Biopolymer Flocculants and Oat Hull Biomass To Aid the Removal of

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Biopolymer flocculants and oat hull biomass to aid the removal of orthophosphate in wastewater treatment Henry K. Agbovi, Lee D. Wilson, and Lope G. Tabil Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04092 • Publication Date (Web): 14 Dec 2016 Downloaded from http://pubs.acs.org on December 16, 2016

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Industrial & Engineering Chemistry Research

Biopolymer assisted coagulation-flocculation processes 338x190mm (96 x 96 DPI)

ACS Paragon Plus Environment


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Biopolymer flocculants and oat hull biomass to aid the removal of orthophosphate in wastewater treatment Henry K. Agbovi,1 Lee D. Wilson,1* Lope G. Tabil2

6 7 8 9 10 11

1

University of Saskatchewan, Department of Chemistry, 110 Science Place, Saskatoon, SK., S7N 5C9,

CANADA. 2

University of Saskatchewan, Department of Chemical and Biological Engineering, 57 Campus Drive, Saskatoon, SK. S7N 5A9 CANADA

12 13

*

Corresponding Author: L. D. Wilson, Email: [email protected]

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Abstract

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This study reports on the removal of orthophosphate (Pi) by coagulation-flocculation with

32

variable combinations of alum, biopolymers and biomass. The combinatorial effects of these

33

coagulant aids were evaluated for single, binary and ternary systems. The role of pH, component

34

dosages, and Pi concentration on the coagulation-flocculation efficacy was evaluated. An optimal

35

dosage of alum (30 mg/L) while alginate and chitosan were 15 mg/L. Pi removal was 86% for

36

alum, and 98% for ternary systems containing chitosan and alginate where [Pi] = 10-11 mg Pi/L.

37

Pi removal for the alum-alginate-chitosan ternary system was more efficient than the binary

38

systems, especially at pH 6-7, where reduced efficiency occurred at pH > 7.5. Pi removal was

39

independent of concentration except at lower levels, [Pi] < 10 mg /L. The alum–refined oat hull

40

binary system was 99% effective for Pi removal, especially when [Pi] = 25 mg/L, with greater

41

removal over the use of oat hulls alone.

42

Keywords: Coagulation; Flocculation; Orthophosphate; Alginate; Chitosan; Oat hull

43 44 45 46 47 48 49 50 51 52 53 54



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1.0 Introduction

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Phosphorous is a key contaminant in water bodies and represents a major environmental

57

concern at elevated levels for many countries globally, especially when wastewater contains ca.

58

10 to 20 mg/L of total phosphorus1. While phosphate is an essential micronutrient in water,

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elevated levels lead to the growth of algae in water bodies. Thus, many countries regulate

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phosphate at threshold levels below 0.05 mg/L to reduce eutrophication2,3. The removal of

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dissolved orthophosphate (Pi) generally involves precipitation at alkaline conditions, followed by

62

removal from the aqueous phase. The removal of Pi has been studied by various methods such

63

as adsorption4, bioremediation5, electro-coagulation6, membrane processes7 and chemical

64

precipitation8,9.

65

The use of inorganic salts such as, aluminium sulphate (alum) and ferric chloride to

66

precipitate phosphate have been widely used due to their cost effectiveness and ready

67

availability10. However, these mineral salts have some disadvantages that include higher dosage,

68

ineffectiveness at lower temperature, pH-dependent performance, excessive sludge production,

69

and potential toxicity to human health11. The utility of synthetic polymers as flocculants and/or

70

coagulant aids for the removal of phosphate species in wastewater was reported8. Polymer

71

flocculants have advantages over inorganic electrolytes that relate to greater efficiency, large floc

72

formation with high density, high mechanical strength, and favourable settling properties.

73

Biopolymer flocculants also possess useful properties over a wide pH range and yield reduced

74

amounts of sludge without metal (aluminium) residuals whilst maintaining the alkalinity of

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water. By contrast, synthetic polymers are non-biodegradable, relatively expensive and pose

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greater potential biological toxicity and mutation12. The use of binary and ternary systems has


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several advantages: cost efficiency, biodegradability, biocompatibility, reduced toxicity, with

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minimal production of pollutants and side effects13.

79

Chitosan is a linear polysaccharide copolymer that consists of N-acetyl-D-glucosamine and

80

glucosamine units with variable acetylation (cf. Figure 1a). Alkaline hydrolysis of chitin yields

81

chitosan where its properties relate to the degree of deacetylation of chitin14. At slightly acidic

82

pH conditions near or below the pKa of chitosan, a cationic form exists due to protonation of the

83

amine groups. In turn, favourable electrostatic interactions occur with anionic species such as,

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phosphate and arsenate15, as evidenced by the use of chitosan as an adsorbent in water

85

treatment16. By comparison, there are sparse reports where chitosan is used for the removal of

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particulate matter and dissolved substances in homogeneous solution17–20. Chitosan has unique

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properties as a coagulant and/or flocculant due to its pH-dependent charge density, variable

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molecular weight, along with its chelation and aggregation behaviour at pH conditions above its

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pKa value12. The unique physicochemical properties of chitosan along with biodegradability,

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biocompatibility, metal chelation and low toxicity favour its use as a biopolymer flocculant13,21.

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Chitosan has been used for the removal of Pi in wastewater using different techniques or

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processes besides coagulation and flocculation22–27. Teh and Wu have also shown that the

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amount of sludge produced using chitosan was significantly lesser as compared to alum28.

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(a)

(b)

95

96

97 98 99

Figure 1: Chemical structures of (a) chitosan and (b) alginate, where m and n denote the degree of polymerization. 4 ACS Paragon Plus Environment


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100 101

Alginate is a natural and linear biopolymer obtained from marine brown algae (seaweed) and

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capsular polysaccharides found in soil bacteria. Alginate is a block copolymer composed of β-D-

103

mannuronate (M) and α-L-guluronate (G) units linked by β-1→4 and α-1→4 glycosidic bonds

104

(cf. Fig. 1b)29 that exists in the form of an anion at pH values above its pKa (3.5). The G and M

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monomer units of alginate can be arranged as hydropolymer G-/M-blocks and heteropolymer

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GM-blocks. Alginate contains carboxylic acid groups on each monomer unit where the ability to

107

chelate with reactive polyvalent metal cations contributes its utility in the treatment of water30.

