Removal of phosphate from aqueous solution by sewage sludge-based activated carbon loaded with pyrolusite

Activated carbons were prepared from sewage sludge by chemical activation with pyrolusite (PAC) to develop an ef ﬁ cient adsorbent for phosphate removal from aqueous solution. One percent (wt.) pyrolusite addition was proved to have an important effect on pore formation of the produced carbon. PAC showed 17.06% larger Brunauer – Emmett – Teller (BET) surface area than the sewage sludge-based activated carbon without modi ﬁ cation (SAC). The adsorption results showed that the phosphate removal by PAC was 13% higher than SAC ’ s. The adsorption experiments also showed that PAC had very good performance with high phosphate removal rate (ca. 90%) in a wide pH range (pH ¼ 4 – 8), and could be stable after 30 min reaction. Adsorption isotherm and kinetics studies demonstrated that phosphate adsorption onto the modi ﬁ ed adsorbent was well ﬁ tted by the Langmuir isotherm and could be described by the pseudo-second-order kinetic model. The modi ﬁ ed sewage sludge-based activated carbons were effective and alternative adsorbents for the removal of phosphorus from aqueous solution due to their considerable adsorptive capacities and the low-cost renewable sources. polymeric exchanger, iron


INTRODUCTION
Phosphorus is crucial in water contamination (Xiong et al. ). Excess discharge of phosphorus into water often causes eutrophication, and results in deterioration of water quality. Phosphorus is released into aquatic environment by various human activities such as mining, industrial and agricultural utilization (Saha et al. ). Phosphorus pollution is ever increasing, and therefore there is an urgent demand to take highly effective, reliable, and economical methods for efficient phosphorus removal.
Phosphate removal is significant for the prevention of eutrophication in natural waters. Various treatment technologies are available for getting rid of phosphorus from wastewater, including chemical precipitation, biological phosphorus removal, and adsorption (Guaya et al. ; Huang et al. ). However, in view of the trace level phosphorus concentration, chemical precipitation often suffers from high cost and low efficiency on residue sludge handling. Moreover, operational difficulties hinder the application of biological phosphate removing methods due to the low phosphorus concentration in water (Long et al. ). In contrast, the adsorption method was found to be superior to other techniques on phosphate removal from aqueous solution in terms of its flexibility and simplicity design, particularly with low phosphate concentration in wastewater (Han et al. ). Thus adsorption has attracted intensive interests on developing dephosphatation technology.
It was well known that adsorption capacity and efficiency were greatly dependent on the adsorbents' materials. Many types of adsorbents for phosphate removal have been investigated, which include aluminum and aluminum hydroxide, polymeric ligand exchanger, iron oxides, dolomite, red mud, zeolite, furnace slag, mesoporous silicates and so on (Pengthamkeerati et al. ; Yue et al. ; Xu et al. ). However, the direct addition of adsorbents in water treatment systems may cause large losses of adsorbents and reduce their reusability because of the adsorbents' tiny powder size, which are easily run out of the system and restrict the application of those absorbents (Shi et al. ). To further improve recyclability and adsorption efficiency, it is necessary to develop some novel phosphate-specific adsorbents.
Activated carbons, with high surface area, microporous structure and great adsorptive capacity, are usually used as adsorbents for the removal of various pollutants from industrial wastewaters. However, the high cost of preparation and utilization of activated carbons is a major drawback of this technology. Currently, there is considerable research on the development of low cost adsorbents from the beneficial use of waste solid materials (Huang et al. ). Sewage sludge, the final production of wastewater treatment plants, is rich in carbonaceous organic matter, and contains a certain amount of inorganic components, including a variety of metal salts and oxides. After chemical activation and pyrolysis treatment, sewage sludge can be transformed into an excellent adsorptive material. The preparation of sewage sludgebased activated carbons has superiority in sludge volume reduction and high adsorbents production with low cost.
Modified materials have been reported to play important roles in the development of pore structures and surface chemistry on the activated carbons (Goswami & Phukan ). Meanwhile, little research is available on activated carbons modified by natural minerals and their adsorption properties on the removal of phosphorus. Pyrolusite is an ordinary and economical natural material widely distributed in Sichuan province, China. In this work, pyrolusite was chosen as the modifier to prepare municipal sewage sludge-based adsorbent. The improvements of their adsorptive abilities on phosphorus removal in aqueous solution were studied. Its characteristics was also evaluated, especially the pore structure and surface chemistry. Operation parameters were investigated for the effective removal of phosphorus from aqueous solution. The equilibrium and kinetic data during the adsorption were analyzed to understand the phosphorus removal process.
It is expected that this work could be useful for an effective sewage sludge management and innovative utilization of sludge residues as low-cost precursors for the adsorbent production, also this work could provide feasibility for phosphorus removal from aqueous environments.

