Chemical Engineering Journal 246 (2014) 383–390

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Chemical Engineering Journal

journal homepage: www.elsevier .com/locate /cej

Coagulation-flocculation of azo dye Acid Orange 7 with green refinedlaterite soil

http://dx.doi.org/10.1016/j.cej.2014.02.1001385-8947/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +60 4 6532215; fax: +60 4 657 3678.E-mail address: ttteng@usm.my (T.-T. Teng).

Yen-Yie Lau a, Yee-Shian Wong a,b, Tjoon-Tow Teng a,⇑, Norhashimah Morad a, Mohd Rafatullah a,Soon-An Ong b

a School of Industrial Technology, Universiti Sains Malaysia, 11800 Gelugor, Pulau Pinang, Malaysiab School of Environmental Engineering, Universiti Malaysia Perlis, Kompleks Pusat Pengajian Jejawi 3, 02600 Arau, Perlis, Malaysia

h i g h l i g h t s

� Acid Orange 7 (AO 7) dye degradationpathway using laterite soil isestablished.� Laterite soil is bi-functional

(coagulation and flocculation).� n-Methylsisiloxane and n-

methyldisilathiane are formed viainterparticle bridging.� The present system controls release

of sulfate ion to be exposed toenvironment.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 December 2013Received in revised form 21 February 2014Accepted 26 February 2014Available online 12 March 2014

Keywords:Laterite soilCoagulationAzo dyeAcid Orange 7Dye degradation

a b s t r a c t

A novel degradation pathway of Acid Orange 7 (AO 7) by laterite soil in coagulation process is suggested.AO 7 has a strong azo bond links with benzene and naphthalene rings. The rings in AO 7 can be cleavedduring coagulation and flocculation by laterite soil. Laterite soil rich in silica component could treat AO 7with 98.43% color removal at pH 2. It could degrade azo dye molecule to the simplest hydrocarbon struc-ture and non-toxic products in coagulation process through a linkage of reactions: CAN bond cleavage,desulfonation, silication, diazene reductase and polymerization. Ascertainment of the degradation path-way and products were characterized by UV–Vis and FT-IR techniques. The results show that laterite soilis bi-functional as a coagulant as well as a flocculant. Sludge Volume Index (SVI) with sludge generated aslow as 21.703 mL/g provides evidence that, laterite soil has polymerization characteristic by formingn-methylsisiloxane and n-methyldisilathiane. Laterite soil detached sulfate ion in AO 7 to formn-methyldisilathiane, indirectly preventing sulfate ion to be exposed to environment.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Degradation pathway of specific textile dyes has recently re-ceived attention from researchers. This happens because effluentsprocessed from textile finishing companies have responded tothe high mutagenic effects impacting the existing treatment plant

and water quality [1]. Degradation intermediates and byproductsmay carry mutagenic and carcinogenic potential that would affecthuman health and environment [2]. Levin et al. [3] and Li et al. [4]reported that several intermediates and byproducts with toxiccharacteristics such as azo bond and aromatic amines have beenreleased into water bodies. Azo dyes characterized by azo bond(AN@NA) structure responsible for the color richness have poseda major pollution problem to environment due to color visibilityand toxicity. Azo dye molecules could not be broken down under

Nomenclature

AO 7 Acid Orange 7ATR Attenuated Total ReflectionCl� chloride ionFT-IR fourier transform infrared spectroscopyH+ hydrogen ionHCl hydrochloric acidHN@NH diazeneHS� sulfanideHSO3

� hydrosulfite

KBr potassium bromidemg/L milligram/litreNa+ sodium ionNaCl sodium chlorideOH� hydroxide ionS2� sulfideSi(OH)4 silicic acidSO3

2� sulfite

384 Y.-Y. Lau et al. / Chemical Engineering Journal 246 (2014) 383–390

the sunlight or radiant energy due to their high photolytic stablecharacteristic, thus critically affecting the aquatic system. Duringdegradation, azo dyes cleavages potentially generate aromaticamines. Some aromatic amines are considered carcinogenic sinceit can accumulate in the food chain and imperil human healthand ecosystem. Therefore, it is necessary to develop mechanismpathway of dye degradation to ensure that the degradation inter-mediates and byproducts yielded are safe to discharge into waterstreams.