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Alginate was studied as a bioflocculant in wastewater treatment by Wu et al., where alginate was

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used with aluminum sulphate to test its coagulation and floc properties31. Similar studies were

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carried out using alginate with ferric chloride, alum and titanium tetrachloride by Zhao et al.31,32.

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Devrimici et al. used calcium alginate to remove turbidity in water33, while Rocher et al.

112

examined the removal of organic dyes by magnetic alginate beads

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Shanthakumar studied the utility of alginate as a coagulant for the treatment of dyes in

114

wastewater35,36.

34

. Vijayaraghavan and

115 116

(a)

(b)

117 118 119 120 121 122 123

Figure 2: Optical images of (a) refined and (b) unrefined oat hull biomass.


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Oat hulls are rich in fibre content relative to wheat and corn bran37, where the cellulose

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content of such biomaterials is of interest for wastewater treatment. Oat hulls contain

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polysaccharides composed of D-glucose monomer units with variable linkages, β-1,3- or β-1,4-

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linkages38. Studies by Bernardo et al. have shown that the functional groups of oat hull relate to

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its utility as a bioflocculant, where the pKa values cover a suitable range of functional groups;

129

carboxyl (3.6 – 4.1), amines (6.6 – 7.2) and the hydroxyl groups (9.1 – 12.6)]39. The -COOH and

130

-OH groups reveal that cellulose, hemi-cellulose, pectin and lignin fractions coexist in oat hull

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biomass 39. Thus far, oat hulls have not been reported as effective flocculant aids for the removal

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of contaminants in wastewater. A recent study on the application of oat hulls as a flocculant

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relates to grafted oatmeal40. Other studies have employed oat hulls as an adsorbent biomass for

134

the removal of chromium ions39,41 and arsenic (V) species40. Herein, oat hulls are examined as an

135

additive to aid the coagulation-flocculation process for the removal of orthophosphate (Pi)

136

species in water.

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The objective of this study is to report on the removal efficacy of dissolved orthophosphate

138

from simulated wastewater using mixtures containing the coagulant (alum) and biopolymer

139

flocculants (alginate and chitosan), and a biomass additive (oat hull). Various single and

140

multicomponent systems were compared: i) single (alum, chitosan or alginate), ii) binary

141

(alum/chitosan or alum/alginate), and iii) ternary (alum/alginate/chitosan) coagulant-flocculant

142

systems to evaluate the Pi removal properties and the role of synergistic effects. This study

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contributes to the field of biomaterials in several ways through the use of alum/chitosan/alginate

144

ternary systems for the removal of phosphate over the removal efficacy of single and binary

145

systems as shown by effective Pi removal at variable pH and concentration conditions.

146



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2. Materials and experimental procedures

148

2.1 Materials and Chemicals

149

All chemicals were of analytical reagent (AR) grade. Hydrated ferric chloride (FeCl3·6H2O),

150

anhydrous monobasic potassium phosphate (KH2PO4), NaOH, HCl, vanadate molybdate reagent,

151

medium viscosity sodium alginate, chitosan (medium molecular weight, 85% deacetylation) and

152

aluminium sulphate (alum) were purchased from Sigma-Aldrich, Oakville, Ontario, Canada. Oat

153

hulls (unrefined) are a by-product of oat processing and were procured from Richardson Milling

154

Ltd., Martensville, SK, Canada. The oat hulls were cleaned using a sieving machine and an

155

aspirator to remove other parts of the oat grain to yield refined oat hulls. Refined and unrefined

156

oat hulls (cf. Fig. 2) were ground using a knife mill (Retsch GmbH 5657 HAAN, West Germany)

157

fitted with a 2.7 mm sieve.

158

2.2 Analytical method

159

Simulated orthophosphate, alum and alginate stock solutions were prepared by dissolving

160

their respective salts in distilled and distilled (ultrapure) water (18.2 MΩ·cm) at 25 °C. A

161

standard chitosan solution was prepared with 0.01 M HCl solution. A Pi calibration curve was

162

obtained using a vanadate-molybdate colorimetric method (λ = 420 nm) at pH 6.542. The

163

coagulation-flocculation experiments were performed using a conventional jar test apparatus

164

with a six-plate (6 × 2000 ml) jar system with stirrers (Phipps & Bird; Richmond, VA. USA).

165

Single, binary and ternary systems were studied. Initially, 1.0 L of Pi solution was added to the

166

respective jars of the system where 5.0 ml of the Pi solution was sampled and prepared for UV-

167

Vis absorbance analysis. After addition of the reagent (1.0 ml of vanadate-molybdate) to the Pi-

168

containing sample, a yellow coloured complex was allowed to develop for 20 mins before the 7 ACS Paragon Plus Environment

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analysis. The pH of the Pi solution (ca 25 mg P/L) was adjusted using 0.1 M NaOH or 0.1 M

170

HCl to the desired value. For the single systems, the coagulant (or coagulant aid) was added to

171

the Pi solution followed by rapid stirring for 3 mins, followed by continuous slow mixing for 20

172

mins. For the binary systems, the alum was added to the Pi solution with rapid stirring for 3 mins

173

followed by slow stirring for 20 mins. The flocculant was added within the first 5 mins of the

174

slow mixing. In ternary systems, alum was added to the Pi solution followed by rapid stirring for

175

3 mins followed by slow stirring for 20 mins. The flocculants were added within the first 10 mins

176

of the slow mixing step. After the rapid mixing, the solution was allowed to settle for 30 mins,

177

where the rapid and slow mixing was 295 rpm and 25 rpm, respectively. After the settling

178

period, 5.0 ml of the solution was sampled from the top region and prepared for UV-vis analysis.

179

The Pi levels were estimated via colorimetry using the vanadate molybdate method where the

180

absorbance (λ = 420 nm) was measured using a double beam UV–Vis CARY-100

181

spectrophotometer before and after the coagulation-flocculation process. The solution pH was

182

measured after a 30 mins settling period. Figure 3 illustrates the sequence of steps used for the

183

study of the coagulation-flocculation process.

