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Journal of Water Process Engineering

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Multiple flocculant prepared with dealkalized red mud and fly ash:Properties and characterization

Xinke Wanga,b, Na Zhanga,*, Yihe Zhanga,c,*, Jingang Liua, Xiao Xiaoa, Ke Menga, Bohua Chua,Chengshan Wangb,**, Paul K. Chuc

a Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Circular Economy Engineering Laboratory, National Laboratory ofMineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing, 100083, Chinab School of Earth Sciences and Resources, China University of Geosciences, Beijing, 100083, Chinac Department of Physics, Department of Materials Science and Engineering, and Department of Biomedical Engineering, City University of Hong Kong, Tat Chee Avenue,Kowloon, Hong Kong, China

A R T I C L E I N F O

Keywords:Multiple flocculantDealkalized red mudFly ashWastewaterTurbidity

A B S T R A C T

A multiple flocculant for the treatment of diatomite simulated wastewater is prepared with dealkalized red mudand fly ash. During the laboratory experiment, the optimal synthetic conditions of the coagulants and mainparameters affecting the flocculation process such as the mass ratio of fly ash to dealkalized red mud, and dosageof polydimethyldiallyl ammonium chloride were investigated. Fourier transform infrared spectrometer andscanning electron microscopy analysis is used to study the molecular structure and morphology of multipleflocculant, respectively. The results show that the optimal mass ratio of sodium carbonate to fly ash is 1:1 toobtain modified fly ash, and that of modification of fly ash to dealkalized red mud is 1:3. The turbidity treatmentof diatomite simulated water decreases from 683.6 NTU to 0.67 NTU with the multiple flocculant. The turbidityremoval rate is up to 98.76 % when the dosage of the multiple flocculant is 90 μL/L. The multiple flocculantprepared with modified fly ash and dealkalized red mud delivers good performance in turbidity treatment ofdiatomite simulated wastewater. Through scale-up experiments, the economic cost of multiple flocculant hasbeen analyzed, and this work provides viable methods to utilizing dealkalized red mud and fly ash efficiently.

1. Introduction

Red mud, a potentially valuable solid waste produced by the alu-mina extraction process, accounts for 2.7 million tons resource pro-duced per year [1,2]. It has revolved around the construction of clay-lined dams or dykes according to the conventional disposal methods,and then red mud slurry is pumped and allowed to dry naturally.However, the alkali liquid permeates farmlands nearby or undergroundwater causing soil alkalization and water pollution, although the redmud has been processed by drying and pressing dehydration [3,4]. Redmud has many applications in the industry such as construction [5,6],wastewater treatment [7–12], and carbon capture [13,14].

Due to porous characteristic of red mud, it can adsorb contaminantsfrom water, and so can be transformed into a high-performance andvalue-adding coagulant. In sewage treatment, red mud is used as anadsorbent to adsorb anions, heavy metals, nonmetallic toxic ions and

dyes from wastewater [15]. Namasivayam and Arasi [16] used red mudas adsorbent for treatment of textile dye wastewater, and the adsorptioncapacity was 4.05 mg/g. Orecanin and Nad [17] synthesized poly-silicate flocculant by activating red mud with dilute sulfuric acid, andthey found that cationic and anionic species could be removed from thewastewater in one step. With one dose of coagulant (activated redmud), five cycles of heavy metals removal could be done. Besides, it isan important research direction to treat sewage by the coagulant pre-pared from modified red mud, such as aluminum ferric silicate floccu-lant and polyaluminium ferric chloride flocculant [18–22].

Fly ash is a solid waste obtained by filtering, deposition, and col-lection by the flue gas in coal combustion. It consists of SiO2, Al2O3,Fe2O3, and trace substances rich in germanium, gallium, nickel, and soon [23–25]. As a porous and loose solid waste with a large specificsurface area and strong adsorption ability, fly ash has been used todispose wastewater [26]. A fly-ash-based magnetic coagulant was

https://doi.org/10.1016/j.jwpe.2020.101173Received 15 October 2019; Received in revised form 27 January 2020; Accepted 28 January 2020

⁎ Corresponding authors at: Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Circular Economy EngineeringLaboratory, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing, 100083, China.

⁎⁎ Corresponding authors at: School of Earth Sciences and Resources, China University of Geosciences, Beijing, 100083, China.E-mail addresses: nazhang@cugb.edu.cn (N. Zhang), zyh@cugb.edu.cn (Y. Zhang), chshwang@cugb.edu.cn (C. Wang).

