# Coagulation and Sedimentation of Concentrated Laterite Suspensions: Comparison of Hydrolyzing Salts in Presence ofGrewia spp. Biopolymer

Authors: Kameni Ngounou M. Bernard; Ndi K. Sylvere; Kofa G. Patrice; Kayem G. Joseph
Journal: Journal of Chemistry
 (2019)

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## Abstract

Coagulation and sedimentation performances of aluminum sulphate and ferric chloride were comparatively investigated in presence ofGrewia spp. biopolymer for the treatment of concentrated laterite suspensions. Jar tests experiments were carried out at different laterite suspension concentrations (10, 20, and 30 g/L) and pH values (5 and 7). The performances of these coagulants were assessed in terms of interfacial settling velocities and sediment concentration factors. Results showed that after addition of Grewia spp. biopolymer, sedimentation velocities were greater when ferric chloride and aluminum sulphate were used alone. When hydrolyzing salts were used alone, the highest settling velocities were obtained with 10 g/L of laterite suspension at pH 5 and settling speeds were 0.22 and 0.28 cm/min for aluminum sulphate and ferric chloride, correspondingly. Addition of Grewia spp. biopolymer led to an increase of settling velocities to 0.56 and 0.57 cm/min, respectively. The sediment concentration factor was also found to be high when Grewia spp. was added. With 30 g/L of laterite suspension, sediment concentration factors at pH 5 were 1.47 and 2.12 for aluminum sulphate and ferric chloride separately. Addition of Grewia spp. biopolymer with aluminum sulphate and ferric chloride produced more compact sludge with sediment concentration factors of 4 and 3.13, respectively. Flocs structures could successfully explain the obtained results.


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## Body

## 1. Introduction

Rivers are one of the main sources of raw water for the production of drinking water in the tropical region. Thus, during the rainy season, river water contains extremely high concentration of suspended matter especially laterite clay particles which are the major component of soil. These particles confer to the brown color of raw water and may act as adsorbent of pollutants [1, 2]; thus they need to be removed from raw water. Classical water treatments include coagulation-flocculation, sedimentation, filtration, and disinfection processes. The coagulation-sedimentation process is used worldwide in water and wastewater treatment as a major pretreatment for filtration [3]. It has the potential to remove colloidal particles like laterite from water [4, 5]. The performance of the coagulation-sedimentation process is widely affected by coagulant types that enhance the aggregation of particles to form large, rapid-settling flocs either through charge neutralization or through chain-bridging mechanisms [6, 7]. Many coagulants are widely used in water treatment processes and can be classified as natural organic (extract of Moringa seed and Opuntia), inorganic (e.g., aluminum sulphate, iron sulphate, and iron chloride), and synthetic organic polymers (e.g., polyacrylamides). Amongst these, hydrolyzing salts and cationic synthetic polymers are very efficient for turbidity removal from water [8–10]. Aluminum-based coagulant is mainly used in municipal water treatment plants because it is relatively cheap [11]. However, its use is associated with residual aluminum in treated water which has been linked to the development of Alzheimer’s disease and certain cancer types in humans [6, 12, 13]. Also, the use of synthetic polymers releases neurotoxic monomers. These synthetic coagulants/flocculants also produce nonbiodegradable sludge [14, 15]. Thus, the search for substitutes to conventional coagulants has become an important defy in the water treatment process, with the aim of lessening the detrimental effects related to their use. Several studies have shown that naturally occurring coagulants/flocculants can be used for the treatment of effluents and drinking water [5, 12, 16]. These natural coagulants/flocculants, in contrast to synthetic ones, are more accessible and nontoxic [17, 18]. All those advantages justify the actual rising interest for natural coagulants [12, 13, 19]. We have shown during our previous work that biopolymer extracted from Grewia spp. bark has flocculating properties [20]. There are several studies on the sedimentation of kaolin suspension with salts alone and in combination with some coagulant aids [21–23]. However, to our knowledge, nothing is known about the sedimentation behavior of laterite suspension when Grewia spp. biopolymer is used in combination with hydrolyzing salts. This study is aimed at examining the effects of hydrolyzing salts as coagulants in conjunction with Grewia spp. biopolymer as natural coagulants aid on the sedimentation behavior of concentrated laterite suspension.


