ARTICLE pubs.acs.org/IECR
Thermochemical Treatment (Thermolysis) of Petrochemical Wastewater: COD Removal Mechanism and Floc Formation Shilpi Verma, Basheshwar Prasad,* and Indra Mani Mishra Department of Chemical Engineering, Indian Institute of Technology, Roorkee 247667, India
bS Supporting Information ABSTRACT: Thermo-chemical precipitation (thermolysis) has been used as a pretreatment method for COD and color reduction from various industrial effluents. This paper reports on the COD reduction from purified terephthalic acid (PTA) wastewater from a petrochemical plant using thermolysis. CuSO4, FeCl3, and FeSO4 were used as chemicals/coagulants for COD reduction. The effect of treatment time, temperature, pH, and chemical dosage on the removal of COD has been studied. FeCl3 was found to be the most effective chemical agent which removed 77.2% COD (from initial COD of 3320 to 765.9 mg/L) at optimum treatment conditions (pH 7, dosage 3 kg/m3 (0.018M), temperature 50 °C, and treatment time 20 min). Temperature played an important role in COD removal. Zeta potential studies depict the change in net surface charge of the particles in all the systems. In the case of CuSO4 3 5H2O, floc size distribution (number density) shows that bigger flocs were formed at higher temperatures. With iron salts, a decrease in floc size was observed with an increase in temperature.
’ INTRODUCTION Purified terephthalic acid (PTA) is used as a basic raw material in the production of various products such as polyethylene terephthalate bottles, polyester textile fibers, and polyester films, etc.1,2 PTA is produced by the catalytic liquid phase oxidation of p-xylene in acetic acid, in the presence of air. Manganese or cobalt acetate is used as a catalyst. The two - step process of PTA manufacture releases various pollutants into the environment. While air emissions generally contain VOCs, unreacted p-xylene, acetic acid, carbon monoxide, carbon dioxide, methyl acetate, nitrogen, and oxygen, the wastewater contains significant amounts of aromatic pollutants including p-toluic acid, benzoic acid (BA), 4-carboxybenzaldehyde, phthalic acid (PA), and terephthalic acid (TA) as major constituents, and 4-formylbenzoic acid, methyl acetate, and p-xylene as minor constituents. Major constituents contribute up to 7080% of COD in the wastewater. The wastewater generation varies from 3 to 10 m3/ tonne of PTA produced and is equivalent to 520 kg COD/ m3.36 The wastewaters generated during manufacturing of PTA are toxic and adversely affect the reproductive capacity of mice.7 The PTA and its products are toxic showing acute, chronic, and molecular toxicity to living organisms. Phthalates, its esters, and intermediate degradation products are found to have endocrine disrupting properties, and affect the reproductive capacity and development of humans.8,9 It is, therefore, essential to treat PTA wastewaters so as to reduce the toxic components to the permissible level. As per the discharge standards prescribed by the Ministry of Environment and Forests (MoEF), Government of India, the petrochemical wastewater should have COD < 250 mg/L and BOD5 e 50 mg/L for release into surface waters.10 Therefore, the petrochemical wastewater must be treated effectively so as to meet the discharge quality standards. Various biological, physicochemical, and thermal treatment technologies are currently being used for petrochemical wastewaters. Biological treatment technologies include both aerobic1115 r 2011 American Chemical Society