A few methods have been proven to be effective for dye degra-dation [5–7]. To date, the pathway on the degradation of dyes incoagulation process is yet to be explored. Researchers have focusedtheir studies on coagulation since it is a physicochemical treatmentprocess that shows high performance in treating dyewastes [8,9].Various inorganic coagulants such as aluminum, ferric, magnesiumsalts and lime had been applied alone to treat dye wastewater.However, the sludge produced from inorganic coagulant is toxic[10]. Flaten [11] reported that aluminum is a neurotoxicant prod-uct that contributes to Alzheimer’s disease. Therefore, naturalcoagulants have recently received high degree of interest in treat-ing dye wastewater.

In the present work, silicon (Si)-rich laterite soil as a naturalcoagulant was used in treating dyeing bath effluent. It was in-tended to investigate the degradation mechanism pathway of AcidOrange 7 (AO 7) using naturally prepared laterite soil. Observationof the AO 7 dye degradation under various pH and dosages of lat-erite soil coagulant was performed. UV–Vis spectra and FT-IR anal-ysis were used to identify the degradation products duringtreatment and to establish a pathway of degradation. Degradationpathway can assist in ensuring zero potential hazards of interme-diates and products formed. Determination of sludge generationin coagulation yielded from laterite soil was carried out by SludgeVolume Index (SVI) test. Besides, the wet sludge was tested forreusability.

Table 1Composition of laterite soil.

Compound Concentration (wt.%)

SiO2 36.3000Al2O3 27.1000Fe2O3 26.8600Others 9.7400

2. Methodology

2.1. Dye

Acid Orange 7 (AO 7) with molecular formula C16H11N2NaO4Swas supplied by Acros Organic. Stock solution of AO 7 was pre-pared by dissolving 1 g of AO 7 with deionized water in a 1 L vol-umetric flask. An appropriate amount of AO 7 from the stocksolution was diluted to 500 mL with deionized water to prepare50 mg/L of AO 7 sample.

2.2. Coagulant

The natural coagulant used in this study was laterite soil col-lected from Bukit Merah, Perlis, Malaysia. The characteristics ofthe laterite soil had been analyzed by using X-ray Fluorescence

(XRF) and the corresponding identified compounds are listed inTable 1. From the characteristic study, laterite soil shows thecapability of the commonly utilized coagulants in treating waste-water. The reddish laterite soil was decolorized before beingused as coagulant to treat synthetic dye wastewater. Flow chartfor the decolorized method of reddish laterite soil is presented inFig. 1. Initially, laterite soil was mixed with ultrapure water andblended, subsequently the mixture was filtered with a nyloncloth to screen out the large particles and impurities. The solu-tion containing dissolved laterite soil was filtered through thenylon cloth. The laterite soil containing filtrate was collectedand left to settle until double layers were observed. Since thehydrolyzed laterite soil has higher density compared to water,the upper layer was aqueous phase whereas the bottom layerwas laterite soil paste. The aqueous phase was drained off andthe laterite soil paste was dried under the sunlight. The proce-dure was repeated until the laterite soil was decolorized. Thedried laterite soil was then crushed into powder and sievedthrough 75 lm sieve. The sieved laterite soil powder was mixedin ultrapure water and blended, followed by filtration to removeexcessive fluid. Laterite soil paste collected on the filter paperwas readily used as coagulant.

