Phosphate removal from aqueous solution by using modified sludge from water treatment plant - Dang Thi Thanh Loc

Vietnam Journal of Science and Technology 56 (2C) (2018) 43-49 PHOSPHATE REMOVAL FROM AQUEOUS SOLUTION BY USING MODIFIED SLUDGE FROM WATER TREATMENT PLANT Dang-Thi Thanh-Loc1, *, Le-Van Tuan1, Hidenori Harada2 1Department of Environmental Science, Hue University of Sciences, Hue University, 77 Nguyen Hue street, Hue city, Viet Nam 2Graduate School of Global Environmental Studies, Kyoto University, Yoshida-honmachi, Sakyo, Kyoto 606-8501, Japan *Email: dangthithanhloc@hueuni.edu.vn Received: 10 May 2018; Accepted for publication: 20 August 2018 ABSTRACT Heat and humic acid modified sludge (MS) from drinking water treatment plant (DWTP) is used as an adsorbent for removal of phosphate (P) from aqueous solution. The MS was characterized by XRD and SEM observation. The effects of pH, adsorbent dosage, initial P concentration, and exposure time on the P removal were studied. Under identical treatment conditions (MS dosage = 10 g/L, initial P concentration = 10 mg/L, pH 7, 120 rpm, and room temperature), a removal efficiency of 91 % was obtained within 240 min. The Freundlich and Langmuir adsorption models were used for the mathematical description of adsorption equilibrium and it was found that P removal was best described by Langmuir model. The maximum adsorption capacity of the adsorbent based on sludge of Quang Te DWTP was 0.90 mg/g. The adsorption process followed pseudo-second-order kinetics (R2 ≥ 0.98). These findings suggest that MS has potential applications as a low-cost adsorbent for P treatment. Keywords: adsorption, modified sludge, phosphate removal, drinking water treatment sludge. 1. INTRODUCTION Phosphorus (P) plays a significant role in the growth of plants but is now widely recognized as a serious environmental issue because of the water eutrophication through wastewater discharge. The P concentration in water medium above 0.02 mg/L can lead to eutrophication, reducing water quality and threatening aquatic ecosystems [1]. To avoid these problems, the excessive amounts of P in wastewater need to be removed or recovered to less than the standard when the wastewater is discharged into the environment. So far, several methods have been developed to remove P in water, i.e. membrane technology, biological treatment, and chemical treatment. The methods based on chemical immobilization of P with di- or trivalent metal salts is one of the common methods that are widely employed for P removal from wastewater as it is efficient and simple to operate [2]. However, the use of conventional coagulants or adsorbents Dang-Thi Thanh-Loc et al. 44 may not be so admirable because of the chemical costs. The soluble P removal methods that reuse industrial by-products high in aluminium and/or ferric content are highly desirable. In recent years, using sludge from drinking water treatment plant (DWTP) to the removal of P from wastewater has received considerable attention [3]. Sludge from DWTP is a sort of by- products in the precipitation process using coagulant. The sludge has a high content of amorphous aluminum and ferric (hydr)oxides, which is able to bond phosphorus via ligand exchange [3]. However, P sorption efficiency of the sludge is greatly dependent on physicochemical properties of the sludge, which is influenced by the quality of water source, type of coagulant, and system of a treatment plant. Annually, Thua Thien Hue Water Supply Joint Stock Company (HueWACO) consumes about 96,442 kg of coagulant and generates about 321,474 kg of dry waste sludge in the form of water treatment residuals [4]. The sludge is dried and directly disposal to Thuy Phuong landfill. Although management of the sludge has received much attention, it is not clear from the research literature whether the modified sludge of HueWACO would be able to reuse as an adsorbent for P removal in water. The purpose of this