Preparation of absorbents
Municipal sewage sludge used as the raw material in this work was taken from a local wastewater treatment plant in Chengdu, China, where wastewater underwent a traditional active sludge process. The proximate analyses of the sludge showed that the moisture content of the raw material was 83.21%, the volatile content, ash content and fixed carbon in dewatered sludge was 66.24, 31.82 and 1.94%, respectively. The ultimate analyses of the dewatered sludge were briefly summarized as 32.61% carbon, 5.10% hydrogen, 5.12% nitrogen and 57.17% oxygen, while the oxygen content was calculated by difference. The ultimate analysis of the raw sludge was taken by Vario MICRO select Elemental Analyzer, Elementar, Germany, in accordance with the standard of JY/T 017-1996 elemental analyzer method. It determines not only the weight percent of carbon content, but also hydrogen and nitrogen contained in organic, inorganic and polymeric materials.
In order to improve the properties of activated carbons, pyrolusite was ground into fine powder and fully blended with the pretreated raw sludge in a certain weight ratio. The pyrolusite used in this study mainly consists of (wt. %): MnO 2 41.3%, SiO 2 30.2%, Fe 2 O 3 13.8%, CaO 3.6%, Al 2 O 3 3.4%.
Sewage sludge was first dried in an electric heating oven at 105 C for 48 h. Dried sludge was then ground into powder by a muller and passed through a 160-mesh sieve as pretreatment.
The activating agents ZnCl 2 (6 mol/L) and H 2 SO 4 (35 wt%) were mixed at a volume ratio of 4:1. The mixture of sludge and pyrolusite power was then impregnated into the activating agent at a ratio of 1:4 (g/mL) and kept for 24 h under room temperature. The impregnated samples were then put into a ceramic crucible and heated in a tubular furnace (KXG-2-13A, Shanghai, China) with a constant heating rate of 10 C/min to 550 C in nitrogen atmosphere (99.99%, 200 mL/min).
After 60 min pyrolysis at 550 C with purified nitrogen flow, the carbonization samples were cooled down to room temperature in the furnace. The samples were taken out and washed several times sequentially with 3M HCl and hot distilled water (70-80 C) to remove any residual chemicals.
The carbon products were then dried in an electric heating oven at 105 C for 24 h, cooled in the desiccator, crushed and sieved through a 200-mesh filter to obtain the final sewage sludge-based adsorbents.
Activated carbons produced from sewage sludge with pyrolusite addition were designated as PAC, while the unmodified sewage sludge-based activated carbon was named as SAC for comparison.

Characterization of activated carbons
The Brunauer-Emmett-Teller (BET) surface area and pore structure of the produced carbons (outgassed under vacuum at 200 C before analysis) were obtained by N 2 adsorption isotherm at 77 K using a surface area analyzer (SSA-4200, China). From the isotherm data, BET surface area, microand mesopore volume and mean pore radius were calculated.
Scanning electron microscopy (SEM) images were recorded to visualize sample's morphology. Pore structure and structural changes after chemical activation could also be observed. In the present work, the surface morphology of the produced carbons (SAC, PAC) were obtained using a JSM-5900LV scanning electron microscope (SEM, Japan).
A Fourier transform infrared spectrometer (FTIR 6700 NEXUS Nicolet, USA) was used to analyze the main functional groups that may be involved in phosphate adsorption on SAC and PAC. The spectra were recorded from 4,000 to 400 cm -1 using a KBr window.
The concentration of total phosphorus (TP) in the solution was analyzed by a UV1800 spectrometer. Iodine and methylene (MB) adsorptive capacity were tested according to the standard methods for the determination of iodine and methylene adsorption value of the produced carbons.