Journal of Water Process Engineering 34 (2020) 101173

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prepared by leaching methods, and the results showed that the mainaction mechanism of fly-ash-based magnetic coagulant is the neu-tralization and magnetic separation [27]. Polymeric aluminum, iron,silicon, and various composite inorganic polymer flocculant have beenused for the application of wastewater treatment [28–30]. It is noticedthat composite inorganic polymer flocculant, as one efficient inorganiccoagulant, possesses the advantages of strong adsorption, rapid for-mation of flocs, rapid settlement, and high removal rate of COD.

In this work, a multiple flocculant was prepared by using deal-kalized red mud and modified fly ash, and the flocculating character-istics were investigated. The parameters affecting the preparation of themultiple flocculant such as dosage of poly-dimethyl-diallyl-ammoniumchloride, concentration of dimethyl diallyl ammonium chloride(DMDAAC), mass ratio of dealkalized red mud to fly ash, and mass ratioof fly ash to Na2CO3 were studied and discussed systematically. Themultiple flocculant prepared by dealkalized red mud and fly ash notonly overcomes the difficulty in processing industrial waste, but alsoenables efficient treatment of wastewater.

2. Experimental details

2.1. Materials

The chemical reagents including sodium carbonate, sulfuric acid(H2SO4), hydrochloric acid (HCl), sodium hydroxide, ammonium per-sulfate (APS), ethylenediamine tetraacetic acid tetrasodium (Na4EDTA)were obtained from Beijing Jingwen Commerce Center, and dimethyldiallyl ammonium chloride was provided by Wanduoxin ShandongChemical Co., Ltd. in Shandong, China. Fly ash was obtained fromShandong Weiqiao Aluminum & Electricity Co., Ltd. Shandong, China.All the solutions were prepared with deionized water. 1 g diatomite and1 L distilled water were weight into a beaker, respectively, and diato-mite simulated water was gotten after stirring.

The dealkalized red mud with pH at 6–7 was obtained after red muddisposed by flue gas, as shown in Fig. 1. The red mud slurry was stirredin a tank firstly, and then it passed through the slurry pipe, controlledby valves into the spray tower. While the flue gas passed through thepipe into the spray tower, the red mud slurry flew through the spraydevice and reacted with the flue gas. It subsequently flew into the se-dimentation pool after reaction with the flue gas, and the flue gas wasdischarged through a chimney [31].

Table 1 shows that the dealkalized red mud mainly includes SiO2,

Al2O3 and Fe2O3, and the contents of these substances reach more than70 wt% for preparation of multiple flocculant. The main constituents offly ash were SiO2, Al2O3, CaO and Fe2O3 as shown in Table 1. Becausethe main mineral phase in fly ash is mullite (Al6Si2O13) which is stable,modification of the fly ash is necessary to prepare the flocculant.

2.2. Preparation of multiple flocculant from dealkalized red mud and flyash

Three kinds of ingredients: fly ash and Na2CO3, fly ash and deal-kalized red mud, fly ash, Na2CO3 and red mud, were collocated. Theywere heated to 800 °C and 900 °C, respectively, followed by naturalcooling. According to molar ratios of Na2O to SiO2 (0.2, 0.4, 0.6, 0.8,and 1), fly ash and Na2CO3 were sintered after mixing to obtain themodified fly ash. The mass ratios of fly ash to dealkalized red mudranged from 3:1, 1:3, 1:1, 2:1, and 1:2. HCl and H2SO4 was stirred andmixed with the solid raw materials in a three-neck round-bottom flaskat 90 °C for 2 h, cooled to room temperature, and filtered. During fil-tration, the products after the reaction were filtered through vacuumfilter with a 9 mm filter paper. The concentrations of HCl and H2SO4

were 30 wt% and 75 wt%, respectively. The solid residue was washedwith 300 mL of distilled water. Through collecting and mixing themother liquid and washing liquid, leaching liquid was obtained.

Polydimethyldiallyl ammonium chloride was prepared by radicalpolymerization with monomer dimethyldiallyl ammonium chloride. Aseries of monomer solutions with different concentrations were addedto a reactor, and monomer initiator-ammonium persulfate (APS) andcomplexing agent B (Na4EDTA) were added under aerobic or anaerobicconditions, respectively. Meanwhile, moderate water was required afterstirring the suspension with the stirring speed at 60 rpm.Polydimethyldiallyl ammonium chloride was obtained with the reac-tion conditions of initial temperature at 45 °C for 3 h. The poly-merization temperature was 50 °C and the curing temperature was 70°C for 3 h, respectively.

