Typical MSW odor abatement using sludge derived carbon prepared by activation with Fenton’s reagent and NaClO

Abstract

Sludge derived carbon (SBC) is a potential resource from sewage sludge disposal, and chemical pre-treatment is a necessary activation method for the improvement of the SBC quality. Two novel activators, i.e. Fenton and NaClO, were introduced to produce SBC precursors through the destruction of cell wall barriers and heterogeneous structures in sludge. High quality SBC-Fenton and SBC-NaClO were produced, with the BET having increased from 38 m² g⁻¹ in the control group to 253 m² g⁻¹ and 423 m² g⁻¹, respectively. The micro-porosity volume increased from 6% to 42% and 46% in the SBC-Fenton and SBC-NaClO, respectively, with the corresponding saturation adsorption capacity increasing from 33.1 mg g⁻¹ to 71.5 mg g⁻¹ and 67.8 mg g⁻¹ based on Langmuir isotherms using methylene blue. The adsorption processes of the typical odorants H₂S and NH₃ were also tested using the SBC-Fenton and SBC-NaClO, which could reduce the landfill volume and odor emissions simultaneously, and around 2.1 mg g⁻¹ and 0.68–2.24 mg g⁻¹ of NH₃ and H₂S were adsorbed under different dosages. SBC could be a promising adsorption carrier and supporting substance for soil cover in landfills.

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1. Introduction

Waste activated sludge (WAS) is the inevitable byproduct from waste water treatment plants, and around 46.5 billion tons of municipal waste water was collected and treated in 2013 in China, with around 35 million tons of sewage sludge generated (80% water content).¹ The unmanageable amount of sludge coupled with the imposition of highly escalating stringent regulations for the sludge disposal has resulted in the exploration of reliable and lasting technological solutions. Sludge has been recognized as a most critical environmental issue and an ecological burden for society, and some traditional sludge disposal routes of incineration, composting and land-use have been tested and applied in several cases, but the results were not satisfactory under the pressure of the emerging environmental concerns.

With the rapid increase of sludge generation, landfill has been used as an emergency route for sludge disposal (over 11.2 million tons annually) since 2005 in China,² with a special provision in the standards for the pollution control of landfill sites of municipal solid waste (GB16889-2008) for sludge landfilling, with the requirement that the water content of the sludge is below 60%.³ However, a huge landfill volume was occupied and an intolerable odor emission was inevitable during the sludge landfilling, and this was strongly opposed and argued against by the local residents.^4,5 The odor emission from the sludge landfilling is different from the municipal solid waste landfilling, with sulfur compounds and ammonia as the two main causes in sludge, while aromatics, sulfur compounds, and other oxygenated compounds are the main components in MSW landfilling.⁶ Sludge volume reduction and the use of sludge as a resource should be considered to retain a balance between the ecological and economic aspects of sludge landfilling.

Soil cover is an enforced requirement for sanitary landfill operation, and soil, compost, aged refuse, and other materials are commonly applied materials, which take up around 1/5–1/3 of the total landfill volume.⁷ The substitution of soil cover with sludge might be a suitable way to ease disposal pressure, although raw sludge has been proven to be impractical for landfill soil cover due to low mechanical strength and serious odor emission.⁸ Some pretreatments might be helpful for the application of sludge as the soil cover, such as pyrolysis. As an aggregation of microorganisms, sewage sludge contains amounts of organic matter, such as lignin, cellulose, hydrocarbon and protein, which provide the basic construction for the preparation of adsorbent material, such as carbon.^9,10

Sludge based carbon has been proven to be of poor quality when made by direct pyrolysis, with a low BET surface area and a surface structure which results in a bad performance as an adsorbent.^11–15 An activation process is the common way to improve the porous structure of SBC, and activation parameters, such as the type of activation, activating agent, and the impregnation ratio (the weight ratio of the activating chemical to the precursor) should be considered carefully. Activators, such as ZnCl₂, KOH, K₂CO₃, H₂SO₄, H₃PO₄ and NaOH, have been proven to be highly efficient in the production of highly porous adsorbent materials.^16,17 KOH has been widely used to activate the activated carbon from sewage sludge. A maximum BET of 310.62 m² g⁻¹ was obtained from textile sewage sludge at a pyrolysis temperature of 700 °C, with an optimum KOH impregnation ratio of 0.5.¹⁸ The BET surface area of SBC has also been found to increase from 3.6 m² g⁻¹ to 328.0 m² g⁻¹ with an oil sludge and KOH mixture,¹⁹ while for sewage sludge, the corresponding BET was as low as 100–200 m² g⁻¹ if the terminal T was below 600 °C.²⁰ Tao et al.²¹ tested the adsorbent preparation from sludge at 800 °C with 60% (v/v) HNO₃ additives, and SBC with a large surface area and abundant functional groups was obtained. The chemical activators would influence the initial types of the sludge precursors, and chemical activators should be selected according to the target pollutant removal.

For SBC preparation and the application for odorant removal in landfill, chemical activators could destruct the macro-molecular weight organic matter in sludge, and provide some special functional groups or adsorbent materials to react with the odorants. Compared to the chemical agents reported, Fenton’s reagent and NaClO might be two promising activators for the precursor of SBC. Fenton oxidants could destroy the cell wall due to the generation of hydrogen radicals (HO˙) with an EV of +2.8 eV,^22,23 which might contribute to the carbon porosities generated. The introduction of NaClO could also enhance the indirect oxidation through the generation of active chlorine (Cl₂, HOCl, and OCl⁻) and NaOH.²⁴ Both of these powerful oxidants could convert the high biopolymer substances in sludge into low-molecular-weight products efficiently. Besides, the intermediate products of Fe₂O₃ and NaOH could react with the carbon in sludge, and produce CO₂ and other gaseous emissions, which will be a benefit for the pores generated in the SBC.

