Tamal Mandal^1*, Uttarini Pathak², Papita Das² and Prasanta Banerjee²

¹Department of Chemical Engineering, National Institute of Technology, Durgapur-713209, India

²Department of Chemical Engineering, Jadavpur University, Kolkata–700032, India

*Corresponding Author:

Tamal Mandal
Department of Chemical Engineering
National Institute of Technology
Durgapur-713209, India
E-mail: tamal.mandal@che.nitdgp.ac.in

Received date: 01 December, 2015; Accepted date: 15 March, 2016

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Keywords

Wastewater; Langmuir isotherm; Gibbs energy; Acid casein

Wastewater from a Dairy Industry represents a complicated system containing different components, including high concentration of organic pollutants generated as a result of the manufacturing process, utilities and service section, the chemicals being used producing residues of additives used in several operations. In this experimental study after determination of the initial parameters of the raw wastewater it was subjected to batch adsorption study using Hydroxyapatite nano-particles. The effect of contact time, initial wastewater concentration, pH, adsorbent dosage, solution temperature and thereby adsorption kinetic, adsorption isotherm, adsorption thermodynamics were evaluated. The results showed that the HAp nanopowders had a remarkable adsorption capability and the adsorption process was fast. The process was favoured at a lower temperature and lower pH in this case. Maximum removal of 93% could be achieved using an adsorbent dosage of 11 g/L, pH of 7.38, temperature of 303 K whereas lowering the temperature could make it upto 99%. The maximum adsorption capacity was obtained as 25.641 mg/g. Adsorption data for kinetics and the isotherm studies showed that the pseudo-second-order model and Langmuir isotherm suited the best to describe the adsorption phenomena. The thermodynamic parameters suggested that the adsorption by HAP was spontaneous and exothermic in nature. The experimental Results were further evaluated using ANN where model suggested a minimum deviation of experimental data from the predicted one.

Introduction

With increasing Urbanization and rapid Industrialization the Food sector becomes one of the largest producer of effluents per unit of production generating a large volume of wastewater which contributes to a great extent to pollution and thereby a threat to the environment. The ever increasing demand for milk and milk products has made the Dairy Industry in India to become the world's largest milk producer consuming almost 100% of its own milk production (Qasim W et al., 2013). Dairy Industry involves conversion of raw milk into into products such as butter, yogurt, cheese and processed milk classified as dries milk in powdered form, consumer milk, condensed milk etc. The processes involved were chilling, pasteurization, and homogenization. Beside these processes it also evolved from sectors like washing and cleaning of equipments like silos, heat exchangers, homogenizers, pipes etc. All these processes consumes a large amount of water engendering effluents with a high organic load indicating large values of biochemical oxygen demand (BOD), oils and grease, nitrogen and phosphorus content and especially Chemical Oxygen Demand. Typical Dairy Waste Water consists of large quantities of proteins basically casein, lactose, fat, salts (inorganic) in addition to use of detergents and sanitizers as a result of washing (Singh NB et al., 2014). They are whitish in colour, slightly alkaline and can be made acidic easily, due to the decomposition of milk sugar into lactic acid by fermentation. The milk waste pollution is characterised by heavy black sludge and strong butryric acid odors due to the decomposition of Casein (Shete BS et al., 2013). Casein and whey are the main components of dairy wastewater out of which Casein is relatively hydrophobic, making it poorly soluble in water, held together by calcium ions, inorganic phosphate, citrate ions and has a negative charge in milk. In milk the casein micelles exists as colloidal dispersion of quite stable molecules.

In this way dairy industries are contributing high pollution load by secreting untreated or partially treated wastewater originating environmental issues. Presently, the Indian government thrusts severe rules and regulations for management of effluent discharges to prevent environmental degradation.