2.3. Coagulation experiments

Experiments on dyewaste coagulation were performed usingJar-Test apparatus (JLT6 VELP Scientifica). Jar test apparatus wasused to determine the optimum pH. The initial pH of the solutionwas measured by 827 pH lab (Methrohm) and then adjusted to arange of values (pH 1, 2, 3, 5, 7, 9) using dilute HCl or NaOH solu-tion (0.01–0.1 M). The mixtures of dye wastewater and laterite soilwere stirred rapidly at 200 rpm for 2 min, then the stirring speedwas decreased to 100 rpm for 15 min. Subsequently, the mixtureswere allowed to settle for 30 min. 20 mL of the samples were col-lected from the top of the settling beaker (2 cm beneath the watersurface). The collected samples were filtered through WhatmanNo. 4 filter papers of 12.5 cm diameter and 20–25 lm pore size.The filtrates were ready for analysis. The determined optimumpH was used and the subsequent experiments were continued byvarying the dosage of laterite soil coagulant from 500 to10,000 mg/L. The procedures were continued as mentioned above.

Laterite soil was collected

from Bukit Merah, Perlis,

Malaysia.

The laterite soil was mixed with ultrapure water

and blended

The mixtures were filtered with nylon

cloth

The paste was collected and

dried under the sunlight

The above procedures was repeated until the paste was decolorized

Dried laterite soil was crushed into powder and

sieved through 75 µm sieve

The sieved powder was

mixed in ultrapure water

and blended

Paste collected was readily used as coagulant

Fig. 1. Flow chart for decolorizing method of reddish laterite soil.

Res

idua

l tur

bidi

ty, N

TU

Col

or r

emov

al, %

pH

Color removal Residual turbidity

Fig. 2. Color removal and residual turbidity of AO 7 after coagulation by laterite soilat various initial pH.

Table 2One-way ANOVA as a function of pH for (a) color removal (b) residual turbidity.

dfa SSb MSc F Significance p�

(a) ANOVARegression 1 5290.7730 5290.7730 16.3424 0.0156Residual 4 1294.9800 323.7450Total 5 6585.7529

Coefficients Standard Error t Stat P-value

Intercept 93.5259 13.8555 6.7500 0.0025X variable 1 �10.5539 2.6107 4.0426 0.0156

dfa SSb MSc F Significance p�

(b) ANOVARegression 1 3129794.2180 3129794.2180 34.2175 0.0043Residual 4 365870.4977 91467.6244Total 5 3495664.7150

Coefficients Standard error t Stat P-value

Intercept 396.5190 232.8920 1.7026 0.1639X variable 1 256.6913 43.8820 5.8496 0.0043

� Significant (p < 0.0500).a df = degree of freedom.b SS = sum of squares.c MS = mean square.

Y.-Y. Lau et al. / Chemical Engineering Journal 246 (2014) 383–390 385

The procedures for the determination of reusability of laterite soilwere exactly the same as the typical jar test mentioned above atthe optimum pH and dosage of laterite soil. Once the fresh lateritesoil had been used to treat the AO 7 dye, the settled wet sludge wascontinuously collected and reused in removing fixed concentrationof AO 7.

2.4. Analysis

For the optimum operating pH determination, the concentra-tion of the dye and turbidity after coagulation by laterite soil atvarious pH were analyzed. The color point of a dye solution wasdetermined by using Shimadzu UV-1601PC UV–Visible ScanningSpectrophotometer. The turbidity in Nephelometric Turbidity Unit(NTU) was determined by using Eutech TN 100 turbidity meter. Forthe optimum dosage determination, the UV–Vis spectra of AO 7 asa function of laterite soil dosage were recorded from 200 to 800 nmusing Shimadzu UV-1601PC UV–Visible Scanning Spectrophotom-eter. After determining the optimum pH and laterite soil dosage intreating AO 7, the sludge obtained at these optimum conditionswas sent for Sludge Volume Index (SVI) measurement and FT-IRanalysis. SVI measurement was carried out in accordance withthe Standard Methods for the Examination of Water and Wastewa-ter, 2005 [12]. It is the volume in milliliters occupied by 1 g of asuspension after 30 min. settling. Equation used to calculate SVIis given below:

