In situ hybridization of waste dyes into growing particles of calcium derivatives synthesized from a Gastropod shell (Achatina Achatina)

Chemical Engineering Journal 171 (2011) 941–950

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In situ hybridization of waste dyes into growing particles of calcium derivatives synthesized from a Gastropod shell (Achatina Achatina) N.A. Oladoja ∗ , I.O. Raji, S.E. Olaseni, T.D. Onimisi Department of Chemistry, Adekunle Ajasin University, Akungba Akoko, Nigeria

a r t i c l e

i n f o

Article history: Received 1 February 2011 Received in revised form 20 April 2011 Accepted 20 April 2011 Keywords: Organic–inorganic hybridization Methylene blue Congo red Calcium hydroxide Calcium carbonate Gastropod shell

a b s t r a c t The necessity to develop a benign and effectual method of water pollution control necessitated the studies on this non-conventional in situ hybridization of organic pollutant into growing particles of CaCO3 and Ca(OH)2 . A biogenic waste, the shell of a gastropod, was used as a precursor for the synthesis of the CaCO3 and Ca(OH)2 , via the sol–gel route, in a medium containing either methylene blue (MB) or Congo Red (CR). Similar trends were exhibited by the growing particles in the abstraction of each dye contaminant from the aqua system. The magnitude of conjugation between the growing particles and dye molecule was higher in CR than in MB medium. The dye removal efficiency was not impacted by the optimization of process variables (initial dye concentration, pH, ionic strength and interfering ion) but the sludge settling property was swayed by the change in ionic strength. Experimental and instrumental (FTIR, XRD and SEM) evidence showed that the two types of co-precipitation reaction–occlusion and surface adsorption were the controlling mechanism of this in situ hybridization process. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The direct toxicity and undesirable colour of dye-contaminated effluents prompted the development of array of methods for the decolorization of dye contaminated aqua system. Due to their chemical structures, dyes are resistant to fading on exposure to light, water and many chemicals [1]. When treated aerobically, using the municipal sewerage systems or biological oxidation, decolorization of dyes and pigments do not occur. Thus a tertiary treatment is required for dye abstraction and safe disposal of the effluent. Plethora of tertiary treatment technologies (e.g., Fenton’s reagent, ozonation, cucurbituril, photochemical and electrochemical destruction, membrane filtration, electrochemical coagulation, irradiation and adsorption) have been developed for this purpose. Amongst these methods, adsorption is a unit process that is preferred but the high cost of the conventional sorbent (i.e., activated carbon) and the stringent operational requirement for the regeneration and management of the used sorbent has made the method unalluring. Bell and Buckley [2] and Fu et al. [3] proffered the use of biological processes for the treatment of colored effluents, alone, or in combination with sorption processes [4]. Some other processes such as photochemical [5,6] and catalytic degradation processes have also been tested but the products generated during these processes are more hazardous to the environment than the original

∗ Corresponding author. Tel.: +234 8055438642. E-mail address: [email protected] (N.A. Oladoja). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.04.044

contaminants [7]. The coagulative precipitation with aluminium or iron salts is a classical methodology, but its removal efficiency is poor for dye contaminants [8]. Increasing numbers of novel environmental materials, such as titanium dioxide photocatalyst, are also being developed for removing organic dyes [9,10]. The electrochemical method has to consume plenty of energy to reach a satisfactory removal of dye [11]. Howbeit, some of these methods are very effective but the applications are limited by high installation and operational costs, high energy consumptions, and management of sludge generated from the operations [12]. Thus, the present trend in water pollution control is in the development of high performance, low cost, low energy consumption and eco-friendly technologies. There has been a great deal of interest in the design and synthesis of inorganic/organic complex materials to achieve specific properties. Various synthesis techniques developed during the last few years have given us access to functional materials with characteristics such as surface modification, inorganic/organic hybridization and functional ligand-loading [13,14]. These materials have been widely applied in various fields such as films, catalysts and pharmaceutical products [15,16,17]. A dye with a functional group that absorbs visible light is often used to prepare hybrid materials for application to solar cells and sensors [18,19]. Adsorptive precipitation was discovered during the mid 20th century and applied mostly in analytical chemistry [20]. The classical co-precipitation method has been extensively applied to the enrichment of metal ions [21] and the synthesis of functionalized materials [22]. Conventionally, inorganic–organic hybridization is employed in the synthesis of functional materials

