemical Engineering Journal 243 (2014) 169–175
PFC showed perfect color removal for disperse yellow and reactive blue dyes, LA addition after PFC gave slight increases in dye removal efficiencies. Compared with single dose of PFC, much larger flocs were formed by the dual-coagulant at a rapider growth rate. However, the dual-coagulant flocs had more open structure because of bridging of LA chains. And dual-coagulant flocs were a little weaker than the PFC flocs when exposed to high velocity gradient for DY, while they were much stronger than those formed by PFC for RB. Sf decreased as hydraulic shear force increased. Additional dose of LA drastically enhanced floc recoverability and regrowth rate, and they were in the order of PFC LA > PFC + LA > PFC. Recovered flocs had relatively open structure for the three coagulants/flocculants. LA played significant roles in charge neutralization, adsorption and bridging in floc regrowth.
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