184

With reference to the binary system containing oat hulls (unrefined or refined) and alum, a

185

standard dosage was prepared by soaking the finely ground powder, with particle size of 50 µm,

186

in ultrapure water and stirred for 30 mins. The coagulation-flocculation process was carried out

187

using the same experimental design according to Figure 3. However, the alginate and/or chitosan

188

flocculants were replaced with the oat hull suspension to afford a binary system containing alum

189

and oat hulls.

190 191

All experiments were performed in triplicate, where the average value and the standard deviation are reported. The Pi removal was estimated using equation (1)42. 8 ACS Paragon Plus Environment


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%Pr emoval =

192

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co − ce ⋅100% co

(1)

193

co and ce refer to the Pi concentration before and after the coagulation-flocculation process,

194

respectively.

195

196

197

Chitosan Jar Test

Alginate

3+

Al

Orthophosphate Fast mixing

Slow mixing

λmax = 420 nm

198

199 Settling

200

201 202

Absorption spectrum

Figure 3: Schematic diagram of the step-wise procedure for the coagulation-flocculation process and experimental setup.

203

204

3. Results and Discussion

205

3.1 Single component systems

206

3.1.1 Effect of alum

207

The results in Figure 4 were obtained by singly varying the dose of alum, alginate and

208

chitosan to understand the effects on the Pi removal. The blue curve in Figure 4 represents the

209

effect of alum dosage at an initial level of Pi ca. 10 mg/L at pH of 5-6. An alum dosage of 5.0

210

mg/L was able to lower the Pi level by 45%. As the dosage of the alum increased, the efficiency

211

of the Pi removal increased steadily until it reached a maximum level of 67% at 35 mg/L. 9 ACS Paragon Plus Environment

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212

Thereafter, the Pi removal decreased to 56% at 50 mg/L, where it remained constant as the alum

213

dosage increased. The decreasing efficiency of Pi removal (%) beyond the optimal dosage was

214

attributed to the stabilization of the Pi solution

215

conditions favour the presence of dianion species or orthophosphate which limits for Pi removal,

216

leading to a decrease in the removal efficiency. In Figure 4b, the Al/P molar ratios range from

217

0.5 to 5.0, with optimal Pi removal at a molar ratio of 3.5. This ratio is within the expected range

218

of Al/P molar ratio for effective Pi removal. The pH of the Pi solution after coagulation-

219

flocculation process was not significantly different from the initial solution.

220

Table 1: Comparison of the removal of phosphate in water and wastewater using metal salts. Wastewater Source

221 222

Coagulant / Flocculant

Optimum Dosage (mg/L)

43

. As the alum dosage increases, the pH

Optimum pH

Efficiency (%)

Synthetic Alum* + Alg 35 5.5 - 7.0 85 ± 0.7 wastewater + Chi Synthetic Alum 30 5.5 - 7.0 67 ± 0.3 wastewater Synthetic Alum and 80, 60 6 85 wastewater CaCl2 Synthetic PACl 6 N/A 94.6 wastewater Aquaculture Alum and effluent ferric 90 7.1 89, 93 discharge chloride Secondary effluent Alum 10 5.7 - 5.9 92 wastewater Activated Alum and sludge effluent 13 N/A 80 FeCl3 discharge * Flocculant dosage was varied while keeping the other variables constant. Chi and Alg refer to chitosan and alginate, respectively.

Reference This work This work Mohammed & Shanshool8 Chen & Luan44 Ebeling et al.45 Banu et al.46 An et al.47

223 224

The reaction between alum with Pi in wastewater likely involves complex formation and

225

charge stabilization; however, the flocculation mechanism is not well established. AlPO4(s) is an 10 ACS Paragon Plus Environment


226

insoluble product formed when alum combines with Pi, (cf. eqn 2), in agreement with the report

227

of Jenkins et al. 48. Aluminum phosphate undergoes precipitation since the thermodynamics and

228

kinetics of the ion association process is favoured over the formation of aluminium hydroxide

229

species.

Al2 (SO4 )3 ⋅ (H2O)14 (aq) + 2 PO43−(aq) → 2 AlPO4(s) ↓+ 3 SO42− (aq) + 14 H2O(l)

230

(2)

231

The optimum dosage of 35 mg/L obtained herein at an initial Pi dose of 10-11 mg/L is lower

232

when compared to values reported in literature (cf. Table 1). For example, Mohammed and

233

Shanshool8 reported an optimal dosage of 80 mg/L, while Ebeling et al.45 reported 90 mg/L for

234

alum while Banu et al.46 report 80 mg/L. 90

90

(a) 80

80

70

70

Pi removal (%)

Pi removal (%)

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60 50 40 30

Alum dose

20

Chitosan dose

(b)

60 50 40 30

Alum dose

20

Chitosan dose Alginate dose

Alginate dose 10

10

0

235 236 237

0

5 10 15 20 25 30 35 40 45 50 55

1

2

3

4

5

[Flocculant / Pi] molar ratio

Flocculant dose / mg/L

Figure 4: Phosphate removal (%) as a function of (a) single coagulant/flocculant dosage and (b) [flocculant/Pi] molar ratio. Initial concentration of Pi is ca. 10-11 mg/L.