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(e.g., photosensitive cells, optical thin films and sensors) but seldom considered for water pollution control. Recently, Zhao et al. [23] developed a facile eco-friendly treatment of dye wastewater mixture by in situ hybridization into a growing calcium carbonate, synthesized from laboratory chemicals. The method was reported to exhibit high level of dye removal and the mechanism of dye removal was alluded to adsorption, flocculation and ionic complexation. Premised on the report of Zhao et al. [23], the present studies aimed at the use of the Shell of African land snail (Achatina achatina) as a precursor to the synthesis of CaCO3 and Ca(OH)2 in the presence of dyes (Congo red, CR and methylene blue, MB). Methylene blue (MB) and Congo Red (CR) have been selected as model dyes in the present studies to evaluate the capacity of the proposed protocol for the removal of these dyes from aqueous solution. Although MB is seen in some medical uses in large quantities, it is also widely used in coloring paper, dyeing cottons, wools, coating for paper stocks, etc. Though MB is not strongly hazardous, it can cause some harmful effects. Acute exposure to MB will cause increased heart rate, vomiting, shock, Heinz body formation, cyanosis, jaundice and quadriplegia and tissue necrosis in humans. CR [1-naphthalenesulfonic acid, 3,3-(4,4-biphenylenebis(azo)bis (4-amino-)disodium salt] is a benzidine-based anionic bisazo dye. This dye is known to metabolize to benzidine, a known human carcinogen. Effluents containing CR are generated from the textile, printing and dying, paper, rubber, plastic, etc. industries. Owing to structural stability, CR is difficult to biodegrade. Physicochemical or chemical treatment of such wastewater is, however, possible. The choice of the shell of this phylum of mollusca as a precursor to the synthesis of the calcium derivatives was predicated on the fact that snail shell (SS) has got the same mineralogical assemblage and basic construction as other Mollusk shells. Basically, the SS consists of mainly CaCO3 , as well as various organic compounds. It consists of three layers thus the Hypostracum, which is the innermost layer; the Ostracum, which is the shell-building layer and the Periostracum, the outermost layer. The Hypostracum is a form of Aragonite, a type of CaCO3 . The Ostracum is built by several layers of prism-shaped CaCO3 crystals with embedded proteid molecules. The Periostracum, the outermost shell layer, is not made of CaCO3 , but of an organic material called Conchin, a mixture of organic compounds, mostly of proteids. Conchin not only makes the outer shell layer, but also embedded between the CaCO3 crystals of deeper layers. The present studies focused on the synthesis of oxide and hydroxide of Ca in the presence of two dyes (an anionic dye, CR and a basic dye, MB), as contaminants, in aqueous system. The effects of process variables – initial dye concentration, presence of interfering anions, ionic strength and pH, on the dye conjugation with the growing particles were studied. The surface functional groups, crystallinity, and the surface microstructure of the sludge, derived from the process in the presence and absence of the dye contaminant shall be expounded using FTIR, XRD and SEM respectively.

2. Materials and methods 2.1. SS preparation and characterization The SS was obtained after the snail has been removed in boiled water. The SS was first washed with tap water then rinsed thoroughly with deionized water, dried in the oven and ground in a wooden mortar and pestle. The ground SS was finally made into powder using laboratory grinding machine and sieved with a laboratory sieve of known mesh size. The procedures for the characterization of the SS were reported in our earlier treatise [24].

2.2. Dye preparation and quantification The dyes used in the present studies, CR (C.I. 2212, chemical formula: C32 H22 N6 Na2 O, MW = 696.68, max = 497 nm), and MB (C.I. basic blue 9, C.I. solvent blue 8, C.I. 52015, chemical formula: C16 H18 N3 ClS; MW = 373.90, max = 661 nm) were accurately weighed and dissolved in distilled–deionized water to prepare the stock solution (500 mg/L) and different working solutions were prepared from the stock by serial dilution. 2.3. Treatment of dye contaminated water via in situ hybridization A typical treatment procedure goes thus calcium chloride solution was prepared by the dissolution of 2 g of snail shell in a dilute HCl (9:1 = H2 O:HCl) and the reaction was allowed to go to completion. Dye solution (100 mL) of known concentration was then mixed with the CaCl2 solution. A known volume Na2 CO3 (0.5 M) and NaOH (1 M) were added, separately and slowly while stirring, to initiate the growth of the CaCO3 and Ca(OH)2 respectively. After 30 min settling time, the supernatant was withdrawn and centrifuged to determine the residual dye concentration by UV/visible spectrophotometer at the respective wavelengths of each dye (661 nm for MB and 497 nm for CR). This treatment procedure was carried out at different initial concentrations (25, 50 and 100 mg/L) 2.4. Influence of pH The effect of pH was monitored between pH 4 and 10. Different pH (4.0–10.0) was controlled by adding either 0.1 M HCl or 0.1 M NaOH solutions to the dye solution before the addition to the CaCl2 solution for the in situ treatment of dye contaminated water. Conditions of initial dye concentration, initial SS dissolved were fixed at 100 mg/L and 2 g, respectively. 2.5. Influence of interfering ions The effect of interfering ions was monitored in the presence of the following anionic species: phosphate (PO4 3− ), borate (BO3 3− ), sulphate (SO4 2− ) and nitrate (NO3 2− ). These anions were derived from the respective soluble salts viz: KH2 PO4 , H3 BO3 , K2 SO4 , and KNO3 . Conditions of initial dye concentration, initial SS dissolved were fixed at 100 mg/L and 2 g, respectively 2.6. Influence of ionic strength The ionic strength effect was tested using NaCl solutions of the following concentrations: 0%, 0.05%, 0.1%, 0.2%, 0.5% and 1%, equivalent to ionic strengths of 0.0 mol/L, 0.0085 mol/L, 0.017 mol/L, 0.0342 mol/L, 0.085 mol/L and 0.17 mol/L. Conditions of initial dye concentration, initial SS dissolved were fixed at 100 mg/L and 2 g, respectively. 2.7. Comparison of amount of dye removed with adsorption process The amount of dye removed, via the hybridization process, was compared with the adsorption process thus a typical experimental procedure was conducted by measuring accurately 100 mL of the dye solution of known concentration, 1.1 g of the calcium derivatives material (the amounts of synthesized calcium derivative from the reaction process) was added and agitated (at 200 rpm) for 120 min, based on the results obtained from the equilibrium time studies, carried out as a preliminary study [results not shown]. Samples were withdrawn, at fixed time intervals, centrifuged, and the supernatant was analyzed for residual dye using a UV/visible