SVI ¼settled sludge volume after 30 min mL

L

� �� 100

total suspended solids gL

� � ð1Þ

The FT-IR measurement on the sludge was carried out at roomtemperature on a Nicolet iS10 Smart OMNI-transmission FT-IRSpectrometer. The sludge produced after coagulation with lateritesoil was collected and dried in the oven at 105 �C for 2 h. Solidifiedsludge was dried and ground into powder and dispersed in a ma-trix to mix with potassium bromide (KBr) solution. The mixturewas transferred to a die with a barrel diameter of 13 mm. Subse-quently the die was pressed to a disc form. The disc was removedcarefully from the die and place in the FT-IR sample holder. Thecontent in the disc was ready for the 4000–450 cm�1 transmissionspectrum analysis.

3. Results and discussion

Fig. 2 demonstrates the effect of pH on decolorization and resid-ual turbidity at laterite soil dosage of 10,000 mg/L. A statistical ap-proach was performed and the results are presented in Table 2. Theone-way ANOVA shows a significant result since the p value forcolor removal (0.0156) and residual turbidity (0.0043) are less than0.0500. It is clear that pH 2 is the optimal pH which gives the high-est removal percentage of 98.43% and the residual turbidity is aslow as 7.64 NTU. Basically, further increase in pH values of dyesolutions to pH 9 led to decrease in color removal and increasein residual turbidity. According to Table 1, the major constituentsof laterite soil are SiO2 (36.3000%), Al2O3 (27.1000%) and Fe2O3

(26.8600%). It tends to develop charges when in contact withwater. Hydrolysis took place forming Si(OH)4, Al(OH)3 and Fe(OH)3.Silica in laterite soil dominated the coagulation performance. Lykl-ema [13] reported that around pH 2 is the point of zero charge forsilica. At the point of zero charge, silica was present in [SiOH+

2] and[SiO3

2�] forms [14,15], which was able to act as coagulant. Cationicand anionic silica present in the system would enhance the

Abs

orba

nce,

abs

Wavelength, nm

10000

8000

7000

6000

5000

4000

3000

2000

1000

500

Original

Fig. 4. Molecular structure of AO 7 and UV–Vis spectra of dye solutions before andafter coagulation by laterite soil at dosages from 500 to 10,000 mg/L.

Acid Orange 7

Silication

C-N cleavage

hydroxynapthalenediazoniumbenzenesulfonic acid

Desulfonation

naphthalenediazoniumbenzene

386 Y.-Y. Lau et al. / Chemical Engineering Journal 246 (2014) 383–390

neutralization reaction in the system. When the pH value was in-creased to alkaline condition with the addition of sodium hydroxide(NaOH), coagulation performance decreased. This was happening be-cause at basic condition, the presence of sodium hydroxide causedthe silica ions to form a stable compound, Na2SiO3 [15] as shownin the equation below:

SiðOHÞ4 þ 2NaOH$ Na2SiO3 þ 3H2O ð2Þ

Thus, the optimum pH should take into consideration to ensureeffective charge neutralization in the system.

Fig. 3 shows the changes of zeta potential and final pH for later-ite soil as a function of pH. Zeta potential measurements were usedto provide an indication of the surface charge present on the later-ite soil particles when they were in aqueous suspension. It wasessential to identify the aggregation and peptization of laterite soilcolloids, as well as their tendency towards coagulation [16]. Whenlaterite soil was introduced into the aqueous dye solutions, electri-cal charge existed at the interface. Therefore, charges at the lateritesoil and dyes interface were rearranged in order to balance thecounter ions. Hence, electrical double layer took place [17]. Thepoint of zero charge (PZC) for the laterite soil was detected at thepH value of 2.5. Similar PZC was reported by Binner and Zhang[18] in their study using silicon carbide and silicon powders.Hence, laterite soil containing the same character of silica formeda dominant layer of SiO2 on the surface. At pH value of 2.5, the sil-ica colloids remained stationary in an electrical field [19]. At thispoint, there was existence of attraction forces between silica col-loids (laterite soil) with dye particles to form highly densed flocs[20]. For the optimum color removal at pH 2, the final pH of thesample was measured at pH 2.11. Only a small increment of pHwas observed.