238 239

3.1.2 Effect of chitosan

240

The effect of chitosan dosage on Pi removal is shown in Figure 4 (red curve) at initial levels

241

of Pi at 10-11 mg/L at pH of 5-6. An increase in chitosan dosage from 0.5 to 20 mg/L enhanced

242

the removal of Pi (50% to 78%). Further addition of chitosan led to a decrease in the efficiency 11 ACS Paragon Plus Environment

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243

of the Pi to 62% when chitosan was 35 mg/L. As the chitosan dosage increased beyond the

244

optimum level, precipitation of dissolved Pi occurs. However, the excess addition of chitosan led

245

to restabilization as positively charged colloidal particles with a corresponding reduction in Pi

246

removal. An increase in the dosage of the chitosan beyond the optimal dosage limit leads to an

247

increase in the amount of cationic charge in the solution. The charge neutralization of Pi anion

248

species with metal cations contribute to agglomeration of the Pi containing particulates by a

249

bridging mechanism with eventual settling of the floc. However, a reverse effect occurs when

250

excess cation species are present in the solution. The excess positive charge stabilizes the neutral

251

Pi containing aggregates and the macroflocs undergo repulsive interactions, resulting in a

252

progressive decrease of the Pi removal efficiency. Similar results were reported by Roussy et

253

al.49 as well as Huang and Chen50. In Fig. 4b, the chitosan to Pi mole ratio ranged from 0.04 to

254

2.74, with optimal Pi removal at 1.6:1.

255

In Table 2, the results of the present study compare favourably to previous results where it is

256

noted that the Pi removal is greater relative to the solid phase when chitosan is dissolved in bulk

257

solution. Filipkowska et al.23 obtained 30% removal, while Fierro et al.24 reported a 60% Pi

258

removal when chitosan was used in its dispersed solid form. Guibal et al.51 reported the

259

effectiveness of chitosan for the removal of suspended and dissolved substances. The variation of

260

efficiency relate to the greater reactivity of the amino groups when chitosan is dissolved in bulk

261

solution. Dissolution of chitosan in aqueous solution improves the accessibility and mass transfer

262

kinetics of the process relative to dispersed solid forms due to ionization of amine groups51.

263

Charge neutralization is a key step in the process of coagulation and/or flocculation, followed by

264

polymer bridging, in agreement with results obtained herein. The type of mechanism is generally

265

governed by the pH, coagulant dose, the type of contaminants


52

. As well, Pi and other


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structurally related oxyanions such as, arsenate have favourable binding affinity to chitosan and

267

its modified forms15,26,53.

268 269 270

Table 2: Comparison of the removal of orthophosphate in water and wastewater using biopolymer flocculants. Water Source Synthetic wastewater Synthetic wastewater Synthetic wastewater Synthetic wastewater Synthetic wastewater Synthetic wastewater Struvite

Optimum Dosage (mg/L)

Optimum pH

Efficiency (%)

Reference

Chi

20

6.2 - 7.0

78 ± 0.1

This work

Alg

25

5.7 - 7.0

38 ± 0.2

This work

Chi* + Alum

49

5.8 - 7.0

88 ± 0.8

This work

Al* + Alum

59

5.8 - 7.0

80 ± 0.8

This work

Chi* + Alg +Alum

15

5.5 - 7.1

98 ± 0.7

This work

Alg* + Chi +Alum

16

5.5 - 7.2

98 ± 1.2

This work

10, 20

N/A

80

Latifian et al.25

N/A

7.5 - 7.9

60

Fierro et al.24

N/A

4.0

30

Filipkowska23

59

Chen and Luan44

Flocculant

Chi and Alg

Synthetic Chi wastewater Synthetic Chi wastewater

271 272

Synthetic Poly(diallyldimethyl) wastewater ammonium chloride 0.5 8.0 Chi 60 Municipal 9.5 Starch 24 wastewater Guar gum 24 * Flocculant dosage was varied while keeping the other constants. Chi and Alg refer to chitosan and alginate, respectively.

89 86 82

Dunets and Zheng19

273 274

3.1.3 Effect of Alginate

275

The influence of alginate dose on Pi removal is shown in Figure 4 (green curve). The use of

276

alginate alone showed poor Pi removal efficiency. By contrast, an alginate dose of 0.5 mg/L had

277

a low Pi removal (13%), where greater Pi removal (39%) occurred gradually up to an optimum 13 ACS Paragon Plus Environment

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278

dose of 25 mg/L. Alginate and Pi are negatively charged at these conditions, resulting in

279

electrostatic repulsions in solution and stabilization of unbound Pi. However, the biopolymer

280

nature of the alginate favours removal of Pi by a bridging mechanism, especially when used in

281

combination with a cationic polyelectrolyte such as chitosan. In Figure 4b, the alginate to

282

phosphate molar ratio ranges from 0.04 to 3.07, with optimal Pi removal at a molar ratio of 2.56.

283

3.2 Binary system

284

The removal efficiency of Pi was studied by combining alum with chitosan or alginate (Fig.

285

5). The Pi removal for the alum-chitosan combination (at constant alum dose of 30 mg/L)

286

increased gradually from 68% to 88% at 49 mg/L and then remained stable with a further

287

increase in the chitosan dosage, as depicted by the red curve in Figure 5a. (a) 85

75

65

Chi* + 30 mg/L Alum dose

55

Alg* + 30 mg/L Alum dose

45

289 290 291

75

65

Chi* + 30 mg/L Alum dose

55

Alg* + 30 mg/L Alum dose

45 0

288

(b)

85 [Pi] removal (%)

[Pi] removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

20

30

40

50

60

Flocculant dosage (mg

70

80

0

P·L-1)

1 2 3 [Flocculant / Pi] molar ratio

4

Figure 5: Phosphate removal (%) as a function of (a) binary coagulant/flocculant dosage and (b) [Alum / Pi] molar ratio. Flocculant with asterisk (*) had variable dosage while the concentration of alum was kept at 30 mg/L. The dosage of Pi was ca. 23 mg/L.

292 293

Comparing this result to the chitosan data in Figure 4, the Pi removal for the binary system is

294

~10% greater than the single component system at comparable optimal dosage values. The single

295

component system had a maximum efficiency of 78% with an optimal dosage of 20 mg/L, where 14 ACS Paragon Plus Environment


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296

the initial dose of Pi was ~10 mg/L. In Figure 5b, the chitosan to Pi mole ratio at a constant alum

297

dose for optimal Pi removal was 2.22. In Figure 4b, the chitosan to Pi molar ratio (without alum)

298

for optimal removal was 3.50. Thus, the combined use of alum and chitosan has synergistic

299

effects according to the enhanced Pi removal. For example, Shak and Wu reported that the binary

300

system consisted of alum and seed gum showed significant removals of total suspended solids

301

and chemical oxygen demand from the real industrial wastewater54.