N.A. Oladoja et al. / Chemical Engineering Journal 171 (2011) 941–950 Table 1 Proximate Physicochemical characteristics of SS. Physicochemical characteristics pHsolution Bulk density (g/cm3 ) PZC Particle size distribution (␮m) (%) >90 90–63 <63 Volatile matter (%) Ash (%) Moisture (%) Ca (%) Mg (%) Na (%) K (%) Cu (%) Pb (%)

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from waters that would ordinarily yield only calcite; they do so by physiological mechanisms that are not fully understood. Values 8.01 1.33 7.9 54.06 41.32 4.61 4.14 93.76 2.10 99.74 0.0002 0.0008 0.0009 0.00002 0.0005

spectrophotometer. The amount of dye sorbed per unit mass of the sorbent [q in mg/g] was calculated using the mass balance procedure. 2.8. Sludge characterization The particles produced from the process, in the presence and absence of the contaminants, were subjected to FTIR, XRD and SEM analysis to elucidate the changes that took place in the surface functional properties, crystallinity and surface microstructure of the synthesized calcium derivatives. The X-ray diffractometric analysis was determined using X-ray mini diffractometer (MD10, Radicon LTD) using copper as the target with Ni as the filter media a K radi˚ Goniometer speed was maintained at ation maintained at 1.54 A. 1◦ min−1 . The SEM was determined using EVO MA/10 model of Carl Zeiss SEM. The FTIR was determined using FTIR spectrophotometer (Model: FTIR 2000, Shimadzu, Kyoto, Japan). The FTIR was recorded in the range from 400 to 4000 cm−1 . 3. Results and discussion 3.1. SS characterization The appositeness of the shell of African land snail (Achatina achatina), as a precursor, for the synthesis of the Ca derivatives (CaCO3 and Ca(OH)2 ) was assessed by the determination of the physicochemical characteristics and the mineralogical assemblage. The results of the physicochemical characterization (Table 1) revealed the very high inorganic fraction of the SS (ash content = 93.76%) and the predominance of Ca2+ (99.74) amongst the selected metal ions determined. The other metal ions determined were present in trace quantities. The fitting of the diffractogram to the data available in the XRD catalogue showed aragonite as the sole mineral constituents of the SS [24]. Aragonite is a stable form of calcium carbonate (CaCO3 ) at high pressures. It may be distinguished from calcite, the commoner form of calcium carbonate, by its greater hardness and specific gravity. Aragonite is always found in deposits formed at low temperatures near the surface of the Earth, as in caves as stalactites, in the oxidized zone of ore minerals (with lead substituting for calcium), in serpentine and other basic rocks, in sediments, and in iron–ore deposits. Aragonite is the mineral normally found in pearls. It is polymorphous with calcite and vaterite, and, with geologic time, probably inverts to calcite even under normal conditions. Aragonite is an important element in the shells and tests of many marine invertebrates. These animals can secrete the minerals