## 2. Materials and Methods

### 2.1. Chemicals

The chemicals used in this study were of analytical grade. Aluminum sulphate (alum) and ferric chloride (FeCl3) were from Prolabo Company. These coagulants were prepared daily to avoid polymerization in solution. Sodium hydroxide and hydrochloric acid both at 0.1 M were used for pH adjustments.


### 2.2. Preparation ofGrewia spp. Biopolymer

The maturedGrewia spp. barks were collected from Mokolo locality in the Far North Region of Cameroon Figure 1. Crude Grewia biopolymer was extracted as described by Somboonpanyakul et al. [24]. The dried and pulverized bark of Grewia spp. was dispersed in water (1 : 80 w/v, pH 4) at 50°C for 4 hours. The fibrous material from the dispersed mucilage was removed by centrifugation at 5300 rpm, 4°C for 20 min. Thereafter, the crude polymer was precipitated with 3 volumes of 95% absolute ethanol and freeze-dried.Figure 1
Grewia spp. leaves (a) and barks (b).
(a)
(b)


### 2.3. Characterization ofGrewia spp. Biopolymer and Laterite Clay

Infrared spectra ofGrewia biopolymer were recorded with a Fourier-transform infrared (FTIR) spectrometer (Bruker, (Vertex-70)). The spectra of the sample were recorded in the range of 4000 cm−1 to 400 cm−1 wavenumber. The molecular mass of the Grewia spp. biopolymer was determined with an AKTA purifier 10 gel filtration chromatography (GFC) system equipped with Superdex_200_10/300_GL columns. The column used was calibrated with four polymer standards: bovine serum albumin (66.6), carbonic anhydrase (29.2), trypsin (23.3), and lysozyme (14.3) kDA; the molecular mass of Grewia spp. was obtained by interpolating the retention times against the calibration graph. XRD (X-ray diffraction) analysis of laterite sample was performed by using desktop X-ray diffractometer Rigaku, Miniflex II. The surface morphology and elemental analysis were performed with a JEOL 6380A scanning electron microscope-energy dispersive spectrometer (SEM-EDX).


### 2.4. Coagulation Experiments

Coagulation tests were carried out on a jar test apparatus, using a coagulation sequence of rapid agitation for 5 min at 150 rpm during which coagulant was introduced, slow agitation for 15 min at 30 rpm, and settling for 20 min. After this sequence, the supernatant was withdrawn from a point located about 2 cm below the liquid surface for turbidity measurements using a turbidimeter (Hach 2100A). The minimum volume of coagulant necessary to obtain the lower residual turbidity in the series of beakers was identified, and the critical coagulation concentration (CCC) was determined as follows:(1)CCCmg/L=CiVcVc+Vi,where Ci is the concentration of hydrolyze salt (mg/L), Vc is the critical volume of coagulation (L), and Vi is the volume of the solution suspension used (L).


### 2.5. Production of Concentrated Laterite Suspension

Coagulated suspensions done at constant pH values of 7 and 5 were kept for 24 hours to decant, the supernatant was removed, and the sediment was recovered and stored. The concentration (C) of solids in the sediment was determined by mass difference after drying the known volume of sediment at 105°C for 24 hours according to the following equation:(2)Cg/L=massgvolumeL.Prior to the preparation of working suspensions, the sediments were diluted with the previously removed supernatant to attain concentrations of 10, 20, and 30 g/L at pH values of 5 and 7.


### 2.6. Sedimentation Test

The prepared suspensions at the concentrations of 10, 20, and 30 g/L at pH values of 5 and 7 were subjected to sedimentation. Sedimentation process was studied in a graduated cylinder of 10 cm diameter and 104 cm height during which the sedimentation front was monitored as a function of time until the compression phase was attained. The sediment concentration factor (CF) was calculated using the following formula:(3)CF=ViVf,where Vi is the initial volume of laterite suspension before coagulation (dm3) and Vf is the final volume of laterite suspension after coagulation (dm3).


## 2.1. Chemicals

The chemicals used in this study were of analytical grade. Aluminum sulphate (alum) and ferric chloride (FeCl3) were from Prolabo Company. These coagulants were prepared daily to avoid polymerization in solution. Sodium hydroxide and hydrochloric acid both at 0.1 M were used for pH adjustments.