302

Similarly, the alum-alginate binary system had an optimum dosage of 59 mg/L with 80% Pi

303

removal. As compared to the single component system in Figure 4, an optimal dosage was

304

obtained using Pi at 25 mg/L with maximum Pi removal at 38%. This illustrates that the binary

305

system displays greater Pi removal over the single component system. Alginate possesses excess

306

negative charge in solution which results in floc stabilization with decreased Pi removal of the

307

alum-alginate binary system. Similar results have been presented for the removal of dyes,

308

turbidity, and other substances using alginate with inorganic salts31–33,35,36,55–57.

309

3.3 Ternary system

310

The results in Figure 6 indicate that Pi removal occurred using a combination of alginate, alum

311

and chitosan (ternary system). However, two parameters were held constant while varying the

312

other component. For instance, the alum dosage varied while the chitosan and alginate dosage

313

were fixed at 10 mg/L. Similarly, the alum (30 mg/L) and alginate (10 mg/L) dosage were fixed,

314

while the chitosan dosage varied. Following the same approach, the dosage of alginate was

315

varied while the level of alum (30 mg/L) and chitosan (10 mg/L) were fixed. The concentration

316

of Pi was fixed at 10-11 mg/L throughout the process unless indicated otherwise.


100

100

95

95

90

90

85

85

[Pi] removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[Pi] removal (%)

Page 17 of 31

80 75 70

Alum* + Chi + Alg

65

Chi* + Alg + 30mg/L Alum

60

Alum* + Chi + Alg

65

Alg* + Chi + 30mg/L Alum

55

5 10 15 20 25 30 35 40 45 50 Flocculant dosage

70 60

55

317 318 319 320 321

75

Chi* + Alg + 30mg/L Alum

Alg* + Chi + 30mg/L Alum

0

80

0

(mg·L-1)

1 2 3 [Flocculant / Pi] molar ratio

4

Figure 6: Phosphate removal (%) at variable conditions: (a) ternary coagulant/flocculant dosage and (b) [flocculant / Pi] molar ratio. Initial concentration of Pi was 10-11 mg/L. Coagulant/Flocculant with an asterisk (*) was varied while the others were kept constant. Chi and Alg refer to chitosan and alginate, respectively.

322 323

The blue curve in Figure 6 represents the trend of alum dosage variation, where a dosage of

324

2.0 mg/L resulted in a reduction of Pi levels by 62% in the presence of chitosan and alginate. As

325

the dosage of the alum increased, the efficiency of the Pi removal effect rose steadily until it

326

reached a maximum (82%) at 30 mg/L. Beyond this point, the efficiency of Pi removal decreased

327

to 70% at 40 mg/L and remained constant with increasing alum dosage. In Fig. 6b, the alum to Pi

328

molar ratio was 1.40, while the chitosan and alginate were fixed to achieve optimal Pi removal.

329

The greater Pi removal was noted for the ternary system over the single and binary systems. The

330

advantage of the addition of organic flocculants is evident by comparing the optimum dosage

331

from this study with those in Table 1. The use of chitosan and alginate as a coagulant-flocculant

332

system leads to the formation of macroflocs once alum undergoes coagulation with the dissolved

333

Pi.

334

The effect of chitosan dosage on the Pi removal without pH adjustment is shown in Figure 6

335

(red curve). An increase of the chitosan dosage up to 15.8 mg/L increased the removal of Pi from 16 ACS Paragon Plus Environment


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Page 18 of 31

336

88% to 98.8%. Further addition of chitosan led to a decrease in the removal of Pi to 92% at 30

337

mg/L of chitosan. In Fig. 6b, the chitosan to Pi molar ratio at a constant alum and alginate dose

338

for optimal Pi removal is 1.40, significantly greater than the values obtained from the single and

339

binary systems.

340

Table 3: Phosphate removal in water using different flocculant systems.

341

Wastewater Flocculant Optimum Flocculant/Pi Source /Adsorbent Dosage (g/L) molar ratio Synthetic Alum 35 3.5 wastewater Synthetic Alum* + Alg + 30 2.6 wastewater Chi Synthetic Chi 20 1.6 wastewater Synthetic Alg 25 2.6 wastewater Synthetic Chi* + Alum 49 2.2 wastewater Synthetic Alg* + Alum 59 2.7 wastewater Synthetic Chi* + Alg 15 1.4 wastewater +Alum Synthetic Alg* + Chi + 16 1.4 wastewater Alum * Flocculant dosage was varied while keeping the other constants.

342

Chi and Alg refer to chitosan and alginate, respectively.

Efficiency (%) 67 ± 0.3 85 ± 0.8 78 ± 0.1 38 ± 0.2 88 ± 0.8 80 ± 0.8 98 ± 0.8 98 ± 1.2

343 344

The influence of alginate on the removal of Pi in the ternary system is shown in Fig. 6a (green

345

curve). When alginate is above its pKa value, it interacts favourably with cationic species such as

346

protonated chitosan to form aggregates which result in the precipitation of Pi species33. Similar to

347

chitosan, the efficiency of the Pi removal increased as the alginate dosage increased to an

348

optimum value (15.0 mg/L) with an efficiency of 98%. Beyond this value, further addition of

349

alginate results in a decrease in the level of Pi removal. Similar to chitosan, the alginate to Pi 17 ACS Paragon Plus Environment

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350

molar ratio was 1.39, while the alum and chitosan levels were fixed to achieve variable Pi

351

removal. The effect is significantly greater over that obtained for the single component and

352

binary systems. While alginate is an anionic polyelectrolyte, it associates with chitosan via

353

electrostatic interactions to yield aggregates58 which facilitate microfloc formation between

354

phosphate and alum. In the absence of chitosan in solution, further addition of alginate beyond

355

the optimum dosage results in charge stabilization due to the formation of negatively charged

356

colloidal suspensions which lowers the Pi removal. The ternary system favours efficient Pi

357

removal relative to the binary and single component systems, as reported in Table 3.