3.2. Treatment of dye contaminated water via in situ hybridization The synthesis of the Ca derivatives began with the preparation of CaCl2 solution. This was achieved via the dissolution of the SS in dilute HCl (9:1 water:HCl, v/v). The reaction was assumed to have gone to completion when the effervescent of CO2 ceases and the complete disintegration of the SS particles was achieved. The reaction of the SS with the diluted acid could be represented thus: SS(CaCO3 ) + 2HCl(aq) = CaCl2 + H2 CO3 Aragonite

(9:1, v/v)

The carbonic acid (H2 CO3 ) produced is not stable hence a spontaneous decomposition into H2 O and CO2 occurred as soon as it is formed. Premised on the findings from the preliminary studies, on the appropriate combination ratio of SS to diluted acid, appropriate amount of SS was dissolved in 50 mL of 9:1 HCl (water:HCl). Dye solution of known concentration was added to the CaCl2 solution, mixed thoroughly, the precipitant (NaOH and NaCO3 ) for each Ca derivative was added slowly and stirred. The conjugated dye–Ca derivative were allowed to settle and portion of the supernatant was withdrawn for the determination of the residual dye concentration. Dye removal, via this process was tested at different initial concentration of the dye (25, 50 and 100 mg/L). The results obtained (Fig. 2) showed that the amount of dye removed (%) were in the same range, irrespective of the initial dye concentration. The two growing particles exhibited similar trend in the removal efficiencies of the two dyes (Fig. 1). The appraisal of the magnitude of removal efficiencies of the two dyes (i.e., MB and CR) showed that a higher magnitude of CR was removed via the in situ hybridization process by the growing particles over MB. Removal efficiencies greater than 95% were obtained for the CR removal while values less than 80% were obtained for the MB removal in both systems. This shows that the hybridization of the dye into the framework of the Ca derivatives favors CR than MB. The mechanism of dye removal via in situ hybridization into the framework of the Ca derivatives synthesized from SS could be assumed to have occurred via precipitation or adsorption. At the inception of the dye–Ca derivative hybridization process, the addition of dye solution to the CaCl2 solution provided a scenario for interaction between the dye molecules and the ionic species (i.e., Ca2+ and Cl− ) present in the mixtures. An interaction between the MB, a cationic dye, and the Cl− ion and an interaction between the CR, an anionic dye, and the Ca2+ ion are feasible because of the opposing charges posses by each of the supposed partnering species. If appreciable interactions occurred, between these opposing ionic species, the neutralization of the dye ionic charge and subsequent precipitation of the neutralized species should occur. Precipitation refers to insolubilization of contaminants by exceeding a solubility product [25] (in this case MBCl or CaCR2 ) but the precipitation of the dye molecules did not occur during this process, hence, neutralization of the dye ionic species by any of the inorganic ionic species in the mixtures could not be ascertained. Premised on the principle of ‘constant solubility products’, if precipitation is occurring, all the residual contaminant concentration after precipitation should be in the same range regardless of the initial concentration and interfering ions. The results obtained from this study agreed with this principle of constant solubility product as similar residual dye concentration values in the product water were obtained at different initial dye concentrations. Thus the option of precipitation as the mechanism of abstraction

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a

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100

Table 2 Comparison of the values of dye removal (mg/g) in the in situ hybridization process with adsorption process at different initial dye concentrations (mg/L).

Dye Removal efficiency (%)

90 80

Dye removal process

70 60 50

CR

40

MB

30 20

Ca(OH)2

CaCO3

MB

CR

MB

CR

In situ hybridization 25 50 100

19.04 34.91 72.34

24.66 49.96 99.62

19.54 39.66 72.75

24.78 49.56 98.62

Adsorption 25 50 100

17.44 32.81 71.07

23.98 45.09 95.87

18.27 33.87 71.84

22.76 47.43 94.11

10 0 25

50

Table 3 Comparison of the adsorption capacity of various adsorbents for removals of MB and CR from aqua medium [27–35].

100

Inial Dye Concentraon (mg/L)

b

100

Dye Removal efficiency (%)

90 80 70 60 50

MB

40

CR

30

Adsorbent

Type of dye

Sorption capacity (mg/g)

Saw dust–pitch pine [27] Coconut coir [28] Class fly ash [29] Fly ash–HNO3 [30] Activated carbon [31] FA–C [32] Baggase fly ash [33] Activated carbon [31] Orange peel [34] Palm kernel seed coat [35] Gastropod shell derived Ca(OH)2−

MB MB MB MB MB CR CR CR CR CR CR MB CR MB

27.78 15.59 4.92 7.74 17.63 31.14 11.84 3.00 22.40 66.23 99.62 72.34 98.62 72.75

Gastropod shell derived CaCO3

20 10 0 25

50

100

Inial Dye Concentraon (mg.L) Fig. 1. (a, b) Effect of initial concentration (mg/L) on the dye removal efficiency (%) in growing particles of Ca(OH)2 (a) and CaCO3 (b).