and anaerobic treatment technologies.1630 The physicochemical treatment processes include adsorption,3134 coagulation and flocculation,15,35,36 and advanced oxidation processes3740 such as catalytic wet oxidation using air, oxygen, and/or hydrogen peroxide, photocatalytic oxidation, and ozonation. All these methods have been used in the treatment of PTA wastewaters. Pretreatment methods such as coagulation and flocculation and thermochemical precipitation make the treated wastewater amenable to biological treatment. Thermochemical precipitation has been used as an effective pretreatment method for pulp and paper mill wastewaters,41 composite wastewater of a cotton textile mill,42 and the distillery wastewater and biodigester effluent of an alcohol production plant.43 Thermochemical precipitation is a process wherein metal salts or chemicals react with the organic and/or inorganic substances present in the wastewater, at an elevated temperature, resulting in complexation and formation of insoluble precipitates. Garg et al.41 studied the removal of COD and color from pulp and paper mill wastewater in the temperature range of 2095 °C. The homogeneous catalyst CuSO4 was found to be the most effective among various homogeneous and heterogeneous catalysts used in the study. A COD removal efficiency of 63.3% was obtained with a catalyst concentration Cw 5 kg/m3 at pH 5. However, maximum color removal (92.5%) was obtained at a Cw of 2 kg/m3. The effect of temperature on COD reduction was not appreciable. Kumar et al.42 used CuSO4, FeSO4, FeCl3, CuO, ZnO, and PAC as catalysts/chemicals for the thermolysis of composite wastewater from a cotton textile mill for the removal of its COD and color. CuSO4 at a Cw 6 kg/m3, pH 12, and temperature of 95 °C was found to be the most effective chemical with ∼77.9% COD removal and 92.8% color removal. Thermolysis Received: September 22, 2010 Accepted: March 15, 2011 Revised: March 9, 2011 Published: March 31, 2011 5352
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was followed by coagulation and flocculation. Potash alum [KAl(SO4)2 3 16H2O] at a coagulant concentration of 5 kg/m3 was found to be the best among the other coagulants tested. Coagulation of the supernatant obtained after treatment by catalytic thermolysis resulted in an overall reduction of 97.3% COD and close to 100% color removal. Choudhary et al.43 treated distillery wastewater by catalytic thermolysis at 80100 °C using CuSO4, CuO, MnO2CeO2, and ZnO as catalysts. CuO was found to be the best among all the catalysts used for COD and color removal. At 100 °C and Cw 4 kg/m3, 47% COD reduction and 68% color reduction were found for distillery wastewater, and 61% COD removal and 78% color removal were found for distillery biodigester effluent. The aim of the present study is to evaluate the scope of thermochemical precipitation as a pretreatment step for PTA wastewater to reduce its COD and BOD. The effect of several parameters such as time, temperature, pH, and chemical concentration on COD removal were also studied. Floc size distribution was also studied at different time intervals during experiments. To the best of our knowledge, no report is available in the literature about the use of inorganic metal salts for the thermochemical precipitation of dissolved and colloidal organics present in PTA wastewater at different temperatures.
higher than the ambient temperature over a temperature range of 4090 °C. Initial pH (pHo) of the wastewater was adjusted by adding 1 N H2SO4 or 1 N NaOH, as the case may be, to the wastewater. A 250 mL volume of wastewater was used in the reactor. The temperature of the reaction mixture was raised to the desired value using a water bath fitted with a digital temperature controller. A vertical water-cooled condenser was attached to the side neck of the reactor to prevent any loss of vapor and the middle neck was fitted with a mechanical stirrer. Thereafter, the chemical/coagulant was added to the reactor. The CuSO4 3 5H2O, FeCl3, and FeSO4 3 7H2O were used as chemicals/coagulants. The FeCl3 was used in the concentration range of 14 kg/m3 (6.16 103 0.024 M). FeSO4 3 7H2O was also used in the concentration range of 14 kg/m3 (3.5 103 0.014 M). CuSO4 3 5H2O was used in the concentration range of 37 kg/m3 (0.0120.028 M). The experimental procedure consisted of three phases: a period of 5 min for the flash mixing of the chemical/coagulant at 250 rpm, followed by slow mixing at 30 rpm. In the final stage, the flocs were allowed to settle for 90 min. The reactor samples were taken out at periodic intervals and also at the end of the settling time. The COD of the supernatant was determined after centrifugation of a sample.