AO 7 has been selected for the study of degradation mechanismsince AO 7 has a strong azo bond which links with benzene andnaphthalene ring. It also consists of sulfonate group (ASO3) and hy-droxyl (AOH) auxochromes responsible for intensifying the colorof chromophore [9]. The molecular structure of AO 7 is shown inFig. 4 along with the UV–Vis spectra of dye solutions before andafter coagulation by laterite soil at dosages from 500 to10,000 mg/L. Untreated AO 7 is characterized by one main bandin the visible region, with its maximum absorption at 483.5 nm,which is originated from color givers of chromophores (azo bond).The other two peaks observed in the ultraviolet region positionedat 310 nm and 228 nm are associated with naphthalene ring andbenzene structures in the AO 7 dye molecule, respectively [21].

The variation of spectra resulted from coagulation as a functionof coagulant dosage (500–10,000 mg/L) can be observed from theUV–Vis spectra in Fig. 4. As the dosage of laterite soil was in-creased, intensities of the peaks in the ultraviolet and visible

Fin

al p

H

Zet

a po

tent

ial,

mV

pH

Zeta potential Final pH

Fig. 3. Zeta potential and final pH of AO 7 after coagulation by laterite soil atvarious initial pH.

n-methyldisilathiane n-methyldisiloxane

Diazene reduction

phenylsilane naphthaleneylsilane

n n

Fig. 5. Degradation pathway of AO 7.

regions decreased gradually, without appearance of any newabsorption bands. Slow decay of the absorbance peaks at 228,

Y.-Y. Lau et al. / Chemical Engineering Journal 246 (2014) 383–390 387

310 and 483.5 nm is attributed to the formation of intermediatesduring degradation of AO 7. As the laterite soil dosage was in-creased to the optimum value of 10,000 mg/L, the absorbancepeaks of benzene and the naphthalene-type rings could hardly beobserved in the ultraviolet regions. This phenomenon indicatesthat aromatic fragment degradation of the AO 7 dye molecule is al-most complete.

3.1. AO 7 degradation mechanism pathway

The degradation pathway for AO 7 is established and presentedin Fig. 5. The pathway has been verified through the UV–Vis spec-tral changes (Fig. 4) and FT-IR products (Fig. 6).

3.1.1. CAN cleavageWhen silica contained laterite soil was introduced into the dye

solution in acidic condition, the CAN single bond between azobond and benzene ring was preferentially removed [22]. This hap-pened because silica tended to destabilize the weaker bond whichsubsequently would enhance the formation of phenylsilane (Eq.(8)). FT-IR (Fig. 6) demonstrates the disappearance of CAN of aro-matic amine between 1353.04 and 1283.66 cm�1 wavenumbers.The CAN cleavage mechanism is shown in Eq. (3). Benzenesulfon-ic acid (C6H6SO3) and hydroxynaphthalenediazonium (C10H8N2O)were formed as a result of the breakage of CAN bond. Meanwhile,sodium ion, Na+ has also been released into the system. Thesecompounds were the primary reaction intermediates which werefurther degraded to form lower-molecular weight products.

benzenesulfonic acid hydroxynaphthalenediazonium Acid Orange 7 (AO 7)

ð3Þ

The unstable hydroxynaphthalenediazonium is spontaneouslytransformed into naphthalenediazonium (C10H8N2), as stated inEq. (4). A similar compound has exemplified in the work done byGhodake et al. [23] in mono azo dye. Hydroxide ion detached fromhydroxynaphthalenediazonium led to the formation of Si(OH)4. Itwas detected in the sludge at the wavenumber of 913.15 cm�1 un-der FT-IR scan as presented in Fig 6(b).

naphthalenediazonium hydroxynaphthalenediazonium

ð4Þ

3.1.2. DesulfonationThe benzenesulfonic acid from the breakage of CAN bond sub-

sequently underwent desulfonation reaction as shown in Eq. (5).The benzenesulfonic acid is an aryl sulfonic group chained to ben-zene. This group is easily cleaved during degradation process dueto the extended valence shells of large sulfur atom which were alsoreported by Hisaindee et al. [24]. Cleavage of aryl sulfonic groupwas promising due to the attack of silica at the benzene ring.