Among the various treatments which are already present in the Dairy Industry are biological treatments including Trickling filters and Activated sludge process. Though effective for complete treatment of the wastewater but is expensive with excessive power requirements and chemical expenditure requiring large infrastructure. Chemical Precipitation methods are also popular due to their short retention times and low capital costs but they become disadvantageous in terms of pH adjustment and generation of chemical sludge that must be treated before disposal (Mehta Saha P et al., 2009). Therefore Adsorption emerges as the new technology in terms of efficiency, economy and operation (Crini G., 2005). Physical adsorption involving use of activated carbon are effective in removal but has a high a high initial cost and need a costly regeneration system. Thus common adsorbents like activated carbons, zeolites, clays, biomass and polymeric materials (Crini G., 2006) faces problems of low adsorption capacities and separation inconvenience. Thus it becomes important to exploit new promising adsorbents. Due to above mentioned complications inorganic adsorbents has become one hot research field. Though the bonding forces between the adsorbent (inorganic) and the adsorbate are usually weaker than those encountered in activated carbon or polymer adsorbents but the inorganic adsorbents also possess excellent thermal stability (Kaili L et al., 2010).

Calcium Hydroxyapatite (HAP), Ca₁₀(PO₄)₆(OH)₂ is a inorganic material and a member of the apatite mineral family with properties of ionic exchange , affinity for adsorption .They have potential towards bonding with organic molecules. These apatite-group minerals are now being considered as of great environmental significance due to their low water solubility, low cost and high stability under oxidizing and reducing conditions (Krestou A et al., 2004). HAP forms a major component of natural bone matrix (60-70%) and has wide biomedical applications in terms of drugs and genes carriers, osteoblast adhesions in addition to recover the contaminated soils, wastewater and fly ashes from heavy metal ions (Slosarczyk JS et al., 2000; Schek RM et al., 2006; Tiselius SH, Levin O et al., 1965; Takagi O, et al., 2004; Del Rio JG et al., 2006; Sandrine B et al., 2007).

It has tremendous potential in removal of long term contaminants from wastewater in adsorption of proteins, catalysts due to high biocompatibility and bioactivity properties with Ca-HAP surface with to P–OH groups acting as sorption sites. The crystal geometry depicts functional groups comprising of positively charged pair of crystal Calcium ions (C-sites) and clusters of 6 negatively charged oxygen atoms with triplets of crystal phosphates (P-sites). These nano composites are not only active biologically but also conductive, innocuous, nonimmunogenic (Murugan R et al., 2005). Moreover their properties such as surface grain size, pore size, wettability, etc. could effectively control protein interactions (Ferraz MP et al., 2004). Phosphoryl groups along with C-sites act strongly on proteins and other solutes.

The objective of this study was to investigate the potential ability of HAP nano-powders as a new biocompatible adsorbent synthesis art higher temperature in a furnace for the adsorption of organic pollutants from Dairy wastewater. The effect of initial concentration, pH, adsorbent dosage, solution temperature on adsorption, and the adsorption kinetic, isotherms and thermodynamic parameters were investigated. Artificial Neural Network Modelling was further developed to study the simulated results generated from the experimental data or using the validated models.

Materials and Methods

Wastewater sample was collected from outfall of a dairy industry at Dankuni near Durgapur express Highway. Wastewater sample collected from the plant was placed in containers to be transported to the laboratory and stored at 4ºC in a refrigerator. All the initial parameters of the wastewater were analysed in the laboratory as per the given standard methods in the handbook (APHA, 2005). All the chemicals were of analytical reagent grades and used as received, without further purifications.

Adsorbent Preparation

The simple precipitation method is the most reputed method for preparing HAP particles. This process is manageable, inexpensive and suitable for industrial production as compared to the processes like sol–gel method (Ferraz MP et al., 2004), Chemical precipitation method (Ferraz MP et al., 2004), microwave-assisted precipitation process (Han J et al., 2006), Double decomposition method (Rey C et al., 1989). The HAP nano powders were prepared by simple precipitation method. In this method (1.11) M ortho phosphoric acid was added drop wise into the (1.5 M) calcium hydroxide solution. It was subjected to strong magnetic stirring at 70°C for 3 h. This sample was agitated until the formation of transparent homogeneous solution. The pH value was raised to 10 using NaOH (2M) by adding dropwise to this solution. The white HAP gel was precipitated after leaving undisturbed for 4 h. The obtained solid products were filtered, washed many times with de-ionized water, and dried under mild temperature. The products in almost powdered form were then heated in a muffle furnace at 700°C for 1 h in air, and finally HAP was obtained (Ragab S et al., 2014) Sintering of hydroxyapatite particles at high temperatures resulted in agglomeration and fusion of the particles into a stable mass.