study was to modify the sludge from DWTP as a low-cost adsorbent for adsorption of phosphate in wastewater. The absorption performance of MS was examined, and the removal mechanisms were elucidated. The finding of this study is helpful for further development of a low-cost adsorbent based on sludge from DWTP to remove P in wastewater and offers a solution reduce water treatment sludge production and disposal fees also. 2. MATERIALS AND METHODS 2.1. Preparation of adsorbent Adsorbents were developed by using sludge taken from Quang Te II DWTP in Hue city, Viet Nam. The sludge was collected from a sludge drying bed, a part of the processes in HueWACO to dry the sludge generated from the sedimentation tank, before the disposal to Thuy Phuong landfill. Poly-aluminum chloride is used as a coagulant in Quang Te II DWTP. The sludge was dried at 105 °C for 24 h and then crushed into powder. Humic acid after calcining produces a large amount of micro-pore structures that can improve adsorption capacity of adsorbent [5]. Therefore, MS was prepared by mixing the sludge with humic acid (Wako, Japan) (w/w = 10:1). After that, distilled water was added into the powder (v/v = 60 %) to become stick material. The mixed material was made balls with sizes in the range of 2.0 to 2.5 mm. The sludge balls were then heated in an oven (Muffle Furnace FP 31, Japan) at 200 °C for 1 hr followed by 600 °C for 2 hr. The final adsorbent was used for adsorption experiments. 2.2. Characterization of adsorbent The structures of MS was examined by X-ray diffraction (XRD), recorded on 8D Advance Bucker, Germany with a CuKα tube (λ = 0.15406 nm, 40 kV, 40 mA). Morphology of MS was observed by scanning electron microscope (SEM. JSM-5300 LV). 2.3. Adsorption experiments Phosphate solutions were prepared artificially by dissolving the pre-weight potassium dihydrogen phosphate (KH2PO4. Merck, Germany) into distilled water. For each batch Phosphate removal from aqueous solution by using modified sludge from water treatment plant 45 adsorption process, 50 mL P solution of known concentration was agitated with MS adsorbent in a shaking incubator (THZ-312) at 120 rpm and room temperature. Investigated influence factors included contact time (0 to 1440 min), initial P concentration (2 to 15 mg/L), adsorbent dosage (5 to 50 g/L), and initial pH of solution (from 2.0 to 11.0). HNO3 and NaOH were utilized to adjust the desired pH values of solutions. After the duration, the suspension was centrifuged (10 min at 10,000 rpm) in a centrifuge (H-15FR, Kokusan Co. Ltd., Japan). The P concentration in the supernatants was measured by Standard Methods for the Examination of Water and Wastewater. All adsorption experiments were conducted in triplicate. The adsorption capacity (qt) was calculated using the Eq. 1 ( ) (mg/g)o tt C C Vq x m − = (1) where C0 and Ct are P concentration (mg/L) at an initial time and any time; V is the volume of solution (L) and m is weight of the adsorbent (g). Kinetic experiment was done in three initial P concentrations (2, 5, and 10 mg PO43-/L) at room temperature, pH 7 and adsorbent dosage 10 g/L. The experimental was continued for 1440 minutes and samples were drawn from the mixture at predetermined time intervals for analysis. Kinetic of P sorption was modeled by the pseudo-first-order, pseudo-second-order equations presented below as Eq. (2)-(3), respectively: 1ln( ) lne t eq q q k t− = − (2) 2 2 1 1 t e e t t q k q q = + (3) where qe and qt are the adsorption capacities (mg/g) at equilibrium and any time, respectively; k1 is the pseudo-first-order rate constant of adsorption (min-1); k2 the pseudo-second-order rate constant of adsorption (gmg-1min-1) [6]. The isotherm of P adsorption was analyzed using the Freundlich and Langmuir [7]. The Freundlich and Langmuir equations are linearized as Eq. (4)-(5), respectively: 1ln lne F eq K C n = + (4) 1 1 1 e m e mq bq C q = + (5) where Ce is equilibrium concentration of P (mg/L) in solution; KF and n are indicators of sorption capacity and sorption intensity, respectively; qm is the monolayer adsorption capacity (mg/g) and b is a constant related to the free energy of adsorption (mL/g). 