’ EXPERIMENTAL SECTION
’ RESULTS AND DISCUSSION
Materials and Methods. Source of Wastewater. The wastewater was collected from the flow equalization tank of the effluent treatment plant (ETP) of a PTA production unit situated in northern India. The wastewater was stored at 4 °C in a refrigerator in the laboratory and was used subsequently in the experiments without any dilution. Chemicals. All the chemicals used in the experiments were of analytical reagent (AR) grade. Copper sulfate (CuSO4 3 5H2O), anhydrous ferric chloride (FeCl3), and ferrous sulfate (FeSO4 3 7H2O) were procured from M/S, RFCL, New Delhi. Terephthalic acid (TA) and benzoic acid (BA) were purchased from Sigma Aldrich, Germany. Analysis of Physicochemical Parameters. The wastewater characteristics such as pH, COD, BOD, total solids (TS), total dissolved solids (TDS), total suspended solids (TSS), total alkalinity and total acidity as acetic acid were determined as per standard methods (APHA).44 The treated wastewater samples were centrifuged (centrifuge model R-24, Remi Instruments, Mumbai) to obtain the clear supernatant and the precipitate. The COD of the effluent, before and after the treatment, was determined by the standard dichromate closed reflux method as per the standard method. The COD value was assayed with a COD analyzer (Aqualytic, Germany). Zeta potential measurements were carried out by using a zeta potential meter (Zetasizer Nano ZS, Malvern, UK). Number density and size distribution of the particles during treatment was determined by a particle size analyzer (PSA) coupled with a computerized inspection system CIS-100 (Ankersmid, Israel) and a GCM-101 magnetic stirring cell module. High-Performance Liquid Chromatography. TA and BA were analyzed through HPLC (Waters, USA). HPLC was operated in an isocratic mode with a C-18 column at ambient temperature with the mobile phase at a flow rate of 0.8 mL/min. The mobile phase consisted of acetic acid (2% vol), 2-propyl alcohol (7% vol), and water (91% vol) with phosphoric acid (300 μL/L solvent). An UV detector (Waters 2487 Dual λ Absorbance) set at 250 nm was used.37 Experimental Procedure. The experimental studies were carried out in a 0.5-L three-necked glass reactor at temperatures
Characteristics of PTA Wastewater. The characteristics of PTA wastewater as analyzed by the petrochemical plant are presented in Table 1. The wastewater consisted of both organics and inorganic substances and is slightly acidic in nature (pH 5.6). Its COD (3320 mg/L) and BOD (963 mg/L) were very high and the benzoic acid (BA) and terephthalic acid (TA) were most predominant. Effect of Temperature on COD Removal. The effect of temperature (4090 °C) on COD removal using CuSO4 3 5H2O, FeCl3, and FeSO4 3 7H2O at a dosage of 4 kg/m3 and pH 5.6 is shown in Figure 1. At ambient temperature, the COD removal was found to be 30.2%, 39.1%, and 8.1% with CuSO4 3 5H2O, FeCl3, and FeSO4 3 7H2O, respectively. Because COD removal was found to be low, further experiments were conducted at higher temperatures (4090 °C) with different inorganic compounds. COD removal shows an increasing trend with temperature up to a certain value and then decreases. This trend is observed for all the chemicals, and shows that there is an optimum temperature at which maximum COD removal is obtained. This is in line with the observation of Meric et al.,45 who conducted experiments on the treatment of wastewater containing reactive black 5 (dye) using Fenton’s oxidation process. All the experiments were carried out in triplicate. The analysis of variance (ANOVA) of the replicates showed the rejection of null hypothesis as the calculated F values showed several fold increase over the tabulated F values (see Supporting Information Appendix I). For every chemical, optimum treatment temperature is different, and beyond the optimum temperature, the COD reduction decreases, as shown in Figure 1. Temperature has a pronounced effect on the electrical double layer of the charged particles and distribution of the hydrolysis/complexation species of coagulants both in solution and on the particle surface.4649 An increase in temperature results in the reduction in particle stability. This is because surface charge density increases with an increase in temperature and electrical double layer thickness decreases. This results in agglomeration of destabilized colloids. When FeCl3 is added as a coagulant to a system at an elevated 5353
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Table 1. Characteristics of PTA Industrial Wastewater parameter
concentration (mg/L), except pH
pH
5.6
COD
3320
BOD
963
terephthalic acid
172
benzoic acid
316
4-carboxy benzaldehyde
ND
methyl acetate
ND
total solids cobalt
4153 4.2
manganese
4.3
chromium
ND
calcium
4.0
magnesium
0.25
sodium
840
aluminum
0.06
zeta potential
23.7(mV)
Figure 2. Effect of CuSO4 3 5H2O dosage and initial pH on COD removal at 80 °C.