358

3.4 pH Effects on phosphate removal

359

Pi removal in water is highly pH-dependent, as shown by the dependence of Pi removal on

360

pH by the addition of 0.1 M NaOH or 0.1 M HCl at different initial dosage levels of Pi in Figure

361

7. The coagulation-flocculation was performed using the optimized condition of the ternary

362

system: alum (30 mg/L), alginate (15 mg/L), and chitosan (15 mg/L). Alum precipitates Pi to

363

form AlPO4, while the chitosan and the alginate biopolymers occur as cation and anion species at

364

these conditions. These biopolymers form cross-linked aggregates which enhance the Pi removal

365

via precipitation of AlPO4 and adsorption processes. The influence of pH on the Pi removal

366

followed a similar trend independent of the initial Pi dosage level. The efficiency increased from

367

pH 2 and reached optimum removal near pH 6-7. Thereafter, the efficiency decreased as the pH

368

increased to pH=10. As alum is added to the Pi-containing-water, a fraction of the alum was

369

precipitated as the hydroxide species and H+ was released according to equation (3).

370

Al3+ (aq) + 3 H2O (l) → Al(OH)3 (s) ↓ + 3 H+ (aq)


(3)


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Page 20 of 31

371

The Al3+ ions are soluble below pH 5.5 and do not precipitate during the dissolution process.

372

In addition, aluminium phosphate (AlPO4) species are formed which are soluble at pH 6 to 7 59.

373

Above pH 7, the addition of alum forms a soluble complex ([Al(OH)4]-), where the efficiency of

374

the coagulation-flocculation process is decreased60. The pH of the solution after the addition of

375

the alum is more important than the initial pH, especially for wastewater with low alkalinity 46.

376

The addition of the alum to low alkalinity water likely results in lower pH which shifts to an

377

optimum range (pH 6-7) for efficient Pi removal61. From the results in Figure 7a, Pi removal

378

occurs at lower dosage at an optimum pH (pH 7). Higher Pi levels reveal optimal removal at pH

379

6-7 and these results are in agreement with other reports8,46,62.

380

Pi removal has a strong dependence on the relative chitosan dosage due to its cationic nature.

381

The amine groups of chitosan are deprotonated at higher pH values, especially at conditions

382

above pH 7.5, since the pKa value for chitosan is near 6.2. Chitosan does not associate

383

favourably with phosphate via attractive ion-ion interactions in its deionized form. By contrast,

384

attractive ion-ion interactions occur below pH 5 due to association of chitosan/phosphate

385

(cation/anion) complexes. The presence of excess chitosan cation species in solution may result

386

in colloidal stabilization at lower pH values below the pKa of chitosan.

387

3.5 Effect of initial phosphate dosage

388

Although pH plays a significant role in Pi removal, the dependence of the initial Pi dosage on

389

the efficacy of the coagulation-flocculation process requires further study. Figure 7b depicts the

390

effect of variable Pi dosage on the Pi removal at optimal pH conditions. In Figure 7b, Pi removal

391

was lower relative to other systems at higher dosage levels when the Pi was fixed at 6.5 mg/L. At

392

low Pi concentration (< 10 mg/L), there is competition due to the formation of hydroxides which

393

prevents the formation of precipitates since the Al3+/ Pi ratio does not favour precipitation at this 19 ACS Paragon Plus Environment

Page 21 of 31

394

condition13. However, a greater Pi dosage (10 to 55 mg/L) indicates there is minor effect on the

395

efficiency of the coagulation-flocculation process. The Pi removal at variable Pi levels (10 to 55

396

mg/L) vary from 96 to 98% at the optimum pH, as shown in Fig. 7b. A Pi dosage of 16.5 mg/L

397

resulted in the highest Pi removal, where typical levels of Pi for contaminated wastewater lie in a

398

similar range (10 to 20 mg/L). Hence, the coagulation-flocculation process is effective for Pi

399

removal in water at relevant levels of Pi found in aquatic and industrial wastewater. With the

400

exception of the lower Pi dosage (< 10 mg/L), it can be concluded that an increase in the Pi

401

concentration does not affect the level of Pi removal. Hence, highly contaminated wastewater

402

laden with Pi above the reported literature values can be effectively treated using the conditions

403

above. pH = 5.53

100

pH = 7.03

pH = 8.55

100 90

95

6.5 ppm 10.0 ppm

90

16.5 ppm 85

21.5 ppm 36.2 ppm

80

41.5 ppm 56.5 ppm

75

2

404 405 406 407

4

6

8

10

70 60 50 40 30 20 10

(a)

70

80

[Pi] removal (%)

[Pi] removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

0 6.5

12

pH

10.6

16.5

20.4

Pi dosage

26.1

41.2

56.4

(mg·L-1)

Figure 7: (a) Effect of pH on Pi (%) in wastewater. The effects of the initial levels of phosphate (ppm) in wastewater on its removal efficiency systems. (b) Effect of phosphate dosage on the Pi (%) in a coagulation-flocculation experiment at variable pH values.

408 409

3.6 Oat hulls as coagulant aids

410

Herein, oat hull suspensions were studied in combination with alum, where the role of oat

411

hulls as a coagulant aid for removal of dissolved Pi in water was evaluated. The efficiency of the 20 ACS Paragon Plus Environment


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Page 22 of 31

412

oat hulls (refined and unrefined) single component and alum-oat hull binary systems for the

413

removal of Pi using coagulation-flocculation process is presented in Figure 8(a, b), at an initial Pi

414

dosage at 25 mg/L. The combinatorial effect of the alum-oat hull binary systems is significantly

415

efficient in the removal of the orthophosphate compared with the use of the single component oat

416

hulls, as observed in Figure 8(a, b). Flocculation was observed to be more effective for the

417

refined oat hull than the unrefined sample in both cases. For the binary systems, as the oat hull

418

dosage reached 0.93 g/L, the Pi removal was 83% (refined oat hull) and 77% (unrefined oat

419

hulls) for an initial Pi dosage of 25 mg/L. Refined oat hulls showed greater Pi removal that

420

increased to 95% at 1.5 g/L with decreased Pi removal to 92% at 1.6 g/L. Thereafter, it increased

421

steadily to 98% at 1.9 g/L and remained constant (95%) up to 2.4 g/L. By comparison, the Pi

422

removal for the unrefined oat hull increased sharply to 92% at 1.8 g/L and decreased to 85% at

423

1.9 g/L. Thereafter, it increased slightly to 89% at 2.1 g/L and remained constant thereafter.