## 2.2. Preparation ofGrewia spp. Biopolymer

The maturedGrewia spp. barks were collected from Mokolo locality in the Far North Region of Cameroon Figure 1. Crude Grewia biopolymer was extracted as described by Somboonpanyakul et al. [24]. The dried and pulverized bark of Grewia spp. was dispersed in water (1 : 80 w/v, pH 4) at 50°C for 4 hours. The fibrous material from the dispersed mucilage was removed by centrifugation at 5300 rpm, 4°C for 20 min. Thereafter, the crude polymer was precipitated with 3 volumes of 95% absolute ethanol and freeze-dried.Figure 1
Grewia spp. leaves (a) and barks (b).
(a)
(b)


## 2.3. Characterization ofGrewia spp. Biopolymer and Laterite Clay

Infrared spectra ofGrewia biopolymer were recorded with a Fourier-transform infrared (FTIR) spectrometer (Bruker, (Vertex-70)). The spectra of the sample were recorded in the range of 4000 cm−1 to 400 cm−1 wavenumber. The molecular mass of the Grewia spp. biopolymer was determined with an AKTA purifier 10 gel filtration chromatography (GFC) system equipped with Superdex_200_10/300_GL columns. The column used was calibrated with four polymer standards: bovine serum albumin (66.6), carbonic anhydrase (29.2), trypsin (23.3), and lysozyme (14.3) kDA; the molecular mass of Grewia spp. was obtained by interpolating the retention times against the calibration graph. XRD (X-ray diffraction) analysis of laterite sample was performed by using desktop X-ray diffractometer Rigaku, Miniflex II. The surface morphology and elemental analysis were performed with a JEOL 6380A scanning electron microscope-energy dispersive spectrometer (SEM-EDX).


## 2.4. Coagulation Experiments

Coagulation tests were carried out on a jar test apparatus, using a coagulation sequence of rapid agitation for 5 min at 150 rpm during which coagulant was introduced, slow agitation for 15 min at 30 rpm, and settling for 20 min. After this sequence, the supernatant was withdrawn from a point located about 2 cm below the liquid surface for turbidity measurements using a turbidimeter (Hach 2100A). The minimum volume of coagulant necessary to obtain the lower residual turbidity in the series of beakers was identified, and the critical coagulation concentration (CCC) was determined as follows:(1)CCCmg/L=CiVcVc+Vi,where Ci is the concentration of hydrolyze salt (mg/L), Vc is the critical volume of coagulation (L), and Vi is the volume of the solution suspension used (L).


## 2.5. Production of Concentrated Laterite Suspension

Coagulated suspensions done at constant pH values of 7 and 5 were kept for 24 hours to decant, the supernatant was removed, and the sediment was recovered and stored. The concentration (C) of solids in the sediment was determined by mass difference after drying the known volume of sediment at 105°C for 24 hours according to the following equation:(2)Cg/L=massgvolumeL.Prior to the preparation of working suspensions, the sediments were diluted with the previously removed supernatant to attain concentrations of 10, 20, and 30 g/L at pH values of 5 and 7.


## 2.6. Sedimentation Test

The prepared suspensions at the concentrations of 10, 20, and 30 g/L at pH values of 5 and 7 were subjected to sedimentation. Sedimentation process was studied in a graduated cylinder of 10 cm diameter and 104 cm height during which the sedimentation front was monitored as a function of time until the compression phase was attained. The sediment concentration factor (CF) was calculated using the following formula:(3)CF=ViVf,where Vi is the initial volume of laterite suspension before coagulation (dm3) and Vf is the final volume of laterite suspension after coagulation (dm3).


## 3. Results and Discussion

### 3.1. Properties of Grewia spp. Biopolymer

#### 3.1.1. Functional Groups ofGrewia spp.

The flocculating action of the bioflocculant depends on the chemical groups in the molecule. FTIR spectra ofGrewia biopolymer are presented in Figure 2. The spectra revealed the typical bands and peak characteristic of polysaccharides [25, 26].Figure 2
FTIR spectra ofGrewia spp. biopolymer.


