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

Phosphorus is crucial in water contamination (Xiong et al. 2017). 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. 2016). 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. 2015; Huang et al. 2015). 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. 2011). 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. 2015). 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. 2008; Yue et al. 2010; Xu et al. 2016). 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. 2011). 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. 2017). 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 sludge-based 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 2017). 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.

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. %): MnO₂ 41.3%, SiO₂ 30.2%, Fe₂O₃ 13.8%, CaO 3.6%, Al₂O₃ 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 ZnCl₂ (6 mol/L) and H₂SO₄ (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.

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 N₂ adsorption isotherm at 77 K using a surface area analyzer (SSA-4200, China). From the isotherm data, BET surface area, micro- and 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.

Stock phosphorus solution was prepared by dissolving KH₂PO₄ 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 HNO₃ or NaOH (1 mol/L). The samples were filtered out and the TP concentration of the supernatant was analyzed through a UV-spectrometer.

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 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.

In this study, pyrolusite was chosen to improve the activated carbons' adsorptive capacity. According to our previous experiments, the most obvious TP adsorption occurred with 1% (wt.) pyrolusite loading on the produced activated carbon. Pyrolusite contains a variety of metal compounds which are favorable for pore formation during the process of carbonization and activation, thus enhancing the adsorption performance.

The iodine and methylene blue (MB) adsorption capacity were investigated for the produced adsorbent. The methylene blue adsorption experiments were carried out with 2 g/L adsorbents dosage under the initial MB concentration of 300 mg/L. After 4 h adsorption at room temperature, the MB adsorption values of SAC and PAC were tested as 132.6 and 144.3 mg/g, respectively.

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. 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 Table 1. A significant increment of the surface physical properties was observed for PAC. The BET surface area, total pore volume, mesopore and micropore volume of PAC were 17.06, 51.57, 57.04 and 43.34%, respectively, higher than those of SAC's. The obvious increase of specific surface area on the PAC was ascribed to the relatively high production of micro- and mesopores compared to SAC. It is expected that the modified carbon could obtain adequate surface areas by mineral loading. Pyrolusite modification could increase adsorbents' porosity by enhancing oxygen functional groups interaction, dehydration and dehydrogenation reactions on the activated carbon. Also, the large pore volume could help mass transfer to easily ingress and egress between absorbents and the external environment in the solution.

Infrared spectroscopy was used to record the chemical changes occurring on the surface of the adsorbent. The FTIR spectra of the dried raw sewage sludge and PAC are presented in Figure 2. After the thermal treatment, the IR spectrum of PAC was obviously simplified compared with the raw sewage sludge's.

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, CH₂, and CH₃ groups from the raw materials (Park & Jung 2002). It could be seen that after pyrolysis, the spectrum of raw material and PAC were obviously changed. There is a trough at 2,852 cm^–1 for raw sludge and it becomes flattened after pyrolysis for PAC. This proved that alkane groups were involved during the carbonization and disappeared after pyrolysis (Demiral & Gunduzoglu 2010). Furthermore, the slight wavelength shifts from 1,540 to 1,419, 3,430 to 3,415 and 1,646 to 1,610 cm^–1 can also be observed from Figure 2. These bands were ascribed to the organic parts of the raw sludge, which were shifted for the produced carbons during activation and carbonization (Xie et al. 2013).

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.

Figure 3 shows the impact of contact time on phosphorus adsorption by PAC at the initial solution pH. To determine the adsorption capacity of an adsorbent, the adsorbent dose is a major factor. The effect of adsorbent dosage on TP removal was studied at two different doses (2 and 4 g/L) at an initial TP concentration of 5 mg/L.

Figure 3 indicates that as the dose increases TP removal rate was also increased from 83% to nearly 100%. Greater availability of active sites and increment of adsorbent's surface area with an increase in adsorbent amount was reported (Deng & Shi 2015). Also, it was noticed that TP removal did not show any obvious increase after 30 min at 4 g/L adsorbent's dosage. This may be due to the fact that all phosphate ions were completely absorbed by PAC at 4 g/L dosage and the removal rate was close to 100%, the absorbents cannot reach the adsorption saturation under this dosage condition and it was not sufficiently economical. 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. 2008).

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 slowed down and controlled by the transport rate from the outside to the inside of the adsorbent, finally reaching equilibrium at 4 h.