The iron and aluminum cations in the solution were polymerized

Fig. 1. Process diagram of dealkalization of red mud by flue gas.

Table 1Components and contents of dealkalized red mud and fly ash (wt%).

Components Al2O3 SiO2 Fe2O3 Na2O CaO MgO TiO2

Dealkalized red mud 22.32 27.45 24.56 2.26 – – 2.29Fly ash 28.67 25.9 5.24 – 19.54 8.15 –

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with NaOH at 60 °C by stirring for 2 h, and the pH was adjusted to2.0–2.5 through pH meter testing. Polydimethyldiallyl ammoniumchloride was added to prepare the multiple flocculant by keeping themixture at 60 °C for 1 h. The process diagram was shown in Fig. 2.

2.3. Characterization and testing of samples

The turbidity of simulated wastewater was determined on the SGZ-2turbidmeter (Shanghai Yuefeng Instruments & Meters Co., Ltd.). X-raydiffraction (XRD) (Bruker Scientific Instruments Hong Kong Co., Ltd.)analysis was conducted to examine the crystalline structure of themultiple flocculant, with the parameters on the D8 Advance (Cu target,2.2 kW and step/time scanning mode of 0.05°/1 s). The morphology ofthe multiple flocculant was examined by JSM-IT300 scanning electronmicroscopy (SEM) (JEOL Ltd). The content of leached metals wereanalyzed by inductively-coupled plasma atomic emission spectrometry(ICP-AES) (Thermo Fisher Scientific, Muffle furnace, JZ-10-1400,Shanghai jingzhao mechanical equipment Co., Ltd.). The flocculant wasdried and analyzed by Fourier transform infrared spectrometer (PerkinElmer instrument Co., Ltd.). The fly ash mixed with sodium carbonateat different temperature was characterized. The modification condi-tions such as different temperature, different proportions of raw ma-terials were analyzed based on the phase transformation under fly ashmodification conditions.

In the leaching liquid test, 5 mL of each sample was analyzed byICP-AES to determine the amounts of aluminum and iron, and leachingrates of these elements. 0.1 g of the leached acid residue was dissolvedby adding hydrochloric acid, nitric acid, hydrofluoric acid, and per-chloric acid. The acid solution was heated and evaporated until fluoridewas completely evaporated. Nitric acid was added to a constant volumein a 250 mL volumetric flask. The process was similar to the leachingliquid test, and the residual aluminum and iron contents in the leachedacid residue were calculated. And leaching rates of aluminum or iron ofsamples are obtained by Eq. (1).

L(Al or Fe) = (L1-L0)/L0*100 % (1)

where L0 is original aluminum or iron content of samples beforeleaching, and L1 is on behalf of aluminum or iron content after leaching.

Based on the self-matching diatomite solution, the solid-liquid ratiowas 1 g : 1 L. The multiple flocculant was added in the diatomite si-mulated water with a specific dosage of 60 μL/L, 90 μL/L, 100 μL/L,and 1 mL/L. 100 mL of the tested solution was firstly placed in a beakerand stirred rapidly at 200 rpm for 2 min. After adding the multipleflocculant at a room temperature, the tested solution was stirred slowlyat 40 rpm for 2 min, and then sedimentation was followed for a while.The turbidity measurement was carried out after the sedimentationtime at 15 min and 30 min, respectively. Each turbidity measurementwas performed under the same operation with the simulated sewagebeing the blank sample. The turbidity removal ratio (W) is obtained byEq. (2).

W = (T1-T0)/T0*100 % (2)

where T0 is on behalf of turbidity before adding the multiple flocculant,and T1 is on behalf of turbidity after adding the multiple flocculant.

3. Results and discussion

3.1. XRD analysis of modified fly ash

The molar ratios of Na2O to SiO2 in the modified fly ash range from0.2 to 1 after sintering at 800 °C (Fig. 3a) and 900 °C (Fig. 3b), whichwas related with XRD patterns from a to e in Fig. 3. As shown in Fig. 3,there is a gradual shift from mullite (Al6Si2O13) and sillimanite(Al2Si2O5) to nepheline (NaAlSiO4) in the modified fly ash at 800 °C and900 °C. Besides, higher the temperature is, the phase shift is relativelycomplete. 900 °C should be selected for sintering when raw materialsare treated. It can also be seen that the degree of conversion frommullite (Al6Si2O13) and sillimanite (Al2Si2O5) to nepheline (NaAlSiO4)increases with the molar ratios of Na2O to SiO2 in the modified fly ash.Hence, it is thought that the optimized molar ratios of Na2O to SiO2 are0.8 and 1, respectively.