of the dye molecule could not be discarded. Precipitation occurs directly by the conversion of the soluble analyte in the matrix to an insoluble form. Precipitation reaction in the presence of contaminants engenders two possibilities viz: post-precipitation and co-precipitation. Thus, the possibilities of any of these options occurring independently or the two occurring contemporaneously in the in situ hybridization of dye molecules into the framework of the growing particles were explored. Post-precipitation is the precipitation which occurred on the surface of the first precipitate after its formation. It occurs with sparingly soluble substances which form supersaturated solutions. They usually have an ion in common with the primary precipitate. Since none of the dyes has a common ion with the primary precipitate, i.e., CaCO3 and Ca(OH)2 the possibility of post-precipitation is not feasible in this regard. Co-precipitation is the incorporation of substances which are soluble in the mother liquor into a precipitate. Two types of coprecipitations are possible. The first is concerned with adsorption on the surface of the particles exposed to the solution and the second relate to the occlusion of the foreign substances during the process of crystal growth from the primary particles. Vogel [26] opined that with regard to surface adsorption this will in general be greatest for gelatinous precipitate and least for those of pronounced macrocrystalline character. Since the precipitate obtained from the different procedure were all crystalline, as confirmed by the responses of the precipitate to X-ray diffractometric analysis,

the possibility of the mechanism of dye removal by occlusion is very high. The diffractogram obtained from the XRD analysis of the sludges obtained from the different processes showed that they were crystalline but the pronounced macrocrystallinity of precipitate; the extreme condition given for the least possibility of adsorption taking place could not be assured in this case. Thus adsorption, another co-precipitation option, was tested as a possible mechanism by synthesizing each of the Ca derivatives (CaCO3 and Ca(OH)2 ), in the absence of the dye contaminants and using each of them as adsorbent for dye abstraction from aqua system. The amount of synthesized particles used as adsorbent was equivalent (in mg) to the amount of particle generated in the in situ hybridization process. It is assumed that if adsorption took a vital role in the abstraction of the dye molecules, a comparable value in the sorption capacity values should be obtained via the two processes. The adsorption capacity, qe (mg/g) of the virgin sorbent was obtained from the analysis of the data obtained from the sorption studies at varying initial dye concentration (25, 50 and 100 mg/L) using the mass balance equation thus: qe =

(Ci − Cr )V m

where Ci and Cr is the initial and residual dye concentration, V is the volume of dye solution, and m is the mass of sorbent used (mg). The amounts of each dye removed per gram of the synthesized Ca derivatives were comparable and in the same range in both the in situ hybridization process and adsorption process (Table 2). The value of the capacity (mg/g) of different sorbents for the model dyes used in the present studies, reported in literatures, were compared with the values obtained in the in situ hybridization process and presented in Table 3.

N.A. Oladoja et al. / Chemical Engineering Journal 171 (2011) 941–950 Table 4 Influence of process variables on the dye removal efficiency (%) of the in situ hybridization process at 100 mg/L initial dye concentration. Parameters

Ca(OH)2

a

200

945

0

0.05

5

10

0.1

0.2

0.5

1

180

CaCO3

MB

CR

MB

CR

160

Ionic strength (%) 0.0 0.1 0.2 0.5 1.0

73.21 71.45 72.48 71.34 69.48

98.34 99.62 99.44 98.64 99.10

75.21 71.26 73.29 74.16 72.92

98.92 99.14 99.21 98.62 99.11

140

pH 4 6 8 10

67.42 74.97 72.45 73.73

99.32 99.62 98.24 99.18

74.19 73.92 67.91 72.68

97.62 99.11 95.34 98.11

Interfering ions PO4 2− BO4 2− SO4 2− NO3 2−

73.42 77.27 75.29 76.87

99.45 99.71 99.48 99.76

73.12 74.21 76.82 76.89

99.72 99.23 99.62 99.74

SVI (mg/L)

120 100 80 60 40 20 0 2

3.3. Process variable optimization

20

30

40

60

Time (min)