3
Figure 1. Effect of temperature (initial pH 5.6 and dosage 4 kg/m ) on COD removal with CuSO4 3 5H2O, FeCl3, and FeSO4 3 7H2O.
temperature, the rate of flocculation and the charge neutralizing ability of the Fe(III) flocs increases.49 Due to the simultaneous decrease of surface charge and increased rate of charge neutralization ability of coagulants at elevated temperatures, the removal of particles from the solution gets enhanced resulting in increased COD removal. The surface potential of the particles determines the repulsion potential between the particles. However, the surface potential of the particles does not decrease with a corresponding decrease of surface charge.50 As the treatment temperature is increased beyond the optimum temperature, the surface potential of the particles is again increased. The increased surface potential of the particles results in increased repulsion potential between the particles. As a consequence of these repulsed particles, the COD reduction from the solution gets decreased. Effect of temperature on flocculation and COD removal is pronounced when floc destabilization reactions rely on enmeshment mechanism. With an increase in floc size, enhanced enmeshing of the particles may happen. This may also increase Brownian motion of the particles which may result in deentanglement of the enmeshed particles and release in the liquid medium.51
Figure 3. Effect of FeCl3 dosage and initial pH on COD removal at 50 °C.
Effect of pH and Dosage on COD Removal by Various Chemicals/Coagulants. The effect of pH and the metal salt
dosage on COD removal at optimum temperature is shown in Figures 24 for CuSO4 3 5H2O, FeCl3, and FeSO4 3 7H2O, respectively. The pH shown in the figures is an initial pH to which the test solution was brought before metal salt addition. pH has a strong effect on the precipitation of organics using metallic salts, an effect which cannot be produced by pH adjustment alone.52 The presence of different contaminants in the wastewater along with the functional groups which provide negative charge, their size or molecular weight, and the hydrophobicity of the contaminants, affect the applicable pH range for the use for different coagulants/ chemicals. When metal salts like CuSO4 3 5H2O, FeCl3, and FeSO4 3 7H2O are added to water, they instantaneously dissociate into their cations and anions. The anions—chlorides and sulfates—are very soluble and nonreactive, while cations undergo hydrolysis, and in the case of Fe(II), possibly oxidation. The hydrolysis of cations produces various monomeric and polymeric species of copper and iron hydroxides; some of which are insoluble in water and precipitate. For the petrochemical wastewater, the optimum pH was found to be 9 at which a maximum of 70.1% COD removal was observed with a CuSO4 dosage of 6 kg/m3 (0.02 M) at the optimum temperature of 5354
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Figure 4. Effect of FeSO4 3 7H2O dosage and initial pH on COD removal at 70 °C.
Figure 5. Time course of pH during treatment at optimum treatment conditions.
80 °C. The optimum pHs for FeCl3 and FeSO4 3 7H2O, were found to be 7 and 8, respectively. At these pH values the COD removals were 77.2% and 37.7% respectively, with FeCl3 dosage of 3 kg/m3 (0.018 M) and FeSO4 3 7H2O dosage of 2 kg/m3 (7.1 103 M) at the optimum temperatures of 50 and 70 °C, respectively. At the optimum pH, CuSO4 and FeCl3 produce Cu(OH)2 and Fe(OH)3 flocs. These metal hydroxides have very low solubility and amorphous structure. An increase in the amount of ferric and cupric hydroxide precipitates results in the formation of flocs and acid anion complexes by electrostatic attraction. Thus the adsorptive surface increases. The Van der Waals forces of attraction drive the formation of bigger flocs through collision and agglomeration. The inorganic and organic substances generally get adsorbed on these flocs of metal hydroxides and thereafter, get precipitated. At the prevailing treatment conditions with FeSO4, some of the Fe(II) may have oxidized to Fe(III). At pH 8, Fe(II) exists as Fe(OH)þ and Fe(OH)2 3 Fe(OH)3 ions form at pH > 10.5.53 Fe(III) mainly exists as Fe(OH)4. With Fe(OH)4 species, it is not possible to remove anions from wastewater and hence the COD. It may be concluded that the charge neutralization by adding FeSO4 to the wastewater at pH 8 is the main driving force for COD removal.52,5456 When the natural pH of the wastewater was adjusted to 2, 3, and 4, without adding any chemical coagulant, enhanced precipitation was observed in comparison to that at higher pH values. At pHs 2, 3, and 4, COD removals were 52.35%, 51.4%, and 49.6%, respectively. No COD removal was observed with CuSO4 3 5H2O, FeCl3, and FeSO4 3 7H2O at pH 2 and optimum temperature and dosage. At pHs 3 and 4, COD removal was 1.1% and 2.5% with FeCl3 at 50 °C temperature and a dosage of 3 kg/m3 (0.018 M). With CuSO4 3 5H2O at 80 °C temperature and a dosage of 6 kg/m3 (0.02 M), 1.2% and 1.9% COD removals were observed at pHs 3 and 4, respectively. FeSO4 3 7H2O did not show any COD removal at pHs 3 and 4. The supernatant of the adjusted pH 2 was again adjusted to pH 7, 8, and 9 using NaOH, which are the optimum treatment pHs for FeCl3, FeSO4 3 7H2O, and CuSO4 3 5H2O, respectively. At pH 7, with temperature at 50 °C and a FeCl3 dosage of 3 kg/m3 (0.018 M), 7.2% COD removal was observed. With CuSO4 3 5H2O, 6.8% COD removal was observed at pH 9 at a temperature of 80 °C and dosage of 6 kg/m3 (0.02 M). 1.4% COD removal was observed with FeSO4 3 7H2O at optimum treatment conditions. Thus, it is inferred that the COD removal is effected at higher original pH as shown earlier.