Batch adsorption experiments
Stock phosphorus solution was prepared by dissolving KH 2 PO 4 in deionized water. All the chemicals used were analytical grade.
The experiments were conducted in a series of 150 mL Erlenmeyer flasks with magnetic stirring (150 rpm) and maintained at 25 C by a thermostatic bath to reach equilibrium. The solution pH was adjusted by HNO 3 or NaOH (1 mol/L). The samples were filtered out and the TP concentration of the supernatant was analyzed through a UVspectrometer.
Batch experiments were carried out by adding a certain amount of the produced carbon into 50 mL synthetic solution with an initial TP concentration of 5 mg/L, to verify the adsorption efficiency at different reaction times.
The amount of adsorption at equilibrium, q e (mg/g), was calculated by Equation (1): where C 0 and Ce (mg/L) are the liquid-phase concentrations of metal ion at initial and equilibrium, respectively. V (L) is the volume of the solution, and m (g) is the mass of the dry adsorbent used.
The accuracy of analytical techniques has been checked by relative standard deviation (RSD), and the maximum standard deviation observed was <5%. All the adsorption experiments were conducted twice, and the results were the mean value of the tests. The surface morphology of SAC and PAC were observed using SEM and is shown in Figure 1. It can be seen from the micrographs that a considerable number of pores, cavities and crevices are present in both adsorbents' surface. Those spaces result from the reagent's evaporation during carbonization, which were previously occupied by the activating agents. As presented in Figure 1(b), the uneven and rough surface on PAC showed dramatically luxuriant pores comparing with SAC, which is probably due to the pyrolusite modification.

Characterization of adsorbents
It should be much more effective on forming the pore structure.
The specific surface area and pore structure of the produced activated carbons were measured and are listed in  As shown in Figure 2, the bands observed at 2,923 and 2,852 cm -1 represented symmetric and asymmetric C-H stretching vibrations of aliphatic CH, CH 2 , and CH 3 groups from the raw materials (Park & Jung ). It could be seen that after pyrolysis, the spectrum of raw material and PAC were obviously changed. There is a trough at  The observation of these oxygenic functional groups' shifting wavelengths can be inferred that the modification has indeed occurred with a certain influence on the raw materials and indicates that modified carbons could improve the removal efficiency toward phosphorus.

Adsorption
Effect of dosage and contact time  In comparison, it was more efficient when absorbent dosage was 2 g/L with more than 91% removal rate after 240 min. It can be determined that the optimum dosage for PAC was 2 g/L (Xiong et al. ).
As seen from Figure 3, the TP removal rate increased with the increasing contact time. The adsorption increased rapidly in the first 30 min, probably due to the readily accessible sites on the carbon surface. When the surface adsorption sites became exhausted, the uptake rate was   Comparing the current work with reported data above showed that maximum TP removal was achieved using PAC, while other adsorbents take more time and higher adsorbent dosages to obtain an efficient TP removal rate.

Effect of pH
Generally, pH values display a significant impact on phos-  Figure 4. Figure 4 shows that the removal efficiency of phosphate increased sharply at the initial stage when the pH value was 2-3. The observed trend is related to phosphate proton dissociation equilibrium. The species of phosphate is different under different pHs, as shown by the following reactions: where pK 1 ¼ 2.13 and pK 2 ¼ 7.20. At pH 2.13, the predominant species of phosphate is H 3 PO 4 which is weakly attached to the sites of the adsorbent. When pH is in the range of 2.13-7.20, PAC did perform well with the main phosphate species of monovalent H 2 PO À 4 . The form of metal active sites is most likely through the protonation represents positive charged surface group. ÀOH þ 2 is easier to be displaced at low pH from the metal binding sites than hydroxyl groups, which can facilitate the ligand exchange process (Qian et al. ). So the electrostatic interactions and ion exchange should be the main mechanisms in this pH range. It also indicated that phosphate sorption onto PAC was primarily the result of an ionexchange between phosphate and hydroxide groups on the adsorbent surface. An inner sphere complex is formed when phosphate is directly chemically bonded to the active sites which are deposited on the absorbent surface. Figure 4 depicts that TP removal was favored in weakly acidic to neutral conditions (pH ¼ 6.0-7.0), and removal efficiency was observed to be 83% and 86% under 6.0 and 7.0 pH conditions for phosphate ion adsorption respectively. Thus the initial solution pH (around 6.4) was within the optimum pH range and no pH adjustment during the PAC adsorption process was necessary. It was also proved that PAC has good pH adaptability toward phosphorus removal.