3.3.2. Influence of interfering ions The effects of the presence of anions in aqua system that can compete with the OH− and CO3 2− counter ions of the Ca derivatives on the dye removal were tested by the addition of 1 M of the following salts: K2 SO4 , KNO3 , H3 BO4 and KH2 PO4 to create the presence of SO4 2− , NO3− , BO3 2− , PO4 2− in the medium. The dye removal process was then carried out in this medium. The results obtained (Table 4) showed that the dye removal efficiency was not impacted in the presence of any of these ions but the volume of sludge generated was larger than when these ions were not present. The inability of these ions to impact on the dye removal efficiency could be ascribed to the underlying mechanism (co-precipitation) of dye removal via this process. Since any of these ions could impede the formation of the primary precipitate, dye removal via this process could not be impacted. The increase in the sludge volume could be ascribed to a synchronous formation of insoluble compounds of these interfering anions (calcium sulphate (CaSO4 ·2H2 O) with sulphate; calcium nitrate (Ca(NO3 )2 ·4H2 O) with nitrate; calcium tetra borate (CaB2 O4 ) with borax; and tricalcium phosphate (Ca3 (PO4 )3 with phosphate) with Ca2+ present in the medium thereby increasing the sludge volume. Aside the increase in sludge volume, the sludge settling characteristics was also affected. The sludge settling rate of the medium containing NO3 and SO4 were faster than the medium containing BO3 2− , and PO4 . The observed increase in the sludge settling rate noticed in the NO3 and SO4 medium may not be caused by the presence of these anionic species but could be ascribed to the increase in ionic strength, occasioned by the difference in the ionization potential and ionization products of the salts (i.e.,

0

b

0.05

0.1

0.2

0.5

1

200 180 160 140

SVI (mg/L)

3.3.1. Influence of pH The results of the effect of pH are presented in Table 4. The removal efficiencies of the dyes were not affected by change in the pH of the dye medium between the pH of 4 and 10. In studying the effect of pH on the removal efficiency, the pH of the dye solution was corrected to a specified pH before it was added to the CaCl2 solution. The pH of the added dye in the acidic region was equipoised by the strongly alkaline precipitants added and neutralized the possible acidic effect on the dye structure. Thus the in situ hybridization mixture also serve as a pool for the neutralization of acidic dye effluent and the in situ hybridization occurred under analogous condition, heedless of the initial dye effluent pH.

120 100 80 60 40 20 0 2

5

10

20

30

40

60

Time (min)

Fig. 2. (a, b) Effects of ionic strength on the sludge settling properties of Ca(OH)2 (a) and CaCO3 (b).

K2 SO4 , KNO3 ) from which the respective anionic species were derived. 3.3.3. Influence of ionic strength The effects of ionic strength on the removal of dyes via the in situ hybridization process were tested with the addition of an electrolyte, NaCl, into the dye solution prior to the dye–Ca derivative in situ hybridization process. Increase in ionic strength had no significant effect on the removal efficiencies and patterns of dye removal (Table 4). The ionic strength effect only manifested in the sludge settling kinetics of the dye hybridized nanoparticles. Thus, the settling rates of the sludges, obtained from medium of different ionic strengths, were assessed using the sludge volume index (SVI) protocol. The results obtained are presented in Fig. 2a and b. The sludge setting rate increased with increase in ionic strength. Solution containing 0.1% of NaCl had the lowest sludge settling rate while 1% solution had the highest settling rate. The point of inflex-

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a

b

Fig. 3. (a) FTIR spectra of Ca(OH)2 , MB–Ca(OH)2 and CR–Ca(OH)2 . (b) FTIR spectra of CaCO3 , MB–CaCO3 and CR–CaCO3 .

ion observed in Fig. 2a and b, obtained around 20 min, could be ascribed to the point at which equilibrium is approached in the settling rate of the derived sludge. After the attainment of equilibrium, the settling rate approached zero. The observed trend in the sludge settling rate could be ascribed to the fact that particle surface charges are strongly impacted by ionic strength and conductivity of the suspension solution. The higher the ionic strength of a particle suspension, the lower its surface charge and thus its zeta potential. Owing to the weaker repulsive forces between particles, the stability of the suspension

tends to decrease and the particles tendency to agglomerates and sediments accretes. 3.4. Sludge characterization 3.4.1. FTIR spectroscopic analysis 3.4.1.1. Ca(OH)2 , MB–Ca(OH)2 and CR–Ca(OH)2 spectra. The difference in the surface functional groups of the synthesized Ca derivatives from the in situ hybridization process and in the absence of dye contaminants (i.e., virgin materials) was stud-

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Fig. 4. (a) XRD diffractogram of Ca(OH)2 , MB–Ca(OH)2 and CR–Ca(OH)2 . (b) XRD diffractogram of CaCO3 , MB–CaCO3 and CR–CaCO3 .