Detachment of the sulfonate group from the benzenesulfonic acidled to formation of hydrosulfite ion (ASO3H) and benzene (C6H6)in the aqueous solution.

benzenesulfonic acid benzene hydrosulfite

ð5Þ

The hydrosulfite ion decomposed to become a more stable sulf-anide ion (HS-). Equilibrium reaction of sulfanide producing dian-ion of S2� is shown below [25].

2HSO�3 $ 2HS� þ 3O2 ð6Þ

HS� $ Hþ þ S2� ð7Þ

3.1.3. SilicationThe intermediate naphthalenediazonium (Eq. (4)) and benzene

(Eq. (5)) are attacked by silica on silication reaction. Silication tookplace in a stepwise manner to yield phenylsilane (C6H8Si) andnaphthalenylsilane (C10H7Si) as shown in Eqs. (8) and (9). FT-IR(Fig. 6(b)) at wavenumber peak of 754.2 cm�1 represents phenylsi-lane and of 1030 cm�1 represents naphthaleneylsilane in thesludge. Once AN@NH of naphthalenediazonium is replaced by sil-ica, it subsequently rendered to naphthalenylsilane (C10H7Si) andAN@NH ion. The released AN@NH ion accepted the excessive H+

ions in the system to form diazene (HN@NH).

phenylsilane benzene

ð8Þ

naphthalenediazonium naphthalenylsilane

ð9Þ

3.1.4. Diazene reductionDiazene, also known as diimide, is a strong reducing agent for

carbon–carbon double bond, (AC@CA) [26]. Diazene reductasehas directed the reduction of benzene and naphthalene rings ofAO 7. The FT-IR sludge scanning in Fig 6(b) has certified that thewavenumber of 1500 cm�1 which represents carbon–carbon

1020

.00

1500.00-1550.00

1530

.00

1353.04-1283.63

1046.83

1639

.03

3690

.00

754.

20

3100

.00

(b)

(a)

1030

.00

913.

15

Fig. 6. FT-IR scanning of (a) AO 7 and (b) sludge.

388 Y.-Y. Lau et al. / Chemical Engineering Journal 246 (2014) 383–390

in benzene and naphthalene rings as labeled in Fig. 6(a)disappeared.

Benzene and naphthalene rings of the unstable phenylsilaneand naphthalenylsilane, respectively, are being altered by diazene.Scission of the benzene and naphthalene rings created linear prod-ucts, hydrocarbon (CH)n as shown in Eq. (10). This has been con-firmed from Fig. 6(b): two peaks at wavenumber of 1639.03 and3100 cm�1 represent AC@CA and @CH, respectively, resulted fromthe rings opening. Naphthalene compound would degrade moreeasily and rapidly than benzene compound since naphthalenecompound is not as stable as benzene [27]. When diazene has suc-cessfully opened the ring of benzene and naphthalene, phenylsi-lane and naphthalenylsilane could crystallize into a regularlattice of n-methylsisiloxane [28] and n-methyldisilathiane [29].Fig. 7 shows the molecular structure of n-methylsisiloxane withsilica bonded to carbon and oxygen atoms whereas Fig. 8 showsthe molecular structure of n-methyldisilathiane with silica bondedto carbon and sulfur atoms.

Fig. 7. Structure of n-methyldisiloxane molecule.

ð10Þ

3.1.5. PolymerizationThe final degradation products, n-methylsisiloxane and

n-methyldisilathiane potentially act as polymers by forming highlydensed floc. This phenomenon is strongly supported by the SludgeVolume Index (SVI) and FT-IR. The SVI of AO 7 lies in the best rangeof the index with a value of 21.703 mL/g. Additionally, the FT-IRwavenumbers of 1070 cm�1 is the evidence of the Si–O–Si

Fig. 8. Structure of n-methyldisilathiane molecule.