#### 3.1.2. Molecular Mass ofGrewia spp. Biopolymer

The molecular mass ofGrewia spp. biopolymer was determined by gel filtration chromatography. The elution profile revealed bimodal molecular mass distributions. The molecular masses of all peaks are presented in Table 1. The obtained result showed that Grewia spp. biopolymer is of high molecular mass, suggesting that it can favor adsorption and bridging effect, encourage the densely packed aggregate nature of floc, and thus enhance floc settling velocity [27, 28].Table 1
Molecular mass ofGrewia spp. biopolymer.
Peak 1
Peak 2
Retentions time (min)
15.31
37.57
Molecular mass (DA)
93598.74
28726.98


### 3.2. Laterite Clay Characteritics

The characterization of the laterite clay was performed by XRD and EDX, and the results are presented in Figures3 and 4.Figure 3
X-ray diffraction pattern of laterite.Figure 4
Energy-dispersive X-ray (EDX) analysis of laterite.The obtained spectra show kaolinite (2θ = 12.26, 24.91, 35.78) as the main component of laterite with triclinic crystal structure. The other subcomponents established were hematite at 2θ value of 33.18, 38.49, 54.33, and goethite. A small portion of quartz mineral (Q) was also confirmed with the peak value of 2θ = 21.0 and 49.53. The mineralogical phases of laterite used in this study are close to laterite used in other researches [29–31].EDX spectra display the presence of Si, Al, Fe, and Ti, which are all elements present in kaolinite, hematite, and goethite composition of laterite. EDX results were consistent with the data obtained by XRD. The chemical composition of laterite obtained is similar to those of Dihang et al. [1].


### 3.3. Influence of pH on the Critical Coagulation Concentration

Coagulation-flocculation of raw water requires large quantity of coagulants (alum and ferric chloride), and the sludge produced is often acidic or near-neutral with pH values around 5.12 and 6.5 [32–34]. The effect of pH on the critical coagulation concentration of concentrated laterite suspensions (10 g/L) was studied at pH values 5 and 7 which are closely similar to the typical pH values of sediments obtained in water treatment plants. Table 2 shows that critical coagulation concentration decreases with lowering of the pH. This behavior may be due to the reduction of repulsive forces between particles; consequently, the colloidal particles approach each other. This would lead to their aggregation [35]. The positive charge of hydrolyzing metals salts decreases with increasing pH [36]. At pH 5 and 7, the concentration demand remains highest for aluminum sulphate when compared with ferric chloride. This could be explained by the difference in specific surface area and surface charge of iron hydroxides and aluminum hydroxides. The specific surface area of Fe and Al hydroxides are in different ranges of 160–230 and 200–400 m2/g, respectively; however, due to higher active metal concentration in ferric chloride and higher molecular weight of Fe, the total available surface is more for ferric chloride than for alum [37, 38].Table 2
Effect of pH on the critical coagulation concentration of laterite suspension 10 g/L.
pH 5
pH 7
Ferric chloride (mg/L)
47.5
112.5
Alum (mg/L)
100
156.25


### 3.4. Scanning Electron Micrograph (SEM) of Laterite Sediment

SEM micrography of uncoagulated, coagulated, and flocculated laterite particles was performed, and results are presented in Figure5. The uncoagulated particles contained laterite organized in an arbitrary manner (Figure 5(a)); this is due to a moderately high surface potential with a dense electrical double layer. When the coagulant was added, its hydrolysis products acted as the counterions to the electric double layer of clay particles, thus favoring particle aggregation confirmed by the increase in particle size as revealed by SEM analysis (Figure 5(b)). Addition of Grewia spp. biopolymer led to thicker and larger flocs (Figure 5(c)), due to its high molecular mass and the presence of several functional groups (Figure 2) which adsorb several laterite particles to form larger, tough, and easily settleable aggregates [39].Figure 5
Scanning electron micrograph (SEM): (a) uncoagulated laterite particles, (b) coagulated laterite particles, and (c) coagulated and flocculated laterite particles.
(a)
(b)
(c)