424

The effect of the oat hulls on the Pi removal relative to the coagulant aid (alum) was

425

determined, as shown in Figure 8c. Accordingly, oat hulls may serve as an effective flocculant

426

aid. In Figure 8c, it is evident that the flocculation efficacy for the refined oat hulls increased

427

from 56% at 0.93 g/L compared with 63% at 1.5 g/L for an initial Pi level of 25 mg/L. Pi removal

428

remained nearly constant (63%) up to 2.4 g/L. In the case of the unrefined oat hulls, the Pi

429

removal increased from 48% to 0.93 g/L to 61% at 1.7 g/L and remained constant up to 2.4 g/L

430

when the initial Pi level was 25 mg/L. Comparing these results with those for other biomass

431

materials in Table 4 reveal the utility of oat hulls as suitable coagulant aids for Pi removal in

432

water.

433 434


Page 23 of 31

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435 436 437 438 439 440

Table 4: Efficiency of Pi removal for different biopolymer materials. Wastewater Source Synthetic wastewater Synthetic wastewater Synthetic wastewater Synthetic wastewater Synthetic wastewater

Optimum Dosage (g/L)

[Pi] (mg/L)

Unrefined oat hull

1.7

25

62 ± 0.9

This work

Refined oat hull

1.8

25

76 ± 1

This work

1.8

25

93 ± 1

This work

1.8

25

99 ± 2

This work

3.4 & 9.1

20

97

34.5 & 46.6

20

90

Barbosa et al.63 Barbosa et al.63

0.7

10

65

Flocculant /Adsorbent

Efficiency Reference (%)

Synthetic wastewater

Unrefined oat hull + alum Refined oat hull + alum Fly ash and bottom ash Fly ash and bottom ash La(OH)3 modified pine needles

Synthetic wastewater

Wheat straw biochars

6.0

3 - 11

88

Carbon residue

5.0

25

25

Sugarcane bagasse

16.0

50

11

pulp and paper mill wastewater

Synthetic wastewater Synthetic wastewater

Wang et al.64 Li et al.65 Kilpimaa et al.66 Hena et al.67 ,

441

442

The removal mechanism for the flocculation of Pi with oat hulls may relate to enhanced

443

polymer bridging and/or adsorption processes, along with synergistic effects. Polymer bridging

444

involves the nucleation of micro- to macro-flocs due to coagulation of alum upon addition of oat

445

hulls. In this process, the electrolyte species are bound to domains which can associate with other

446

particles68. The mechanism is favoured and optimal results are obtained when long-chain 22 ACS Paragon Plus Environment


447

polymers without a high level of ionic charge are used69. Here, the cellulose and the hemi-

448

cellulose fractions of the oat hull are uncharged at these pH conditions which may favour the

449

formation of flocs with other species by polymer-bridging effects. The flocs formed after the

450

coagulation process depend on the metal salt dose, where the floc size increases the interaction

451

between polyelectrolytes of opposite charge

452

interact favourably with domains of another floc in a uniform manner. In this process,

453

Coulombic and dispersion forces cause the biomass (oat hull) biopolymers to co-adsorb onto two

454

or more flocs. This occurs when the surface is partly covered, during the initial stages of the

455

process or when the polymer dosage is low. The effect occurs immediately upon addition of the

456

biomass and depends on the mixing conditions 71.

457

70

. Bridging takes place when the adsorbed chains

80

(a)

75

[[Pi] removal (%)

458

459

460

70 65 60 55

ROH UOH

50 45

461

0.8

1.3

1.8

2.3

Oat hull dosage (g·L-1)

462 65

100

465 466 467 468 469

(c) ∆ [(ci - co) / ci] (%)

464

(b)

95

463 [Pi] removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

90 85 80

25 mg/L Pi (ROH)

75 25 mg/L Pi (UOH)

70 65 0.8

1.3

1.8

2.3

2.8

60

55

25 mg/L Pi (ROH)

50

25 mg/L Pi (UOH) 45 0.8

Oat hull dosage (g·L-1)

1.3

1.8

Oat hull dosage


2.3

(g·L-1)

2.8

Page 25 of 31

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470 471 472 473


Figure 8: (a) Pi removal (%) in water as a function of oat hulls, (b) Pi removal (%) in water as a function of alum-oat hull flocculant system, and (c) The dependence of Pi removal (%) in water as a function of oat hull dosage. UOH is the unrefined oat hull and ROH is the refined oat hull. Alum dosage was kept at 30 mg/L and initial Pi dosage was 25.0 mg/L in (b) and (c).

474 475

476

4.0 Conclusions

477

Coagulation and flocculation of orthophosphate (Pi) using alum in conjunction with

478

biopolymers (chitosan and alginate), along with oat hull biomass are reported in this study. The

479

efficiency of the Pi removal was related to the dosage of the coagulant-flocculant system.

480

Variable alum dosing in the alum-chitosan-alginate ternary system revealed an optimum alum

481

dosage of 30 mg/L when the initial Pi level was 10-11 mg/L. Pi removal was more effective for

482

the ternary system relative to the binary- and single-component systems. The binary system

483

(alum/alginate or alum/chitosan) had a lower Pi removal (80% and 88%) relative to the ternary

484

system (98%). The presence of flocculants and the smaller value of alum to phosphate molar

485

ratios (Table 3) indicate that Pi removal depends on pH, with an optimum value at pH 6–7, in

486

agreement with a process that occurs by charge neutralization and polymer-bridging. The initial

487

level of Pi was found to be relatively independent of the Pi removal efficacy. Refined and

488

unrefined oat hulls aid the flocculation process by further removal of dissolved Pi. Variable

489

removal of Pi occurs for the refined (99%) and unrefined (93%) oat hulls at an initial Pi dosage of

490

25 mg/L in the binary system. By considering other properties of alginate and chitosan such as

491

sorption and chelation of metal ions, biocompatibility, biodegradability, non-toxic nature and

492

antimicrobial activity, the use of such biopolymers can enhance Pi removal. The wide availability

493

of oat hull biomass reveal its utility for wastewater treatment, as evidenced by its enhanced



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Page 26 of 31

494

pollutant removal when used in conjunction with conventional flocculation-coagulation

495

processes.