0

10

20

30

40

50

60

70

80

90

100

1 2 3 4 5 6 7 8

Col

or r

emov

al, %

Times, n

Fig. 9. AO 7 Reusability of laterite soil.

Y.-Y. Lau et al. / Chemical Engineering Journal 246 (2014) 383–390 389

asymmetry stretching vibration and 3690 cm�1 representsorganosilicon structure [30]. Surprisingly, laterite soil is a novelbi-functional material which is able to act as a coagulant andflocculant spontaneously.

Here is another beneficial part of the n-methylsisiloxanepolymer formation. Sulfate ion is a risky substance potentiallyendangering the environment and human due to their good watersolubility, poor degradability and latent carcinogenicity character-ization. The n-methylsisiloxane accepted sulfate ion from theconversion of sulfonate groups as derived in Eqs. (5)–(7), indirectlyminimizing exposure of this hazardous substance to theenvironment.

To finalize the AO 7 degradation, the original AO 7 stretchingvibration peaks at 1500–1550 cm�1 for azo bond (AN@NA),1530 cm -1 for carbon–carbon (ACACA) in aromatic compound,1046.83 cm�1 of sulfonate group and 1353.04–1283.63 cm�1 ofCAN stretching of aromatic amine group [31,32] as stated inFig. 6(a) are entirely absent in the FT-IR spectrum of the sludge(Fig. 6(b)) after coagulation with laterite soil at optimum condition.Furthermore, for the aqueous phase, UV–Vis spectrum shows thatthe color of AO 7 has been reduced after treated with laterite soil.The mechanisms of the present process consist of charge neutral-ization followed by interparticle bridging. Charge neutralizationoccurred when hydrochloric acid was used to adjust the pH ofthe solution to 2. Laterite soil coagulant received sufficient amountof positively charged hydrogen ions to neutralize the AO 7 dyestructure and the reactions involved are: CAN cleavage, desulfona-tion and diazene reduction [33]. Subsequently, interparticle bridg-ing happened when silica had covered the particle surface asreported by Depasse [34]. This mechanism took place at the poly-merization stage by forming organosilicon structure.

3.2. Reusability of laterite soil

Fig. 9 shows the reusability of laterite soil in treating fixedconcentration of AO 7 dye bath (50 mg/L). Initially, the fresh lat-erite soil could reach color removal of 98.47%. For the subsequentcoagulation by sludge to treat AO 7, gradual decrease of percent-age color removal was observed. As the sludge was reused for the1st time, it could reach 98.05% of color removal, only 0.40%decrement compared to the initial usage of fresh laterite soil.When the sludge was reused for the 8th time, 32.97% of colorremoval was observed. Gradual decrement of removal efficiencyis conceivable as the wet sludge contamination has minified thecoagulation capacity of laterite soil after being reused for severaltimes. Based on the above findings, reusing the laterite soil sludgeis a better strategy to completely utilize the coagulation capacityof laterite soil.

4. Conclusion

AO 7 dye degradation pathway by laterite soil in coagulation isestablished. AO 7 dye has been degraded to the simplest hydrocar-bon structure through reactions of CAN bond cleavage, desulfona-tion, silication, diazene reductase and polymerization. Despite thefunction of coagulation, laterite soil also acts as efficient flocculantby forming n-methylsisiloxane and n-methyldisilathiane withsludge generated as low as 21.703 mL/g. The present system con-trols release of sulfate ion to be exposed to environment by detach-ing sulfate ion in AO 7 to form n-methyldisilathiane.

Acknowledgements

The authors would like to express their thanks for the supportby the Project for Environmental Technology of University SainsMalaysia (USM) and School of Environmental Engineering of Uni-versity Malaysia Perlis (UniMAP).

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