Adsorbent Characterization

The crystallinity and crystal phases of the synthesized nano-powdered HAP were examined by X-ray diffraction (XPERT-PRO-11014322) method. For detail study of the composition and functional groups present, the prepared samples of HAP nano-particles were examined by Fourier transform infrared (FTIR) spectroscopy. Size and morphology of the synthesized powders were characterized by field emission Scanning Electron Microscopy (ZEISS EVO-MA 10, Germany) at an electron acceleration voltage of 10 kV. The Atomic microscopy (Innova,Veeco, Bruker AXS Pte Ltd.) analysis of the nano adsorbents were done to investigate the particle size and morphology in order to compare with the XRD results, where layer-by-layer technique (deposition using mica substrate) was used to prepare the samples.

Batch Adsorption Study

The sorption studies were carried out at 30°C. Solution pH was adjusted with HCl or NaOH (0.1N). pH had been measured by following electrometric method using a digital pH meter. A known amount of adsorbent was added to samples and was agitated at 172 rpm agitation speed, allowing sufficient time for adsorption. Then, the mixtures were filtered through filter paper and membrane and the final concentration were determined in the filtrate using UV/VIS spectrophotometer The effects of various parameters on the percentage removal were observed by varying adsorbent dosage, initial pH of wastewater, temperature and concentration. The adsorption capacity was measured by the following equation,

   (1)

where Co = initial concentration (mg/L), C_t = concentration at time t (mg/L), V is the volume (Litre) of wastewater and m = mass of HAP adsorbent (g)

Adsorption Isotherm

In the present study the adsorption behaviour was investigated since they provide the most important piece of information in understanding the adsorption process. They give some idea about the underlying sorption mechanism as well as the surface properties and affinity of the sorbent (Chowdhury S et al., 2011). The Langmuir model (Langmuir I., 1916) assumes that the uptake of ions occurs on a homogenous surface by monolayer adsorption of each molecule having equal activation energy and that sorbate– sorbate interaction is negligible. The equation is stated as following:

   (2)

Where C_e is the equilibrium concentration , q_e is the amount of ions or molecules adsorbed (mg/g), Q_o is q_e for a complete monolayer (mg/g), K_L is sorption equilibrium constant. A plot of C_e/q_e versus C_e should indicate a straight line of slope 1/Q_o and an intercept of 1/K_L Q_o.

On the other hand the Freundlich isotherm (Freundlich HMF., 1906) states that uptake occurs on a heterogeneous surface by monolayer adsorption (Bulut Y., 2006) and is expressed as

   (3)

Where Ce is the equilibrium concentration, qe is the amount of ions or molecules adsorbed (mg/g), K_f and n are Freundlich constants related to the adsorption capacity and adsorption intensity respectively.

Adsorption Kinetics

Several Kinetic models are in use to explain the mechanism of the adsorption processes in order to be able to design industrial scale separation processes. A simple pseudo second order equation was used.

   (4)

Where q_t and q_e are the amount of adsorption at equilibrium and at time t respectively and K2 is the rate constant of the pseudo second order adsorption process.

Adsorption Thermodynamics

The thermodynamics of an adsorption process is obtained from a study of the influence of temperature on the process. The standard Gibb’s Energy was

   (5)

The equilibrium constant Kc was evaluated at each temperature using the following relationship

   (6)

K_c = distribution coefficient for adsorption.

C_a = equilibrium concentration on the adsorbent.

C_e = equilibrium concentration in solution.

Other thermodynamic parameters such as change in standard enthalpy ΔH° and standard entropy ΔS° were determined using the following equations.

   (7)

ΔG° = Gibbs free energy change

ΔH° = enthalpy of reaction.