3.2. Aluminum and iron contents after leaching experiment

The aluminum and iron leaching rates of modified fly ash are shownin Table 2. According to the molar ratios of Na2O to SiO2 in the

Fig. 2. Preparation process diagram of multiple flocculant.

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modified fly ash, when the proportions of sodium carbonate and fly ashare 5.3:15 and 6.63:15, the aluminum and iron leaching rates are re-latively high.

Table 3 shows the aluminum and iron leaching rates determinedfrom the liquid for fly ash and dealkalized red mud. It can be seen thatwhen a certain amount of dealkalized red mud is added, the leachingrate of aluminum and iron is improved, and the iron leaching rate caneven reach up to 50 %. When the mass ratio of fly ash to dealkalized redmud is 1:3, the aluminum and iron leaching rates could reach 33.49 %and 65.68 %, respectively. For the same proportion and comparisonwithout dealkalized red mud, the leaching rate of aluminum increasesby 37.67 %, but the iron leaching rate increases by 68.95 %.

Table 4 shows the aluminum and iron leaching rates determinedfrom the liquid for modified fly ash and dealkalized red mud. It can beseen that the leaching rate of aluminum and iron is greatly improved,and the iron leaching rate can even reach up to 90 %. When the massratio of sodium carbonate : fly ash : dealkalized red mud is 1:1:3, theleaching rate of aluminum and iron could reach 49.36 % and 91.37 %,respectively. Compared with the above result in Table 3, without theaddition of sodium carbonate, the aluminum and iron leaching ratecould only reach 33.49 % and 65.68 % when the mass ratio of fly ash todealkalized red mud is 1:3, which are less than those results in Table 4.Hence, the mixture system of sodium carbonate, fly ash and dealkalizedred mud can obtain high aluminum and iron leaching rates.

From the above results listed in Tables 2–4, it is found that alu-minum and iron leaching rates from fly ash could be improved whenthe molar ratios of Na2O to SiO2 are 0.8 and 1. When the fly ash was

modified with Na2CO3, the aluminum and iron in the fly ash could beextracted effectively.

3.3. Evaluation of flocculation on the simulated wastewater

3.3.1. Effects of synthesis conditions of polydimethyldiallyl ammoniumchloride on flocculation

The monomer concentration has a great influence on the polymermolecular weight and monomer conversion, as well as flocculation ofmultiple flocculant. Fig. 4 shows the effect of monomer concentrationon the flocculation of multiple flocculant. It can be seen from Fig. 4 thatthe turbidity removal has a promoting effect under the differentmonomer concentration synthesis conditions after sedimentation timeof 30 min. When the concentration of the monomer is 80 %, the visc-osity increases and implosion may occur easily during the reaction,causing the detrimental effect of generation of polydimethyldiallylammonium chloride. The turbidity of diatomite simulated wastewaterafter disposal by multiple flocculant at the monomer concentration of

Fig. 3. XRD spectra of modified fly ash at different temperatures of 800 °C (a) and 900 °C (b).

Table 2Leaching rates of aluminum and iron determined from the liquid for themodified fly ash.

Na2CO3: fly ash Molar ratio of Na2O to SiO2 Aluminum (%) Iron (%)

1.92:20 0.2:1 18.98 18.983.53:20 0.4:1 7.62 6.693.98:15 0.6:1 5.49 24.685.30:15 0.8:1 11.54 34.376.63:15 1:1 22.56 63.23

Table 3Leaching rates of aluminum and iron determined from the liquid for fly ash anddealkalized red mud.

Fly ash : dealkalized red mud Aluminum (%) Iron (%)

3:1 22.98 19.891:3 33.49 65.681:1 31.69 55.422:1 24.54 28.371:2 29.56 45.23

Table 4Leaching rates of aluminum and iron determined from the liquid for sodiumcarbonate, fly ash and dealkalized red mud.

Na2CO3 : fly ash : dealkalized red mud Aluminum (%) Iron (%)

1:5:0 14.30 29.992:5:0 27.80 26.801:1:0 11.69 22.421:5:15 27.36 98.631:1:3 49.36 91.37

Fig. 4. Effects of monomer concentration on flocculation of multiple flocculantby disposing diatomite simulated wastewater.