Because the metal ion hydrolysis continues after the addition of metal salts, the pH of the supernatant of the treated wastewater decreased from the initial pH 9 to 4.4 for CuSO4 3 5H2O, from 7 to 3.5 for FeCl3, and from 8 to 5.1 for FeSO4 3 7H2O, as shown in Figure 5. Role of Zeta Potential in COD Reduction. Most of the colloidal dispersions in aqueous media carry an electrical charge. This electrical charge may be intrinsic to the particles due to imperfections in solid crystal or they may arise as a selective adsorption or desorption of ionic constituents of the particle potential determining ions or of specific adsorption of other ionic species.57 The zeta potential of the PTA wastewater was found to be 23.7 mV. The 1 N NaOH used for pH adjustment for each chemical/coagulant changed the magnitude of charge of the electrical double layer surrounding the particle, resulting in a change of zeta potential of the wastewater. As the pH was adjusted to 7, 8, and 9 for FeCl3, FeSO4 3 7H2O, and CuSO4 3 5H2O, respectively, the zeta potential changed to 19.5, 17.3, and 18.4 mV, respectively. This increase in zeta potential caused the colloids to flocculate to a small extent, which led to a COD reduction by 13.4%, 15%, and 17.6%, respectively, at adjusted pHs of 7, 8, and 9. The change in zeta potential is faster with FeCl3 than that with CuSO4 3 5H2O and FeSO4 3 7H2O. The zeta potential increased from 19.5 to 1.6 mV within 5 min of FeCl3 addition during flash mixing. However, in the case of CuSO4 3 5H2O and FeSO4 3 7H2O, the zeta potential increased from 18.4 to 15.6 mV and 17.3 to 13.9 mV after 5 min of flash mixing. After 5 min of flash mixing, slow mixing was initiated. At the end of the slow mixing phase, the zeta potential values for FeCl3, FeSO4 3 7H2O, and CuSO4 3 5H2O were obtained as 5.89 mV, 11.1 mV, and 0.87 mV, respectively, as shown in Figure 6. The first sample for COD analysis was withdrawn after 20 min of adding the chemical/coagulant. As seen from the electrochemical curve, the isoelectric point was attained only for FeCl3 after 10 min from the start of the experiment. In the case of CuSO4 3 5H2O, the zeta potential almost approached the isoelectric point at the end of slow mixing. This was achieved either by adsorption of Hþ ions or other positively charged ions, such as ferric ions or cupric ions, on negatively charged surfaces. When the zeta potential of particles tends toward zero, the coagulation efficiency improves. 5355
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Figure 6. Time course of zeta potential at optimum treatment conditions.