3. RESULTS AND DISCUSSION 3.1. Characterization of modified sludge The SEM image of MS shown in Fig. 1 reveals the surface texture and porosity of the adsorbent. The MS consists of very fine particles less than 500 nm in sizes (Fig. 1). The availability of the pores and internal surface is requisite for an effective adsorbent. Figure 2 shows XRD of MS. It shows that the present MS possess amorphous structure. The small amount of crystalline phase could be identified as quartz - SiO2, (pattern: 01-085- Dang-Thi Thanh-Loc et al. 46 0796) and halloysite - Al2O3.2SiO2.4H2O (pattern: 00-002-0043). The MS with large amount of aluminum oxides should be expected the high capacity of adsorption for phosphate. 3.2. Effect of pH The effect of solution pH was examined between pH 2 and 11 and the results are shown in Fig. 3. Increasing the pH from 2 to 3, an increase in adsorption capacity was observed. The largest adsorption capacity (0.72 mg/g) reached at a pH of 3. Then the uptake of P decreased from 0.66 to 0.48 mg/g when the pH increased from 4 to 11. Figure 3. Adsorption performance of MS at various solution pH values (initial P concentration = 10 mg/L, MS dose = 10 g/L, 240 min, 25 oC). The error bars represent the standard deviation from the mean. The increase of pH values from 3 to 11 lead to reduce the P adsorption capacity of MS. This finding agrees with Elkhatib et al. [8]. In the solution, phosphate exists under various forms such as H2PO4-, H3PO4, HPO42-, and PO43-. H2PO4- predominantly exists at low pH levels of 3-6, which form may easy to metal oxides containing in the MS. Meanwhile, the increase in pH values may change the MS surface to more negatively charged surfaces [8]. As solution pH increases, hydroxyl ions (OH-) mainly exists in the system, accelerating the competition for Faculty o f C hemistry , H US, VNU , D 8 AD VANC E-Bruker - SKA 00 -0 02 -0 04 3 ( D) - Ha llo ys ite - Al2O 3 ·2 S iO 2·4 H2 O - Y : 31 .21 % - d x b y: 1 . - W L: 1 .5 4 06 - 01 -0 85 -0 79 6 ( A) - Q ua rtz - SiO2 - Y : 10 0.0 0 % - d x b y : 1. - W L : 1 .54 06 - He xag o na l - a 4 .91 1 80 - b 4 .91 18 0 - c 5.4 03 40 - a lph a 9 0 .0 0 0 - b eta 90 .00 0 - g a mm a 12 0 .0 0 0 - P rim itiv e - P 32 2 1 (1 5 4) - 3 - 1 12 .89 6 - I/I c P DF 3 .1 - 00 -0 05 -0 14 3 ( D) - K ao lin ite - A l2S i2 O 5( OH) 4/A l2 O 3·2 SiO 2 ·2H2 O - Y : 1 9 .5 1 % - d x by : 1. - W L : 1.5 40 6 - Tr icli nic - a 5 .1 4 00 0 - b 8.9 30 0 0 - c 7 .37 00 0 - alp ha 91 .1 30 - be ta 1 04 .80 0 - ga m m a 9 0.0 00 - 32 6 .9 9 3 - F3 0= 4 (0 .0 6 0 F ile : Tu Hu e S K A.ra w - Typ e: 2Th /Th l ocke d - Sta rt: 5.0 00 ° - En d : 7 0.0 10 ° - S tep : 0 .03 0 ° - S te p tim e: 0.3 s - T e mp .: 2 5 °C ( Roo m) - Tim e S ta rte d: 1 3 s - 2 -Th eta : 5.0 00 ° - T he ta: 2 .50 0 ° - Ch i: 0.0 0 ° - P h i: 0.0 0 ° - X : 0 .0 m Li n (C ps ) 0 10 0 20 0 30 0 40 0 50 0 60 0 70 0 80 0 90 0 10 00 2-Theta - Sc ale 5 1 0 20 30 4 0 5 0 6 0 7 0 d= 13 . 40 2 d= 11 . 55 7 d= 10 . 14 4 d= 5. 04 7 d= 4. 49 0 d= 4. 21 2 d= 3. 54 0 d= 3. 34 7 d= 3. 18 5 d= 2. 92 6 d= 2. 07 0 d= 1. 81 9 d= 1. 78 8 d= 1. 65 9 d= 1. 54 3 Figure 1. Representative SEM image of MS. Figure 2. XRD of modified sludge. Phosphate removal from aqueous solution by using modified sludge from water treatment plant 47 binding sites among phosphate ions and OH- on the MS surface, resulting in a reduction in the P adsorption capacity of MS [8]. Taken together, the findings suggest that the P adsorption was favored at lower pH values. Nevertheless, acidification is not interesting from a practical viewpoint. Besides, the pH value of domestic wastewater is typically in the range of 6.8 to 7.8 [9]. Hence, solution pH of 7 was used for subsequent experiments. 