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80 % is higher than those values at the monomer concentrations of 60% and 70 %. While monomer concentration decreases to 50 %, thedegree of polymerization is low due to a low productivity of poly-dimethyldiallyl ammonium chloride, and the turbidity of diatomite si-mulated wastewater after disposal by multiple flocculant is still higher.

When the monomer concentration is 70 % and polydimethyldiallylammonium chloride is prepared, the turbidity of diatomite simulatedwastewater after being disposed by multiple flocculant is the lowestranging around 40 NTU. While the monomer content is low, the pro-duct conversion rate is low. With the increase of monomer content, freeradical polymerization in the reaction system becomes more efficiently,resulting in a higher viscosity and larger polymerization reaction heat.Moreover, it causes implosion phenomenon obviously with highmonomer content, leading to an incomplete aggregation and low pro-duct yield, which has disadvantage effect on the flocculation of multipleflocculants [32–35].

According to actual conditions, preparation of polydimethyldiallylammonium chloride was carried out under aerobic and anaerobicconditions, respectively, and the turbidity removal effect of multipleflocculant was investigated during polydimethyldiallyl ammoniumchloride added into the multiple flocculant. Fig. 5 shows the effect ofaerobic and anaerobic conditions on the flocculation of multiple floc-culant through disposing the diatomite simulated wastewater. It can beseen from Fig. 5 that the turbidity removal has a promoting effect underthe anaerobic synthetic condition after sedimentation time of 30 min,and the residual turbidity is low for a dosage of polydimethyldiallylammonium chloride of 0.05 mL. After sedimentation without anyflocculant, the turbidity is 89 NTU, which is higher than those ofaerobic condition. Under aerobic conditions, preparation of poly-dimethyldiallyl ammonium chloride is difficult, as the reaction involvesfree radical polymerization reaction, and it has negative effects on thereaction.

3.3.2. Treatment of simulated wastewater using multiple flocculantWhen the multiple flocculant is prepared with fly ash and deal-

kalized red mud, the flocculant dosage is 90 μL/L, and the turbiditydrops from 683.6 NTU to 4.68 NTU after sedimentation for 15 min and0.67 NTU after 30 min. The turbidity removal rate is 98.76 %. When thedosage is more than 90 μL/L, the treatment results is unsatisfactory inFig. 6. Fig. 6 shows that preparation of multiple flocculant using fly ashmodified by Na2CO3 and dealkalized red mud is necessary with ex-cellent flocculation performance.

3.4. Characterization and flocculation mechanism of multiple flocculant

In order to verify the flocculation of multiple flocculant, the

morphology of the multiple flocculant prepared with dealkalized redmud and modified fly ash is presented in Fig. 7(a) and (b). Roughsurfaces with ridges and wrinkles are observed due to dehydrationduring the preparation of multiple flocculant in vacuum. Fig. 7(a) and(b) reveal pleated structures with the form of flowers providing ampleabsorption sites, suggesting that this unique microstructure improvesthe flocculation performance.

The morphology of multiple flocculant in Fig. 7(a, b) shows com-posite flocculant after the acicular crystal with desultory grid structureis pulled down. At this point, it can form colloidal and flocculationparticles in the suspension.

Fourier transform infrared spectra of the multiple flocculant pre-pared by the modified fly ash and dealkalized red mud is shown inFig. 7(c). There is a large absorption peak at 3387 cm−1, which iscaused by the expansion of −OH, as shown in Fig. 7(c). The peak at2109 cm−1 is associated with stretching vibration of C]N. The peak at1615 cm−1 is related to bending vibrations of adsorbed water or thecrystal water. The small band at 1107 cm−1 is caused by asymmetricvibration of the Fe-OH-Fe or Al-OH-OHFe or Al-OH-Al, and the floc-culant is mainly caused by the bending vibration of Al-OH at 960 cm−1.It is inferred that during the flocculation process, Fe-OH-Fe or Al-OH-OH-Fe or Al-OH-Al may form complex structure to adsorb particles.

Above the analysis of morphology and structure of the multipleflocculant, the flocculation mechanism of multiple flocculant preparedby the modified fly ash and dealkalized red mud is inferred in Fig. 8.This may be accomplished by adsorption of coagulant or double layercompression, owning to electric charge of the multiple flocculant sur-face, colloidal particles could link together with the multiple flocculant.When adding small dosage of multiple flocculant, colloidal particlesadsorbs onto multiple flocculant in such a manner that an individualchain can be attached to two or more particles thus “bridging” themtogether, long chain forms with suspension of colloidal particles. Whenthe length of floc is large enough, more effective repulsive force isgenerated in the colloidal particle solution, the branched chain canoccur between the flocs leaving the role of bridging larger flocs con-tinue.