ied using FTIR. The FTIR spectra of the virgin Ca(OH)2 , MB and CR loaded Ca(OH)2 (i.e., MB–Ca(OH)2 and CR–Ca(OH)2 ) are presented in Fig. 3a. The virgin Ca(OH)2 spectra showed characteristic peak related to the H-bonding stretching vibration of the OH at 3641 cm−1 . Peak at 3419 cm−1 was assigned to N–H stretching vibration of amine while peak at 2924 cm−1 was assigned to OH stretching of carboxylic acid. These two peaks (i.e., the amine and carboxylic groups) were assumed to have originated from the precursor (i.e., SS) of the synthesized Ca(OH)2 . Basically, the SS consist of CaCO3 as well as varied organic compounds, mostly of proteids molecule (Conchin). Thus the presence of these functional groups could be ascribed to the proteids molecule whose building block is amino acids. The peak at 1455 cm−1 was assigned to the CO3 2− band of CaCO3 . Though the precursor is made up of aragonite, which is a form of CaCO3 , but the conversion of the SS to nano Ca(OH)2 , via the sol–gel

techniques, is expected to get rid of the CO3 2− group. The presence of the carbonate peak portends two possibilities: the incomplete dissolution of the precursor in the medium and subsequent inclusion in the end product and; the fractional conversion of the CaCl2 , formed from the dissolution of the precursor in diluted HCl solution to CaCO3 , via the reaction with dissolved CO2 , from the atmosphere, in the reaction medium. The band at 679 cm−1 and 872 cm−1 is the characteristic bending and rocking vibration bands of CO3 2− and CaO. The CR–Ca(OH)2 spectra showed a reduction in the intensity of the H-bonding stretching vibration of the OH at 3641 cm−1 . The OH stretching vibration of the carboxylic group disappeared in the spectra. The reduction in the intensity of the OH peak and the disappearance of the carboxylic peak showed that these functional groups were involved in the CR hybridization into the growing Ca derivative. In aqueous solution, the sulphonate group of the anionic

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dye (D–SO3 Na) are dissociated and converted to anionic dye ions thus: D–SO3 Na = DSO3 − + Na Considering the negatively charged nature of the product of the dye molecule and the surface chemistry of the Ca(OH)2 , dye removal could be assumed to have taken place not via a straitlaced mechanism. The preparation of CaCl2 solution occurred in acidic medium and dye was added to the CaCl2 solution in this acidic medium. The formation of the Ca(OH)2 (with the addition of NaOH) also commenced in this acidic medium. The gradual formation of the Ca(OH)2 , at the acidic pH, is concomitant with the protonation of the surface thus: Ca(OH)2 = Ca–OH2 + + OH− The positively charged surface, created by the protonation, enabled electrostatic attraction between the negatively charged dye molecule and the surface of the Ca derivative. However, as the pH value increased, with the addition of more NaOH solution, the adsorbent surface become negatively charged and the mechanism of the CR uptake changes. This initial electrostatic interaction could have resulted in the reduction in the intensity of the OH peak. An increase in the intensity of the CO3 2− peak (1455 cm−1 ) and a shift of the wavelength of absorption from 1455 cm−1 to 1408 cm−1 was also observed. The peak at 1106 cm−1 in the virgin Ca(OH)2 disappeared and a shoulder at 1170 cm−1 appeared which was ascribed to C–N in aromatic amine of CR. An increase in the intensity of the peak at CO3 2− and CaO was also observed. A sharp peak at 711 cm−1 was ascribed to the out of plane C–H deformation of aromatic ring of CR. The MB–Ca(OH)2 spectra revealed no additional peak and showed close similarity to the spectra of the virgin Ca(OH)2 . This showed that no chemical interaction occurred between the Ca derivative and the MB molecules. 3.4.1.2. CaCO3 , MB–CaCO3 and CR–CaCO3 spectra. The characteristic band of CaCO3 at 1463 cm−1 was observed in the virgin CaCO3 spectra (Fig. 3b). The peak was assigned to the CO3 2− stretch vibration of CaCO3 . The middle strong peak at 872 cm−1 and small peak at 715 cm−1 are the characteristic bending and rocking vibration bands of CO3 2− and CaO stretch and bending vibration respectively. Comparison of the spectra of the virgin CaCO3 with the dye loaded dye showed the broadening and an increase in the intensity of the carbonate peaks. A band shift from 1463 cm−1 to 1393 cm−1 in the MB–CaCO3 and 1394 cm−1 in the Cr–CaCO3 occurred. An increase in the intensity of the CO3 2− and the CaO peaks were also observed 3.4.1.3. X-ray diffractometric analysis. The XRD pattern (Fig. 4a and b) of the Ca derivatives showed that they were crystalline materials. The diffractograms were matched with the data in the XRD catalogue and the presence of each material synthesized was confirmed. Comparison of the XRD pattern of the MB and CR loaded materials (i.e., CaCO3 and Ca(OH)2 ) showed an increase in the basal spacing when the dye molecules were hybridized into the material framework. Increases in the number of peaks in the dye loaded materials were also observed which showed the presence of other material aside the host Ca derivative. The increase in the basal spacing showed the intercalation of the dye molecule in the crystalline phase of the growing particles. The lift shift of peaks of CR–Ca(OH)2 and CR–CaCO3 that were more obvious than MB–Ca(OH)2 and MB–CaCO3 at 30 ◦ is a pointer to the magnitude of intercalation of guest (dye) material in the host particle which was higher in the CR medium than in the MB medium.