### 3.5. Settling Characteristics

The height of interface of suspensions coagulated with aluminum sulphate, ferric chloride, andGrewia spp. biopolymer as coagulant aid is shown in Figures 6 and 7. For the purpose of comparison, the settling characteristics are presented at pH 5 and pH 7. At each concentration of laterite suspension, settling curves have the same trends. These curves are divided into three sections as those described by Kynch [40]. The first section is followed by the regular height reduction of the solid/liquid interface—settling zone regime—with a constant settling velocity [41], followed by the transition-settling period, with decreasing settling velocity. Finally, the compression settling takes place, characterized by a very low variation of the interface, as reported by Zodi et al. [42]. For each concentration, the settling curves obtained using iron salts were below the aluminum sulphate ones. This could be explained by the difference of affinity between the anions of these salts and laterite particles. Iron salt contains Cl−, and alum contains SO42−. It was shown that the ability of SO42− to facilitate the dispersion of the clay suspension is higher than that of Cl− [43, 44]. Due to more favorable attraction forces between laterite particle and iron, the flocs formed by iron salt are denser compared to those derived from aluminum sulphate. This confirms that the ferric salts are more effective than aluminum sulphate.Figure 6
Settling curves of coagulated laterite suspension: pH 5 (a–c); pH 7 (d–f); Al (alum); Fe (FeCl3); Al + G (alum + 25 mg/L Grewia spp. biopolymer); Fe + G (FeCl3 + 25 mg/L Grewia spp. biopolymer).
(a)
(b)
(c)
(d)
(e)
(f)Figure 7
Concentrated laterite suspension undergoing sedimentation after adding alum (Al), FeCl3 (Fe), and Grewia spp. biopolymer (G).Figure6 also shows that an increase in solid concentration also increases the settling time. As solid concentration increases, hindered settling becomes more significant, which leads to the decrease of velocity as found by Guibai and Gregory [45]. For the same reason, increasing critical coagulation concentration may result in a significant gel formation whose settling is hindered by local viscosity and low-density difference [42]. Some published papers [21, 46] reported similar results on settling characteristics of kaolinite.


### 3.6. Sedimentation Velocity

The settling velocities of the interface upon addition of the salt andGrewia spp. biopolymer were calculated (Figure 8). Initial settling rate was calculated from the slope of the linear portion of the settling curves. The coagulation performance of a particular salt was correlated with the settling velocity. The greater the settling velocity of the interface, the greater the coagulation performance. For a given salt, settling velocity was appreciably greater at lower solid concentration. Results also show that settling velocity increased after adding the Grewia spp. biopolymer. For the 10 g/L laterite suspension, settling velocity at pH 5 was 0.22 and 0.28 cm/min for alum and ferric chloride, respectively. Addition of Grewia spp. biopolymer resulted in larger and dense flocs with settling velocities of 0.56 and 0.57 cm/min for alum and ferric chloride, correspondingly. In the presence of Grewia spp. biopolymer, bridging effects led to the formation of larger aggregates which settled quickly [47]. Similar observations with other biopolymers have been reported [12, 15, 48]. These authors showed that biopolymers increase the particle size and settling rate. It was also observed that settling characteristics of laterite particles vary with pH of the suspension; thus, the settling velocity at pH 5 was higher than at pH 7. Laterite is a hydrous aluminum silicate (Figures 3 and 4). Under an acidic environment (pH < 6), aluminum at the edges binds with hydrogen ions to reach positive charges. Such electrostatic conditions cause edge-to-face desirability that generates a card-house structure. Under alkaline conditions, the edges become negatively charged, thereby reducing attraction forces between particles, consequently reducing the settling velocity of particles.Figure 8
Settling velocity of laterite suspension at different experimental conditions: a (10 g/L); b (20 g/L); c (30 g/L); Al (alum); Fe (FeCl3); Al + G (alum + 25 mg/L Grewia spp. biopolymer); Fe + G (FeCl3 + 25 mg/L Grewia spp. biopolymer).
(a)
(b)
(c)


### 3.7. Sediment Concentration Factor

Generally, the amount of the sludge produced during the coagulation-flocculation process depends on the products used. The obtained results of sediment concentration factors are shown in Figure9. At different laterite concentrations, for a given coagulant, the sediment concentration factor at pH 5 was higher than that obtained at pH 7. This difference may be attributed to the floc structure obtained. When the pH decreased, the repulsive forces between the particles were reduced giving priority to attractive forces. Under these conditions, the laterite particles agglomerate along their basal surface in the form of layers [49]. In contrast, when the pH increased, the face-to-face energy barrier is very high, thereby leading to reduced particle settling velocity [31]. Figure 9 also shows that when Grewia biopolymer was added, the sediment concentration factor was higher for both pH values. For 30 g/L of laterite suspension, sediment concentration factors at pH 5 were 1.47 and 2.12 for alum and FeCl3, respectively. The presence of Grewia spp. biopolymer resulted in more compact sludge with sediment concentration factors 4 and 3.13 for alum and ferric chloride, respectively. When the particles were flocculated in the presence of Grewia spp., they all settled together and consolidated at their own weight. Thus, the thickness of the sediment decreased. The sediment concentration factor of aluminum sulphate was smaller than that of ferric chloride. Both in terms of critical coagulation concentration and settling velocity, ferric chloride was more effective than aluminum sulphate.Figure 9
Sediment concentration factor vs. concentration of laterite suspension.