496 497

Acknowledgements The authors are grateful to the University of Saskatchewan (U of S) and the Government of

498 499

Saskatchewan through the Ministry of Agriculture (Agriculture Development Fund; Project

500

#20140260), for supporting this research. Dr. Majid Soleimani from the Department of Chemical

501

and Biological Engineering (U of S) is also acknowledged for providing complimentary samples

502

of oat hulls.

503 504

References

505

(1)

Rybicki, S. Phosphorus Removal From Wastewater - A Literature Review; 1997, 1-106

506 507

(2)

Benyoucef, S.; Amrani, M. Adsorption of Phosphate Ions onto Low Cost Aleppo Pine Adsorbent. Desalination 2011, 275, 231-236.

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(3)

Painting, S. J.; Devlin, M. J.; Malcolm, S. J.; Parker, E. R.; Mills, D. K.; Mills, C.; Tett, P.; Wither, a.; Burt, J.; Jones, R.; et al. Assessing the Impact of Nutrient Enrichment in Estuaries: Susceptibility to Eutrophication. Mar. Pollut. Bull. 2007, 55, 74-90.

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(4)

Sø, H. U.; Postma, D.; Jakobsen, R.; Larsen, F. Sorption of Phosphate onto Calcite; Results from Batch Experiments and Surface Complexation Modeling. Geochim. Cosmochim. Acta 2011, 75, 2911-2923.

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Gautam, R. K.; Banerjee, S.; Gautam, P. K.; Chattopadhyaya, M. C. Remediation Technologies for Phosphate Removal From Wastewater: An Overview. Adv. Environ. Res. 2014, 36, 1-23.

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Bektaş, N.; Akbulut, H.; Inan, H.; Dimoglo, A. Removal of Phosphate from Aqueous Solutions by Electro-Coagulation. J. Hazard. Mater. 2004, 106, 101-105.

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Peleka, E. N.; Mavros, P. P.; Zamboulis, D.; Matis, K. a. Removal of Phosphates from Water by a Hybrid Flotation-Membrane Filtration Cell. Desalination 2006, 198, 198-207.

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Mohammed, S. A. M.; Shanshool, H. A. Phosphorus Removal from Water and Waste Water by Chemical Precipitation Using Alum and Calcium Chloride. Iraqi J. Chem. Pet. Eng. 2009, 10, 1-6. 25 ACS Paragon Plus Environment

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Sowmya, A.; Meenakshi, S. Effective Removal of Nitrate and Phosphate Anions from Aqueous Solutions Using Functionalised Chitosan Beads. Desalin. Water Treat. 2014, 52, 2583-2593.

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Teh, C. Y.; Budiman, P. M.; Shak, K. P. Y.; Wu, T. Y. Recent Advancement of Coagulation-Flocculation and Its Application in Wastewater Treatment. Ind. Eng. Chem. Res. 2016, 55, 4363-4389.

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Tzoupanos, N. D.; Zouboulis, A. I. Coagulation-Flocculation Processes in Water / Wastewater Treatment : The Application of New Generation of Chemical Reagents. 6th IASME/WSEAS Int. Conf. HEAT Transf. Therm. Eng. Environ. 2008, 309-317 .

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Renault, F.; Sancey, B.; Badot, P. M.; Crini, G. Chitosan for Coagulation/flocculation Processes - An Eco-Friendly Approach. Eur. Polym. J. 2009, 45, 1337-1348.

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Bratby, J. Coagulation and Flocculation with an Emphasis on Water and Wastewater Treatment. 1980, 55-58.

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Zemmouri, H.; Drouiche, M.; Sayeh, A.; Lounici, H.; Mameri, N. Chitosan Application for Treatment of Beni-Amrane’s Water Dam. Energy Procedia 2013, 36, 558-564.

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Pratt, D. Y.; Wilson, L. D.; Kozinski, J. A. Preparation and Sorption Studies of Glutaraldehyde Cross-Linked Chitosan Copolymers. J. Colloid Interface Sci. 2013, 395, 205-211.

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Varma, A. .; Deshpande, S. .; Kennedy, J. . Metal Complexation by Chitosan and Its Derivatives: A Review. Carbohydr. Polym. 2004, 55, 77-93.

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Divakaran, R.; Pillai, V. N. S. Flocculation of Algae Using Chitosan. J. Appl. Phycol. 2002, 14, 419-422.

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Bratskaya, S. Y.; Avramenko, V. A.; Sukhoverkhov, S. V.; Schwarz, S. Flocculation of Humic Substances and Their Derivatives with Chitosan. Colloid J. 2002, 64, 681-686.

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Dunets, C. S.; Zheng, Y. Combined Precipitation / Flocculation Method for Nutrient Recovery from Greenhouse Wastewater. HORTSCIENCE 2015, 50, 921-926.

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Fortin–Chevalier, T.; Leduc, R. Removal of Phosphorus and Residual Aluminium with the Simultaneous Use of Chitosan and Alum on the Effluent of an MBBR Biological System during Start–up. Int. J. Environ. Waste Manag. 2014, 14, 358-375.

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Crini, G.; Badot, P.-M. Application of Chitosan, a Natural Aminopolysaccharide, for Dye Removal from Aqueous Solutions by Adsorption Processes Using Batch Studies: A Review of Recent Literature. Prog. Polym. Sci. 2008, 33, 399-447.

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Aguilar-May, B.; Del Pilar Sánchez-Saavedra, M. Growth and Removal of Nitrogen and Phosphorus by Free-Living and Chitosan-Immobilized Cells of the Marine Cyanobacterium Synechococcus Elongatus. J. Appl. Phycol. 2009, 21, 353-360.

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