As the pH was adjusted to 2, 3 and 4, the zeta potential changed to 2.53, 4.2, and 6.93 mV, repectively, from its initial values of 23.7 mV. At pH 2, zeta potential is positive. Due to the protonation of the functional groups of the compounds present in wastewater, there is no neutralization by metal salts and, therefore no COD removal. At pH 3 and 4, the zeta potential is approaching zero. When the metal salts were added to the wastewater, the COD removal was found to be very small. Effect of Thermochemical Precipitation on Concentration of Terephthalic Acid (TA) and Benzoic Acid (BA). When metal salts like CuSO4 3 5H2O, FeCl3, and FeSO4 3 7H2O are added to water, they instantaneously dissociate into their cations and anions. The anions—chlorides and sulfates—are very soluble and nonreactive, while cations undergo hydrolysis. The hydrolysis of cations produces various monomeric and polymeric species [MOH2þ, M(OH)þ2, M2(OH)4þ2, M(OH)5þ4, M(OH)03(s), and M(OH)4 ]41,55 of copper and iron hydroxides, some of which are insoluble in water and precipitate. Insoluble metal complexes are formed at alkaline pH. The resulting metal hydroxide polymers are positively charged having very large surface area and an amorphous structure. The hydrophobic nature of the metal hydroxide polymer renders them to get adsorbed onto the organic anionic particle surface and become insoluble. The removal of terephthalic acid and benzoic acid during coagulation with metal salts takes place through various mechanisms, commonly referred to as charge neutralization, sweep flocculation, and adsorption. In the present case, the two carboxylic groups of terephthalic acid and one carboxylic group of benzoic acid are important. The negative charge imparted on particles through the presence of these carboxylic groups gets neutralized by the hydrolysis products of metal ions. The neutralized organics are able to form bigger flocs by Van der Waal forces of attraction. These bigger flocs then settle down. This induces the adsorption and sweep flocculation.36 TA and BA are the main components of PTA wastewater, having a concentration of 171.9 and 316.2 mg/L, respectively. At the optimum treatment conditions, 76.7% TA removal (residual concentration 40 mg/L) and 81.3% BA removal (residual concentration 59.2 mg/L) were obtained with CuSO4 3 5H2O. Using FeCl3 at the optimum treatment conditions, the concentration of TA reduced to 31.4 mg/L (81.7% removal) and that of BA reduced to 73.8 mg/L (76.7% removal). At pH 9 and Cw 6 kg/m3
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(0.02 M) and 40 °C temperature, the use of CuSO4 3 5H2O gave 26.5% TA removal and 69.4% BA removal. When the temperature was increased to 60 °C, the percentage removal of TA increased to 66.1% and that of BA increased to 76.2%. With FeCl3, the corresponding reductions in the concentration of TA and BA were, respectively, 57.1% and 12.3% at 30 °C and 63.8% and 68.3% at 40 °C at pH 7 and Cw 3 kg/m3 (0.018 M). The acid removal data confirm the effectiveness of the thermochemical precipitation at the optimum temperature of the corresponding chemicals in the removal of TA and BA from the petrochemical wastewater. Particle/Floc Size Distribution. The process of coagulation starts generally, with an initial rapid (or) flash mixing of the coagulant or the chemical for its dispersion in the liquid medium containing colloidal or dissolved matter for a period of 5 min. After this period, the stirring was reduced considerably (from 250 to 30 rpm) which helps in mechanical stimulation for the destabilized colloidal particles to come into contact with each other or with the coagulant and grow into larger flocs, with settling tendency. The slow mixing creates the desired velocity gradients in the mixture for flocculation to proceed. During this slow stirring period, the process of agglomeration takes place because of various mechanisms: part of the organic compounds and colloidal particles are absorbed by the coagulant hydrolysate, and a part of them are swept with the coagulant hydrolysate. In case of CuSO4 3 5H2O and FeCl3, the charge neutralization is accompanied by adsorption and sweep flocculation. During adsorption and sweep flocculation, colloidal particles get adsorbed on the surface of flocs with the formation of new chemical bonds and some of the colloidal particles get entrapped in the settling flocs. As the formation of new chemical bonds continues, the floc size also increases concurrently. However in the case of FeSO4, charge neutralization is the major phenomena. When charge neutralization dominates the coagulation mechanism, the negative charges in the colloid surfaces are neutralized by the positively charged coagulants and the destabilized colloids aggregate to form flocs. The charge neutralization process is a physical process with no formation of new chemical bonds. As the adsorption and sweep flocculation are the