## 3.1. Properties of Grewia spp. Biopolymer

### 3.1.1. Functional Groups ofGrewia spp.

The flocculating action of the bioflocculant depends on the chemical groups in the molecule. FTIR spectra ofGrewia biopolymer are presented in Figure 2. The spectra revealed the typical bands and peak characteristic of polysaccharides [25, 26].Figure 2
FTIR spectra ofGrewia spp. biopolymer.


### 3.1.2. Molecular Mass ofGrewia spp. Biopolymer

The molecular mass ofGrewia spp. biopolymer was determined by gel filtration chromatography. The elution profile revealed bimodal molecular mass distributions. The molecular masses of all peaks are presented in Table 1. The obtained result showed that Grewia spp. biopolymer is of high molecular mass, suggesting that it can favor adsorption and bridging effect, encourage the densely packed aggregate nature of floc, and thus enhance floc settling velocity [27, 28].Table 1
Molecular mass ofGrewia spp. biopolymer.
Peak 1
Peak 2
Retentions time (min)
15.31
37.57
Molecular mass (DA)
93598.74
28726.98


## 3.1.1. Functional Groups ofGrewia spp.

The flocculating action of the bioflocculant depends on the chemical groups in the molecule. FTIR spectra ofGrewia biopolymer are presented in Figure 2. The spectra revealed the typical bands and peak characteristic of polysaccharides [25, 26].Figure 2
FTIR spectra ofGrewia spp. biopolymer.


## 3.1.2. Molecular Mass ofGrewia spp. Biopolymer

The molecular mass ofGrewia spp. biopolymer was determined by gel filtration chromatography. The elution profile revealed bimodal molecular mass distributions. The molecular masses of all peaks are presented in Table 1. The obtained result showed that Grewia spp. biopolymer is of high molecular mass, suggesting that it can favor adsorption and bridging effect, encourage the densely packed aggregate nature of floc, and thus enhance floc settling velocity [27, 28].Table 1
Molecular mass ofGrewia spp. biopolymer.
Peak 1
Peak 2
Retentions time (min)
15.31
37.57
Molecular mass (DA)
93598.74
28726.98


## 3.2. Laterite Clay Characteritics

The characterization of the laterite clay was performed by XRD and EDX, and the results are presented in Figures3 and 4.Figure 3
X-ray diffraction pattern of laterite.Figure 4
Energy-dispersive X-ray (EDX) analysis of laterite.The obtained spectra show kaolinite (2θ = 12.26, 24.91, 35.78) as the main component of laterite with triclinic crystal structure. The other subcomponents established were hematite at 2θ value of 33.18, 38.49, 54.33, and goethite. A small portion of quartz mineral (Q) was also confirmed with the peak value of 2θ = 21.0 and 49.53. The mineralogical phases of laterite used in this study are close to laterite used in other researches [29–31].EDX spectra display the presence of Si, Al, Fe, and Ti, which are all elements present in kaolinite, hematite, and goethite composition of laterite. EDX results were consistent with the data obtained by XRD. The chemical composition of laterite obtained is similar to those of Dihang et al. [1].