Yang et al. (2013) used a tablet porous material (TPM) developed as adsorbent for phosphate removal and the results showed that the experimental maximum adsorption removal rate was 21.9% when the initial phosphate concentration was 80 mg/L (TPM dosage: 4 g/L). Karimaian et al. (2013) used natural and engineered waste pumice for phosphate removal and while the initial phosphate concentration was 5 mg/L and the pumice dosage was 6 g/L, the removal rate reached 63.6% after 140 min. Xiong et al. (2011) used powdered freshwater mussel shells for phosphate removal and after 720 min with 20 g/L dosage and an initial phosphate concentration of 20 mg/L, the removal rate was 96.01%.

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.

Figure 4 depicts that TP removal was favored in weakly acidic to neutral conditions (pH = 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 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 selected for the description of the adsorption experimental results.

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 R² values were greater than 0.99 for TP adsorption, while PAC (R² = 0.9995) had a higher R² value compared with SAC (R² = 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.

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.

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 phosphorus concentrations of 1–50 mg/L, and the contact time of the experiments lasted 240 min. The calculated isotherm constants are presented in Table 3. In terms of the correlation coefficient R², the experimental data could be well fitted to the Langmuir isotherm rather than the Freundlich isotherm model. Thus, it showed that the adsorption process occurred on the carbons' surface and was monolayer adsorption. The Langmuir model assumes that adsorbent is structurally homogenous and the adsorption process ends up in monolayer coverage with uniform adsorption energies. The maximum TP adsorption capacity on PAC calculated by the Langmuir model was 10.78 mg/g.

A comparative test was carried out to evaluate the performance among sewage sludge carbon (SAC), pyrolusite modified sludge carbon (PAC), commercial carbon prepared by coconut shell (AC), lithium-silicon powder (LS-Powder) and walnut shell-based carbon (WS-AC) for TP removal. It can be seen in Figure 5 that PAC had the highest removal rate compared with other absorbents under the same experimental conditions (initial pH 5.0, contact time of 4 h, 25°C, and initial TP concentration of 5 mg/L). While the dosing of PAC (2 g/L) was only half the amount of other adsorbents (4 g/L) and much less than AC (6 mg/L), it also showed outstanding performances on TP removal. With 91.1% of TP removal rate by PAC, it is 4% higher than SAC, and 81.9, 84.8 and 87.14% higher than LS-powder, WS-AC and AC, respectively. According to previous research, lithium-silicon powder, a common nature mineral, had a significant influence on the activated carbon modification. Lithium-silicon powder had good performance on some specific pollutants treatment. Similarly, walnut shell-based carbon had a massive removal effect and is widely used in wastewater treatment. The commercial activated carbon prepared by coconut shells had massive specific surface area and mesopore volume based on the BET surface area examination, which had a considerable effect toward the adsorption of dye molecules and flue gas. However, as seen in Figure 5, LS-AC, WS-AC and AC were not suitable for phosphorus adsorption. It can be concluded that sewage sludge-based activated carbons modified with pyrulosite had a significant effect on phosphorus removal compared with other absorbents.

According to our previous study, PAC also had preeminent adsorption performance on the removal of heavy metal ions (e.g. Cu(II), Pb(II) and Cd(II) etc.) (Xie et al. 2013) and the treatment of dye wastewaters (Chen et al. 2012). This proved that PAC has relatively outstanding adaptability on various pollutants. The exploitation and utilization toward PAC has enormous economic benefit and potential for practical application.

Activated carbons produced from municipal sewage sludge with 1% (wt%) pyrolusite addition were proved to be efficient on the removal of phosphorus from aqueous solutions. Pyrolusite displayed an important role in the preparation of high-quality carbonaceous adsorbents. The FTIR result showed that after pyrolysis the hydroxyl and carboxyl groups on modified activated carbon's surface were shifted compared with the original sewage sludge and was thus beneficial for TP removal.

The produced adsorbents with pyrolusite loading (PAC) showed higher BET surface area and more micropores and mesopores than SAC, while PAC had much better adsorptive performance than SAC. The surface chemistry of the PAC was changed, and resulted in an enhanced TP removal rate with an economical and optimum dosage of 2 g/L. The pH adsorption experiments showed that PAC had a stable performance with a high phosphate removal rate in a wild pH range, and TP adsorption under the original solution pH (around 6.4) was efficient. The pseudo-second-order kinetics provided a good correlation of the adsorption data, and the experimental adsorption data was well fitted by the Langmuir isotherm model. The comparison of PAC with other absorbents manifested that PAC had observable advantages towards TP removal. Modified carbons with pyrolusite addition had a massive improvement on the phosphorus adsorption from aqueous solution.

The authors would like to acknowledge the funding support by the National Natural Science Foundation of China (grant No. 51208323).