Results and Discussions

Characterisation of Wastewater

The Dairy wastewater collected from the input of an effluent treatment section has the following characteristics presented in Table 1. Various physical and chemical parameters where effluents whitish in colour along with obnoxious odor were observed. The results showed though the wastewater did not have a very high COD value but it was above the permissible limit. The COD value was found to be larger than the BOD value suggesting that the organic compounds in wastewater are slowly biodegradable (Qasim W et al., 2013) .The pH was found to be slightly alkaline. The chlorides may be present due to the use of detergents and sanitizers in the cleaning of equipments but the value was not above the permissible value. Oil and grease was high due to presence of fats, lactose and proteins. The amount of total suspended solids and total dissolved solids were quite high and the removal of the earlier was usually targeted. The presence of Calcium ions as a prime constituent of Casein was indicated by the high value of alkalinity. Electrical Conductivity of the wastewater was also recorded to be quite high.

Table 1: Characteristics of dairy wastewater

Characterisation of Adsorbent

The typical X-ray patterns of the synthesized hydroxyapatite nanostructure is depicted in Figure 1a which demonstrated that all pattern of diffraction peaks can be connected to the well-crystallized single phase hydroxyapatite such that only HAP reflections, comprising of any impurity are detected in the figure which further confirmed that the synthesized hydroxyapatite nanoparticles contained pure hexagonal form. Figure 1b shows the general morphologies of synthesized spherical-shaped hydroxyapatite nanostructures which concluded that the particles are uniform size and highly aggregated spherical-like structure with average diameter of 135.43 nm and also the microstructure possessed similarity to that of the natural bone. The FTIR spectra model of the prepared HAP nanoparticles are depicted in Figure 1c which indicates that it is clear that the hydroxyl stretching and vibrational modes of OH^- are due to the presence of a hydrated layer or a organized water structure in addition to PO₄^- groups. Thus it is concluded that the distinguishable peaks belonging to the hydroxyl and phosphate groups in the HAP spectrum of FTIR sample support the well-crystallized apatite structure. The Atomic microscopy analysis of the nano adsorbents are demonstrated in the Figure 1d. Aggregates on the surface appeared and AFM images demonstrated the size of the particles indicating anisotropic with ellipsoidal geometry.

Fig 1a: XRD Analysis of HAP

Fig 1b: SEM analysis of HAP

Fig 1c: FTIR Spectra of HAP

Fig 1d: Atomic Force Microscopy Analysis of HAP (2D and 3D resp.)

Batch Adsorption Study

Effect of Adsorbent Dosage: Result on the effect of adsorbent dosage at temperature of 30°C, pH 7.38 and agitation 172 rpm is presented in Figure 2a. The adsorbent dosage was varied between 4-12 g/L and the percentage removal as a function of adsorbent dosage was calibrated. It was observed that the percentage removal increased with an increase in adsorbent dosage from 4-10 g/L at the beginning. The initial increase was mainly because of the increase of the contact surface of adsorbent particles and the availability of more binding sites increase for adsorption (Kaili L et al., 2010). But after a dosage of 10 g/L there was hardly any increase in the percentage removal. Thus a dosage of 11 g/L was chosen as an optimum dosage for the rest of the experiments. It also indicated that a satisfying removal can be achieved using small amount of the nano-adsorbent.

Fig 2a: Effect of Adsorbent Dosage on Percentage Removal by HAP

Effect of pH: In adsorption studies pH of the solution is an important monitoring parameter. It not only influences the surface charge of the sorbent but also the degree of ionization of the organic substances present in the solution. In this study the pH was varied between 2 to 10 from highly acidic range to high alkaline range keeping other parameters like adsorbent dosage at 11 g/L, temperature of 30°C and rotational speed of 172 rpm. The removal was favoured at a lower pH and there is a sharp decrease in the removal capacity with the increase of pH shown in Figure 2b. It has been reported that in case of organic removal, at higher pH the adsorbent surface carries a net negative charge while at lower pH a net positive charge (Castilla MC., 2004). At low pH values, the HAP would be protonated and became positive, the surface of the HAP will be surrounded by the hydrogen ions, which enhances the interactions between the organic substances and HAP through attractive force .Thus the carboxyl groups gets attracted to C sites. On the other hand in alkali medium the carboxyl groups (proteins)/C-sites (HAP) did not interact much resulting in electrostatic repulsion between the molecules and the binding sites. Due to the enfeebility of electrostatic force of attraction between the oppositely charged adsorbate molecules and adsorbent surface ultimately led to the reduction in removal.