## 3.3. Influence of pH on the Critical Coagulation Concentration

Coagulation-flocculation of raw water requires large quantity of coagulants (alum and ferric chloride), and the sludge produced is often acidic or near-neutral with pH values around 5.12 and 6.5 [32–34]. The effect of pH on the critical coagulation concentration of concentrated laterite suspensions (10 g/L) was studied at pH values 5 and 7 which are closely similar to the typical pH values of sediments obtained in water treatment plants. Table 2 shows that critical coagulation concentration decreases with lowering of the pH. This behavior may be due to the reduction of repulsive forces between particles; consequently, the colloidal particles approach each other. This would lead to their aggregation [35]. The positive charge of hydrolyzing metals salts decreases with increasing pH [36]. At pH 5 and 7, the concentration demand remains highest for aluminum sulphate when compared with ferric chloride. This could be explained by the difference in specific surface area and surface charge of iron hydroxides and aluminum hydroxides. The specific surface area of Fe and Al hydroxides are in different ranges of 160–230 and 200–400 m2/g, respectively; however, due to higher active metal concentration in ferric chloride and higher molecular weight of Fe, the total available surface is more for ferric chloride than for alum [37, 38].Table 2
Effect of pH on the critical coagulation concentration of laterite suspension 10 g/L.
pH 5
pH 7
Ferric chloride (mg/L)
47.5
112.5
Alum (mg/L)
100
156.25


## 3.4. Scanning Electron Micrograph (SEM) of Laterite Sediment

SEM micrography of uncoagulated, coagulated, and flocculated laterite particles was performed, and results are presented in Figure5. The uncoagulated particles contained laterite organized in an arbitrary manner (Figure 5(a)); this is due to a moderately high surface potential with a dense electrical double layer. When the coagulant was added, its hydrolysis products acted as the counterions to the electric double layer of clay particles, thus favoring particle aggregation confirmed by the increase in particle size as revealed by SEM analysis (Figure 5(b)). Addition of Grewia spp. biopolymer led to thicker and larger flocs (Figure 5(c)), due to its high molecular mass and the presence of several functional groups (Figure 2) which adsorb several laterite particles to form larger, tough, and easily settleable aggregates [39].Figure 5
Scanning electron micrograph (SEM): (a) uncoagulated laterite particles, (b) coagulated laterite particles, and (c) coagulated and flocculated laterite particles.
(a)
(b)
(c)


## 3.5. Settling Characteristics

The height of interface of suspensions coagulated with aluminum sulphate, ferric chloride, andGrewia spp. biopolymer as coagulant aid is shown in Figures 6 and 7. For the purpose of comparison, the settling characteristics are presented at pH 5 and pH 7. At each concentration of laterite suspension, settling curves have the same trends. These curves are divided into three sections as those described by Kynch [40]. The first section is followed by the regular height reduction of the solid/liquid interface—settling zone regime—with a constant settling velocity [41], followed by the transition-settling period, with decreasing settling velocity. Finally, the compression settling takes place, characterized by a very low variation of the interface, as reported by Zodi et al. [42]. For each concentration, the settling curves obtained using iron salts were below the aluminum sulphate ones. This could be explained by the difference of affinity between the anions of these salts and laterite particles. Iron salt contains Cl−, and alum contains SO42−. It was shown that the ability of SO42− to facilitate the dispersion of the clay suspension is higher than that of Cl− [43, 44]. Due to more favorable attraction forces between laterite particle and iron, the flocs formed by iron salt are denser compared to those derived from aluminum sulphate. This confirms that the ferric salts are more effective than aluminum sulphate.Figure 6
Settling curves of coagulated laterite suspension: pH 5 (a–c); pH 7 (d–f); Al (alum); Fe (FeCl3); Al + G (alum + 25 mg/L Grewia spp. biopolymer); Fe + G (FeCl3 + 25 mg/L Grewia spp. biopolymer).
(a)
(b)
(c)
(d)
(e)
(f)Figure 7
Concentrated laterite suspension undergoing sedimentation after adding alum (Al), FeCl3 (Fe), and Grewia spp. biopolymer (G).Figure6 also shows that an increase in solid concentration also increases the settling time. As solid concentration increases, hindered settling becomes more significant, which leads to the decrease of velocity as found by Guibai and Gregory [45]. For the same reason, increasing critical coagulation concentration may result in a significant gel formation whose settling is hindered by local viscosity and low-density difference [42]. Some published papers [21, 46] reported similar results on settling characteristics of kaolinite.