Adsorption kinetic
Adsorption kinetic modeling is very useful for better understanding phosphate adsorption mechanisms onto modified activated carbon, such as the importance of the chemical reactions and the intra-particle diffusion processes. Kinetic studies are generally used to determine the uptake rate of the adsorbates, and to control the residual time of the whole adsorption process. In the present study, three kinetic models, viz. the pseudo-first-order, pseudo-second-order and intra-particle diffusion, were  The pseudo-first-order kinetic model is the most widely used rate equation for the sorption of a solute from a liquid solution and is generally expressed as Equation (2): log (q e À q t ) ¼ log q e À k 1 t 2:303 ( 2) where q e and q t (mg/g) are the amounts of total phosphate adsorbed at equilibrium and time t (h), respectively, and k 1 (1/h) is the rate constant of pseudo-first-order adsorption.
The values of k 1 and q e can be obtained from the linear plot of log (q e À q t ) vs. t plots.
The pseudo-second-order kinetic model based on equilibrium adsorption can be represented as Equation (3): where k 2 (g/mg·h) is the rate constant of the second-order adsorption. The values of q e and k 2 are calculated from the linear plot of t/qt vs. t.
The intra-particle diffusion model is generally expressed as Equation (4): where k p (mg/(g·min 1/2 )) is the adsorption rate constant for the intra-particle model (Gunay et al. ; Subbaiah & Kim ).
The calculated parameters for all the kinetic models are listed in Table 2. In general, the adsorption process can be described as a series of steps: mass transfer from fluid phase to the particle surface across the boundary layer, diffusion within the porous particle, and adsorption itself onto the surface. According to Table 2, the correlation coefficients were relatively low lying between 0.76 and 0.96 for the pseudo-first-order model and intra-particle model. These results state a bad fit between the model and the experimental data. As a consequence, phosphates adsorptions by SAC and PAC are not ideal pseudo-first-order or intra-particle diffusion reactions. The highest correlation coefficients for phosphate adsorption by SAC and PAC were obtained through the pseudo-second-order kinetic model. The R 2 values were greater than 0.99 for TP adsorption, while PAC (R 2 ¼ 0.9995) had a higher R 2 value compared with SAC (R 2 ¼ 0.9955). In addition, the difference between the experimental and theoretical adsorbed masses at equilibrium is very small (less than 1%). The applicability of the pseudo-second order kinetic model demonstrated the adsorption process well. It revealed that the limiting stage of the process was the adsorptive mechanism rather than the mass transfer from the solution to the adsorbent's surface. It also reflected that the adsorption behavior may involve the valence forces through sharing electrons between the phosphate and adsorbents.

Adsorption isotherms
The adsorption isotherm indicates how the adsorbate molecules are distributed between the liquid phase and the solid phase at given pH values and temperatures, which was obtained by varying the initial concentrations of P (1-50 mg/L) at room temperature. The capacity of an adsorbent can be described by its equilibrium sorption isotherm, which is characterized by certain constants that express the surface properties and affinity of the adsorbent. To analyze the validity of the adsorption data, Langmuir and Freundlich adsorption isotherm models were discussed.
The Langmuir isotherm equation, which is the most commonly used for monolayer adsorption with a finite number of identical sites on the surface, is represented as Equation (5): where C e (mg/L) is the equilibrium concentration of phosphate, q e (mg/g) is the amount of TP adsorbed onto the adsorbent at specified equilibrium, q m (mg/g) is the theoretical maximum adsorption capacity of TP onto the adsorbent, and K L (L/mg) is the Langmuir adsorption equilibrium constant related to the affinity of the binding sites.
The Freundlich isotherm equation, the most important multilayer adsorption isotherm for heterogeneous surfaces, is described by Equation (6): where K F ((mg/g)(L/mg) 1/n ) and n are the Freundlich adsorption isotherm constants which are indicators of adsorption capacity, and adsorption intensity, respectively (Oguz ).
Equilibrium adsorption isotherms could provide useful data for understanding the mechanism of a given adsorption process. In the present study, adsorption isotherm experiments were conducted at room temperature with initial