Fig. 5. SEM image of (a) Ca(OH)2 , (b) MB–Ca(OH)2 , (c) CR–Ca(OH)2 .

3.4.1.4. SEM analysis. The SEM images of the virgin materials and the dye loaded materials are presented in Figs. 5 and 6. The surface of the virgin Ca(OH)2 as presented in Fig. 5a is a fine particle of irregular shape, size and pores. The SEM images of the dye loaded Ca(OH)2 (i.e., MB–Ca(OH)2 and CR–Ca(OH)2 ) did not imply an alteration in the morphology and microstructure of the material. The presence of the dyes on the material surface was attested to by the brighter nature of the SEM images.

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face became very smooth and the edges were well defined and sharp. 3.4.1.5. Mechanism of in situ hybridization of dye contaminant. The quantitative appraisal of the dye removal from the aqua system, via the in situ hybridization of the contaminated dye molecules showed that appreciable amount of the dye was removed from the aqua system. A similitude of trend was exhibited by the two calcium derivatives in the abstraction of the two types of dyes studied. This is an indication that the pattern of interaction between the carbonate (CO3 2− ) and the hydroxide (OH) group with the different dye molecules is similar in this regard. The magnitude of CR removed from the aqua stream was higher than that of the MB which showed that the difference in the molecular structure of the dye molecule affected the magnitude of dye removal. A near complete dye removal was attained in the medium containing CR. Premised on the evidence of the results from the present studies, the underlying mechanism of in situ hybridization of dye molecules into the framework of growing Ca derivative could be assumed to have occurred via precipitation, co-precipitation, and adsorption. The precipitation of the dye molecule by the opposing ions (Ca+ and Cl− ) from the CaCl2 solution was discarded because no precipitation occurred in this medium. The idea of post-precipitation was also jettisoned because for post-precipitation to occur, there should be occurrence of common ion between primary precipitant (i.e., the NaOH or Na2 CO3 ) and the dye contaminant (CR and MB). In this regard, no such common ions exist hence the possibility of the reaction mechanism being post-precipitation was discarded. Co-precipitation, which involves two options – occlusion and adsorption, is a possible mechanism of dye abstraction via the in situ hybridization process that was greatly favoured by experimental evidence. The possibility of co-precipitation, via occlusion, was very high because of the crystalline nature of the precipitate which was attested to by amenability of the synthesized materials to XRD analysis. The co-precipitation mechanism, via occlusion, is analogous to the sweep coagulation mechanism in coagulation/flocculation process. Sweep coagulation or enmeshment in the precipitate occurs when precipitating coagulant traps suspended particles within a colloidal floc as it forms or settles. The prospect of the mechanism also occurring via adsorption, another co-precipitation option, was explored by carrying out adsorption studies using the synthesized adsorbent. It is assumed that if the removal mechanism is via adsorption, the magnitude of dye removal should be comparable. The comparison of dye abstraction via the two processes showed that the value of the amount of dye removed in each case was comparable. Premised on the foregoing experimental evidence the mechanism of dye removal via the in situ hybridization process could be assumed to have taken place via the co-precipitation process. During the co-precipitation process, both occlusion and adsorption were fundamental mechanism occurring contemporaneously 4. Conclusion

Fig. 6. SEM image of (a) CaCO3 , (b) MB–CaCO3 , (c) CR–CaCO3 .

Comparison of the SEM images of the CaCO3 in the presence and absence of the dye contaminants showed a change in morphology and microstructure of the material. The SEM image of the virgin CaCO3 (Fig. 6a) showed an aggregation of different rough surface rectangular particulates with obtuse edges. The rectangular prism aggregate wedged each other together. The SEM image of the dye loaded materials (Fig. 6b and c) showed the disaggregation of the rectangular prism and the sur-

In situ hybridization of dye molecules into growing particles of Ca(OH)2 and CaCO3 could be used as cleaning protocol for dye contaminated water. Change in initial dye concentration, variation in pH of the medium, presence of interfering anion and increase in ionic strength did not bear on the dye removal efficiency but the sludge settling property was swayed by the change in the ionic strength of the medium and the presence of interfering anions. Two types of co-precipitation reaction (occlusion and surface adsorption) were assumed, from the experimental and instrumental evidence, as the mechanism of dye removal via the in situ hybridization process.

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