## 3.6. Sedimentation Velocity

The settling velocities of the interface upon addition of the salt andGrewia spp. biopolymer were calculated (Figure 8). Initial settling rate was calculated from the slope of the linear portion of the settling curves. The coagulation performance of a particular salt was correlated with the settling velocity. The greater the settling velocity of the interface, the greater the coagulation performance. For a given salt, settling velocity was appreciably greater at lower solid concentration. Results also show that settling velocity increased after adding the Grewia spp. biopolymer. For the 10 g/L laterite suspension, settling velocity at pH 5 was 0.22 and 0.28 cm/min for alum and ferric chloride, respectively. Addition of Grewia spp. biopolymer resulted in larger and dense flocs with settling velocities of 0.56 and 0.57 cm/min for alum and ferric chloride, correspondingly. In the presence of Grewia spp. biopolymer, bridging effects led to the formation of larger aggregates which settled quickly [47]. Similar observations with other biopolymers have been reported [12, 15, 48]. These authors showed that biopolymers increase the particle size and settling rate. It was also observed that settling characteristics of laterite particles vary with pH of the suspension; thus, the settling velocity at pH 5 was higher than at pH 7. Laterite is a hydrous aluminum silicate (Figures 3 and 4). Under an acidic environment (pH < 6), aluminum at the edges binds with hydrogen ions to reach positive charges. Such electrostatic conditions cause edge-to-face desirability that generates a card-house structure. Under alkaline conditions, the edges become negatively charged, thereby reducing attraction forces between particles, consequently reducing the settling velocity of particles.Figure 8
Settling velocity of laterite suspension at different experimental conditions: a (10 g/L); b (20 g/L); c (30 g/L); Al (alum); Fe (FeCl3); Al + G (alum + 25 mg/L Grewia spp. biopolymer); Fe + G (FeCl3 + 25 mg/L Grewia spp. biopolymer).
(a)
(b)
(c)


## 3.7. Sediment Concentration Factor

Generally, the amount of the sludge produced during the coagulation-flocculation process depends on the products used. The obtained results of sediment concentration factors are shown in Figure9. At different laterite concentrations, for a given coagulant, the sediment concentration factor at pH 5 was higher than that obtained at pH 7. This difference may be attributed to the floc structure obtained. When the pH decreased, the repulsive forces between the particles were reduced giving priority to attractive forces. Under these conditions, the laterite particles agglomerate along their basal surface in the form of layers [49]. In contrast, when the pH increased, the face-to-face energy barrier is very high, thereby leading to reduced particle settling velocity [31]. Figure 9 also shows that when Grewia biopolymer was added, the sediment concentration factor was higher for both pH values. For 30 g/L of laterite suspension, sediment concentration factors at pH 5 were 1.47 and 2.12 for alum and FeCl3, respectively. The presence of Grewia spp. biopolymer resulted in more compact sludge with sediment concentration factors 4 and 3.13 for alum and ferric chloride, respectively. When the particles were flocculated in the presence of Grewia spp., they all settled together and consolidated at their own weight. Thus, the thickness of the sediment decreased. The sediment concentration factor of aluminum sulphate was smaller than that of ferric chloride. Both in terms of critical coagulation concentration and settling velocity, ferric chloride was more effective than aluminum sulphate.Figure 9
Sediment concentration factor vs. concentration of laterite suspension.


## 4. Conclusion

This study examined the effects of hydrolyzing salts as coagulants in combination withGrewia spp. biopolymer as natural coagulants aid on sedimentation behavior of concentrated laterite suspension. Based on the results, the following conclusions are made. Settling characteristics of laterite suspension is a function of solid concentration, pH, and the type of coagulant. Addition of Grewia spp. biopolymer led to the formation of larger flocs which settled faster than those formed by hydrolyzing salts alone. The best settling regime was obtained with 10 g/L of laterite suspension at pH 5, and settling velocities were 0.22 and 0.28 cm/min for aluminum sulphate and ferric chloride, respectively. Addition of Grewia spp. biopolymer resulted in increased settling velocities of 0.56 and 0.57 cm/min for aluminum sulphate and ferric chloride, respectively. There was an inverse correlation between sediment concentration factor and pH of the laterite suspension. When the Grewia biopolymer was added, the sediment concentration factor was higher for both pH values. The sediment concentration factor of 30 g/L of laterite suspension was 1.47 and 2.12 for aluminum sulphate and ferric chloride, respectively. Upon addition of Grewia spp., the values moved to 4 and 3.13 for aluminum sulphate and ferric chloride, respectively.



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*Source: 1431694-2019-02-07.xml*