minor driving forces here, the floc size remains small. Normally, such flocs show weaker floc strength and are prone to break with external shear force.5861 The floc size distribution in terms of number density with respect to flocculation time and temperature is shown in Figures 68 and Table 2. In the case of CuSO4, an increase in floc size was observed as the treatment temperature was increased from 40 to 80 °C. Floc size reached its maximum in 25 min at 40 and 60 °C, while the maximum floc size was observed in 15 min at 80 °C (Figure 7). At this temperature, the number densities of particles are larger in larger floc size range. At a higher temperature, bigger organic molecules would break up into the smaller ones and the Brownian motion of particles increases. This results in an increased interparticle collision, and therefore, enhanced rate of flocculation. An interesting trend was observed with FeCl3 and FeSO4 3 7 H2O as the temperature was increased from 30 to 50 °C and 40 to 70 °C, respectively. It was found that the floc size decreased with an increase in treatment temperature. With FeCl3 at the treatment temperature of 50 °C, 50% of the flocs were in the size range of 0.540.99 μm after 5 min. As the slow mixing progressed, the floc size increased. The maximum average floc size was 1.80 μm after 15 min, and thereafter the floc size decreased (Figure 8). 5356
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Table 2. Floc Size (μm) Median Value (d50) at Various Temperatures and Time Intervals CuSO4
FeCl3
FeSO4
time (min) 40 °C 60 °C 80 °C 30 °C 40 °C 50 °C 40 °C 60 °C 70 °C 5
0.63
0.73
1.93
1.69
0.98
0.87
1.44
1.35
0.72
15 25
0.84 0.88
1.06 1.08
2.54 1.96
1.78 2.16
1.92 2.47
1.80 0.87
1.45 1.39
1.34 1.92
0.95 1.27
35
0.87
0.92
1.87
1.75
2.29
0.86
1.28
1.58
1.60
Figure 7. Change in number density of flocs at various time intervals treated with CuSO4 at pH 9, dosage 6 kg/m3 (0.02 M), and temperature 80 °C.
Figure 9. Change in number density of flocs at various time intervals treated with FeSO4 3 7H2O at pH 8, dosage 2 kg/m3 (7.19 103 M), and temperature 70 °C.
Figure 8. Change in number density of flocs at various time intervals treated with FeCl3 at pH 7, dosage 3 kg/m3 (0.01 M), and temperature 50 °C.
As shown in Figure 9 and Table 2, the flocs formed with FeSO4 3 7H2O were found to be smaller than those found with FeCl3 and CuSO4 3 5H2O. This indicates that the charge neutralization was the major driving force during the initial 5 min period of flash mixing. During flocculation at 40 °C the floc size did not change significantly. However at 60 and 70 °C, the floc size increased continuously with time, suggesting that the adsorption and sweep flocculation played significant roles in the floc formation at elevated temperatures. At a given shear rate, the floc size and its growth were not consistent for all the coagulants. During the course of treatment, the growth and breakage of the flocs take place simultaneously. Thus the rate of aggregation is the result of equilibrium between floc formation and floc breakage.59,62,63 When a certain optimal floc size is reached, an increased internal shearing will break the largest and the most fragile flocs.64
’ CONCLUSIONS Among several chemicals/coagulants used for thermochemical precipitation treatment of PTA wastewater, FeCl3 was found to be the most effective. Temperature played a significant role in COD removal and the concentration reduction of TA and BA in the wastewater. With FeCl3 at the optimum treatment conditions (pH 7, dosage 3 kg/m3 (0.018 M) and temperature 50 °C) 77.2% of COD was removed. TA and BA removal was 81.7% and 76.7%, respectively, at 50 °C which was much higher than 57.1% and 12.3% at 30 °C. Comparable COD removal was observed with CuSO4 at its optimum treatment conditions (pH 9, Cw 6 kg/m3 (0.02 M), and temperature 80 °C) with 70.1% COD removal and a reduction of 76.7% in TA concentration and 81.3% in BA concentration. At 40 °C, 26.5% TA and 69.4% BA removal were observed. The high removal of TA, BA, and COD with FeCl3 and CuSO4 3 5H2O at elevated temperatures signifies the influence of temperature on the treatment efficiency of the PTA wastewater. Zeta potential analysis of all the chemicals/coagulants showed that the zeta potential approached the isoelectric point very fast in case of FeCl3. This showed that FeCl3 is a very effective coagulant for the removal of COD from PTA wastewater. Floc size distribution of CuSO4 system showed an increase in size as flocculation proceeded with temperature. The maximum average floc size (d50) was 2.54 μm at a temperature of 80 °C and a treatment time of 15 min. However, in the case of FeCl3 and 5357
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’ ASSOCIATED CONTENT
bS
Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ91 1332 285323(O)/285169(R). E-mail: bashefch@ iitr.ernet.in.
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