Fig 2b: Effect of Solution pH on Percentage Removal by HAP

Effect of Temperature: Figure 2c shows the effect of different temperatures on the adsorption process. Keeping other parameters same as above (like adsorbent dosage at 11 g/L, pH of 7.38 and rotational speed of 172 rpm) the temperature was varied between 20°C to 40°C. The percentage removal decreased with increasing temperature. This decrease in removal was because the binding capacity decreases with increasing temperature which may be due to the weakening of the bonds between the molecules and the binding sites of the sorbent (Chakraborty S et al., 2011) Since the sorption capacity of the adsorbent was greater at lower temperature, it can also be said that the sorption might be an exothermic process. With regard to the effect of temperature on the adsorption, an increasing uptake of organic molecules is expected when the adsorption temperature decreases since adsorption is a spontaneous process.

Fig 2c: Effect of Temperature on Percentage Removal by HAP

Effect of Concentration: The concentration of the wastewater was varied by the method of dilution keeping the other parameters same as above (adsorbent dosage at 11 g/L, pH of 7.38 temperature of 30°C and rotational speed of 172 rpm). It was observed that on increasing the dilution or reducing the concentration the percentage removal remains also constant upto 220 mg/L after which there was a sharp decrease. It is evident from the Figure 2d. This decrease was mainly due to fact as in case of lower concentrations, the ratio of the initial number of moles of ions to the available surface area of adsorbent is large and subsequently the fractional adsorption becomes independent of initial concentration. However, at higher concentrations, the available sites of adsorption become fewer, and hence the percentage removal of decreases (Yu LJ et al., 2003).

Fig 2d: Effect of Concentration on Percentage Removal by HAP

Adsorption Isotherm Study

Adsorption isotherm, is utilized to develop an equation representing the results and can be used to design of adsorption systems. Among the different models for interpreting the data, the Langmuir and Freundlich models are the most frequently employed models. In this study, the Langmuir and Freundlich isotherm models were used to explain the equilibrium between the adsorbed molecules on HAP nanopowders and molecules in solution at the constant temperature. Data for Langmuir, Freundlich were plotted for adsorption of molecules into the nano-adsorbent presented in the Figure 3a and 3b. The parameters obtained from the Langmuir (C_e/q_e versus C_e), Freundlich (log qe versus log C_e) were evaluated. To compare the accuracy of the models quantitatively, the correlation coefficients (R2) were also calculated whose analysis suggested that the Langmuir isotherm model provides best fit to the adsorption data as compared to Freundlich model. Q₀ is 25.641 for Langmuir isotherm and K_L is 0.1054. For Fruendlich, K_F is 6.9240 and n is 3.26. This indicated monolayer coverage of the molecules onto the adsorbent with ach molecule has equal activation energy and that sorbate–sorbate interaction is negligible. The essential features of Langmuir and Freundlich adsorption isotherms can be expressed in terms of separation factor (dimensionless constant) or equilibrium parameter (RL). The RL value indicates the shape of the isotherm to be irreversible (RL = 0), favorable (0<RL<1), linear (RL = 0) or unfavorable (RL >1) (Hall K et al., 1966). In this case RL was found to be 0.0866 and thus favourable. On the other hand the freundlich isotherm showed that the situation n>1 (n = 3.2679) is populars and may be due to a dispension of surface sites or any other factor that leads to increasing surface density and decrease in adsorbent-adsorbate interaction (Va´ zquez I et al., 2007). It was found similar to this case.