3.5. Scale-up experiments and economic analysis of multiple flocculant

Based on the above laboratory experiments, scale-up experiments ofthe multiple flocculant were processed as shown in Fig. 9. The multipleflocculant was prepared in reaction tanks as shown in Fig. 9(a) and (b),the liquid flocculant was stored in the plastic drum as presented inFig. 9(c), and the solid flocculant was collected in the woven bag aspresented in Fig. 9(d). In the scale-up experiments, waste sulfuric acidand hydrochloric acid were adopted to prepare the multiple flocculant.Except for this difference, there is not much difference between thescale-up experiments and laboratory experiments. The turbidity re-moval results of multiple flocculant prepared in the scale-up experi-ments can be guaranteed after testing. The turbidity removal rate canbasically reach 95.00 % by using the multiple flocculant prepared bydealkalized red mud, fly ash and waste acid. Importantly, this saves alot of natural resources and improves the urban environment. More-over, considering that waste sulfuric acid and hydrochloric acid can beused to produce the multiple flocculant, all of the raw materials cost forproducing the multiple flocculant is estimated at 350 ¥/ton. Taking thecost and sale price into consideration, it brings considerable economicbenefits when sale price of multiple flocculant is 700 ¥/ton.

4. Conclusions

Fly ash and dealkalized red mud are used as main raw materials toprepare the multiple flocculant. The optimal synthetic conditions of thecoagulants and main parameters affecting the flocculation process suchas mass ratio of dealkalized red mud to fly ash, dosage of poly-dimethyldiallyl ammonium chloride were investigated. The multiple

Fig. 5. Effects of aerobic and anaerobic conditions on the flocculation of mul-tiple flocculant by disposing diatomite simulated wastewater.

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flocculant as a coagulation reagent for the treatment of diatomite si-mulated wastewater was described. From the above studies, the fol-lowing conclusions can be drawn:

(1) The multiple flocculant for the treatment of diatomite simulatedwastewater has been prepared with dealkalized red mud andmodified fly ash. The optimal mass ratio of sodium carbonate to flyash is 1:1 to obtain the modified fly ash, and that of the modified flyash to dealkalized red mud is 1:3. Owing to the rough surfaces ofmultiple flocculant with ridges and wrinkles, it is able to adsorbparticles in the wastewater.

(2) Polydimethyldiallyl ammonium chloride was prepared under theanaerobic conditions and the concentration of monomer at 70 %.The residual turbidity is low for a dosage of polydimethyldiallylammonium chloride of 0.05 mL.

(3) The optimal dosage of the resultant multiple flocculant for the besttreatment is 90 μL/L. The turbidity treatment of diatomite simu-lated water decreases from 683.6 NTU to 0.67 NTU by using themultiple flocculant prepared with the modified fly ash and deal-kalized red mud, and the turbidity removal rate could reach 98.76% in the laboratory experiment.

(4) The scale-up experiment of multiple flocculant enables efficientutilization of dealkalized red mud and fly ash as well as the in-dustrial waste acid. Taking the cost and sale price into considera-tion, it brings considerable economic benefits with the profit at 350¥/ton for the multiple flocculant.

The prepared multiple flocculant is an efficient inorganic coagulantwhich is environmentally acceptable. It possesses the advantage of thepresence of both Al and Fe polymers to overcome the problems asso-ciated with the individual coagulant during process of treating waste-water. As wastewater treatment is a complex and systematic process, itis thought that the multiple flocculant can be applied to pretreat in-dustrial wastewater such as oily sewage, dyeing wastewater and so on.This research is at a preliminary stage, and other methods could be usedduring the subsequent stage. Therefore, further studies would be car-ried out in our next work.

Declaration of Competing Interest

The authors declare that they have no known competing financialinterests or personal relationships that could have appeared to

Fig. 6. Flocculation results for multiple flocculant after (a) 15 min and (b)30 min.

Fig. 7. The morphology and structure of the multiple flocculant: (a) and (b) morphology of the multiple flocculant, (c) FTIR spectra of the multiple flocculant.

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influence the work reported in this paper.

Acknowledgements

This research was supported by National High Technology Researchand Development Program (863 Program No. 2012AA06A109) ofChina, Fundamental Research Funds for the Central Universities (No.2652017339) as well as City University of Hong Kong StrategicResearch Grant (SRG No. 7005105.

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