3.3. Effect of MS dosage Figure 4 presents the influence of MS dosage (5 to 50 g/L) on the P adsorption performance. The P adsorption capacity increased with the increase of adsorbent dosage from 5 to 10 g/L and reached the larges capacity (0.58 mg/g) at 10 g/L of adsorbent. This finding suggests that the increase of adsorbent dosage leads to the greater surface area. Whereas, the increase in the adsorbent mass from 20 to 50 g/L leaded to the adsorption capacity gradually decreased from 0.45 to 0.20 mg/g, respectively. This result is expected due to the saturation level attained during an adsorption process. Hence, 10 g/L of MS dosage was selected at an optimum condition for further experiments. 3.4. Effect of exposure time and initial phosphate concentration Figure 5 shows the effect of exposure time on the P adsorption capacity of MS at different initial P concentrations (2, 5, and 10 mg/L). The higher initial P concentration led to enhanced adsorption capacity (0.18 to 0.90 mg/g) but reduced the P removal efficiency (95 % - 91 %). For all the concentrations, the P adsorption capacity rapidly increased at the first 240 min and was then relative stable after 900 min when its equilibrium was reached. 3.5. Isotherm adsorption Figures 6 show isotherm modeling of P adsorption by linear plots of Langmuir (Fig. 6a) and Freundlich (Fig. 6b). The Langmuir model described the isotherm of P adsorption with high correlation coefficient (R2=0.97) and better than Freundlich model, indicating that the adsorption process occurred on the surface of MS should be monolayer adsorption. Kinetic experiments presented that maximum time required to reach equilibrium was 900 minutes and that the maximum adsorption capacity (qm) gained 3.24 mg/g. Wang et al. [5] reported that 1440 min was required for 1.21 mg/g of qm with a filter substrate (made from sludge:kaolin:humic acid in Figure 5. Effect of initial phosphate concentration on P adsorption (absorbent dose of 10 g/L, pH 7, and 25oC). The error bars represent the standard deviation from the mean. Figure 4. Effect of adsorbent dosage on P adsorption (initial P concentration of 10 mg/L, pH 7, 240 min, and 25oC). The error bars represent the standard deviation from the mean. Dang-Thi Thanh-Loc et al. 48 a weight ratio of 10:7:2) at 25 °C and pH 7. Despite shorter contact time (900 min), the qm obtained in the present study (3.24 mg/g) was greater than that obtained by Wang et al. [5]. These findings affirm the superior performance of MS. Figure 6. Adsorption isotherm parameters for modified sludge. 3.6. Adsorption kinetic Kinetic parameters of P adsorption are shown in Table 1. The pseudo second-order model described the adsorption with high correlation coefficient (R2 ≥ 0.98) and better than the pseudo- first order equation. The predicted qe values were close to the experimental qe results, suggesting that the P adsorption kinetic by MS could be well described by a pseudo second-order model and that the P adsorption was a chemisorption process. Table 1. Kinetic parameters for the adsorption of phosphate onto modified sludge. The pseudo-first order equation (Lagergren equation) C0 (mgP-PO43-/L) Equation R2 k1 ×103 (min-1) qe (mg/g) Experiment Predicted 2 Log(qe – qt) = -0.8512 - 0.0022t 0.96 5.07 0.18 0.14 5 Log(qe – qt) = -0.4563 - 0.0013t 0.98 2.99 0.43 0.35 10 Log(qe – qt) = -0.1603 - 0.0011t 0.94 2.53 0.90 0.69 The pseudo-second order equation C0 (mgP-PO43-/L) Equation R2 k2 × 103 (mgg-1min-1) qe (mg/g) Experiment Predicted 2 0.99 77.36 0.18 0.18 5 1.00 15.25 0.43 0.45 10 0.98 8.74 0.90 0.89 Phosphate removal from aqueous solution by using modified sludge from water treatment plant 49 4. CONCLUSIONS Sludge modified by humic acid (in a weight ratio of 10:1) and heated at 600 °C showed a great chemically and physically activated adsorbent. SEM image indicated that the porous structural aspects of the MS suitable for removing P from aqueous solution. 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