Fig 3a: Langmuir Isotherm Plot for adsorption onto HAP

Fig 3b: Freundlich Isotherm Plot for adsorption onto HAP

Adsorption Thermodynamics Study

In any process involving adsorption and engineering practice, both the values of energy and entropy are the actual indicators for practical application of a process parameters and thereby these thermodynamic parameters should be considered in order to determine the spontaneous nature of the processes. The thermodynamic parameters such as Gibbs energy (ΔG), enthalpy (ΔH) and entropy changes (ΔS) for the adsorption process can be determined using Van’t

Hoff equation. The enthalpy change is determined graphically by plotting ln (keq) versus 1/T which gives a straight line and the values of ΔG and ΔS computed numerically from the slope and intercept are presented in the Figure 3c and Table 2. Gibbs energy values are negative and large and increases with increase of temperature. Furthermore, decrease in the negative value of ΔG with increasing temperature suggests that the adsorption process was more favorable at lower temperatures and thermodynamically favourable. This indicated that better removal is obtained at lower temperature. Negative value of ΔH indicate that the process is exothermic. Standard entropy determines the disorderliness of the adsorption at solid-liquid interface. The negative value of ΔS shows the feasibility of the adsorption and the increased randomness at the sorbent/solution interface during the adsorption of molecules onto HAP. The negative value of ΔS also suggests that the process is enthalpy driven (Saha P et al., 2012).

Fig 3c: Van’t Hoff Plot for estimation of thermodynamic parameters for adsorption onto HAP

Table 2: Thermodynamic Parameters for adsorption onto HAP

Adsorption Kinetics

The experimental data for the adsorption kinetics showed that it was found to be well suited with the pseudo Second Order Model. It is presented in the Figure 3d. The pseudo-second-order model constants, k₂ and qe were calculated from the values of slope and intercept of the plot of t/qt versus t. The calculated model parameters along with the correlation coefficient values (R2) are calculated with q_e = 27.78 mg/g and K₂ = 0.0762 g mg⁻¹ min⁻¹. It can be also concluded that the rate limiting step may be a chemisorption process.

Fig 3d: Pseudo Second Order Kinetic Model for adsorption onto HAP

Artificial Neural Network

For Training and testing of ANN a computer programming using MATLAB 7 was done. The best model was generated after a number of trials including crucial parameters like training algorithms, transfer function and number of neurons in various layers. Though all algorithms and transfer functions may not be suitable for all processes. The present work utilises input variables like pH, temperature, dosage, concentration. The percentage removal was chosen the experimental output variable. A coefficient of correlation (R) between the model prediction and experimental results with a value of 0.97147 for training data sets (25), was considered to be suitable and hence selected. The comparison is demonstrated in Figure 3e and in Table 3. It was observed that the model provides a good agreement between the predicted and the experimental data with minimum error.

Fig 3e: Comparison of experimental results with simulated results from ANN

Table 3: A comparative study between observed and theoretical percentage removal for adsorption onto HAP using ANN model

Conclusion

The present study shows that Nano composites especially Hydroxyapatite particle can be effectively used as a adsorbent for treatment of Dairy Wastewater as it could bring about a removal upto 93% could be achieved using an adsorbent dosage of 11g/L, pH of 7.38, temperature of 30°C whereas lowering the temperature could make it upto 99%. Moreover it is a cost effective process since it was prepared from cheaply available raw materials. The entire process was favoured at lower temperature and lower pH with a little adsorbent dosage. The solution pH controls the adsorptive–adsorbent and adsorptive– adsorptive electrostatic interactions, which can have a profound effect on the adsorption process. The organic removal was favoured at lower temperature which concluded that the process is exothermic. Thermodynamic parameters stated that the process was spontaneous and enthalpy driven. Langmuir isotherm and pseudo second order models fitted best. Further ANN modelling suggested that the removal was favoured within the experimental ranges adopted in the fitting model. However there is always a future scope of study for recovery of the valuable proteins present in the wastewater in addition to treatment with Hap nano-materials.

Acknowledgement

The authors would like to be grateful to the governing authorities of Jadavpur University and National Institute of Technology for providing immense support and facilities both financially and instrumentally towards experimental analysis and thereby for its successful completion.

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