To produce suitable SBC for landfill, two activation reagents of Fenton’s reagent and NaClO were introduced to prepare the precursor, and the morphology and structural properties of the SBC were compared. The adsorption capacity was tested using methyl blue (MB) simulated dyeing wastewater, and the potential utilization route of SBC in landfill was proposed and tested by the simulation of typical odor removal.

2. Materials and methods

2.1 Sludge samples

Sludge samples were collected from the secondary sludge tank of the Minhang municipal wastewater treatment plant in Shanghai, China, 5 times, with a typical A/O activated sludge treatment process. Around 100 L of sludge was sampled each time, and then dewatered using a centrifuge at a rate of 4000 rpm (revolutions per minute) for 5 minutes in the laboratory for sludge with a set water content. The sewage sludge properties are listed in Table 1.

Table 1 Properties of sewage sludge

TS (%)	pH	VSS/TSS (%)	TCOD (mg L^−1)	TN (mg L^−1)	TP (mg L^−1)
0.8–1.05	6.00–6.66	68.00–69.58	25 000–37 000	92–130	217–268

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2.2 Activation and preparation processes

SBC preparation involves 4 steps: pre-drying, chemical activation, carbonization, and a washing process.¹⁶ Sewage sludge was activated using NaClO and Fenton’s reagent under the optimum conditions based on our previous work,²⁰ and then were stored as the precursor for the next step. The dewatered sludge was dried at 105 °C until a constant weight was obtained. The sludge was then soaked in 0.8 mol L⁻¹ NaClO solution (Sinopharm Chemical Reagent Co., Ltd, active chlorine, 5.68% w/v, aqueous solution), with an optimum ratio of 0.5 according to our previous work.²⁰ For Fenton-SBC preparation, 500 mL of raw sludge solution was placed in the reactor at room temperature and stirred with the dropwise addition of 1.0 M H₂SO₄ until a desired pH of 3 was reached, and then the Fenton reaction was carried out under the operation conditions of H₂O₂/FeSO₄·7H₂O at a molar ratio of 5 : 1 and different H₂O₂ dosages based on our preliminary work.²³

The dried mixtures were pyrolyzed in a horizontal quartz glass tube furnace (HTL1100-60, HAOYUE, Shanghai, China) at 600 °C for 2 h, and N₂ was used as a protective gas with a flow rate of 400 mL min⁻¹. After the carbon-like material was prepared and cooled down, the carbon was washed using 10% (v/v) HCl and successive soaking in distilled water until a constant pH was reached. All of the samples were stored in a zip-lock bag and labeled with the number and reagent. The control SBC sample was prepared according to the same processes, except to omit the reagent impregnation.

2.3 Characterization of SBC

The porous structure and surface functional groups influenced the quality of the SBC greatly. The porous structure was observed using scanning electron microscopy (SEM) at 15.0 kV. Pore size distribution and specific surface area were measured using N₂ adsorption and desorption isotherms at 77 K using Quantachrome Instruments. The desorption data of the N₂ isotherm were used to determine the pore size distribution with the Barrett–Joyner–Halenda (BJH) method. The SBC surface was an interconnecting network of micropores, mesopores, and macropores.²⁵ The specific surface area of the activated SBC was calculated using the BET function, and the Dubinin–Raduskevitch (DR) method was used to evaluate the micropore volume. Surface functional groups were measured using a FTIR spectrometer (Nicolet 6700, ThermoFisher) at 25 °C, in which the samples were diluted in potassium bromide (KBr) and compacted into a thin membrane at 8.0 T cm⁻² for 2 min.

The percentage of the elements carbon (C), hydrogen (H), and oxygen (O) in the SBC were determined. C was oxidized to CO₂ and analyzed using a CS analyzer (CS-3000, NCS Testing Technology, Shanghai, China), and H and O were tested using an ONH analyzer (ONH-3000, NCS Testing Technology, Shanghai, China), in which H in the samples was released in the form of H₂ and the content was determined using a thermal conductivity cell, while O was converted into CO at 2300 °C and measured with infrared spectroscopy.

2.4 Adsorption capacity and adsorption isotherms

2.4.1 MB adsorption process. A varying dose (0.5 to 2.5 g L⁻¹) of SBC was added into 100 mL of Methylene Blue (MB) solution (with an initial concentration of 20–125 mg L⁻¹) in a 250 mL flask and shaken at 100 rpm for 60–120 min until equilibrium was obtained at 25 °C. The exhausted adsorbent was filtered using a 0.45 μm filter. Solution samples were taken at a given time and immediately centrifuged at 14 000 rpm for 3 min to remove the adsorbent, and the MB concentrations were measured using a UV spectrophotometer (Unico, UV 2102, Shanghai) at 664 nm. The effect of the dose of SBC was determined accordingly using:where q_e (mg g⁻¹) is the amount of MB adsorbed at equilibrium, and C₀ and C_e (mg L⁻¹) are the initial and equilibrium MB concentrations respectively. V (L) is the volume of the solution and W (g) is the mass of adsorbent.

MB removal efficiency is estimated as:

where C₀ and C_e (mg L⁻¹) are the initial and equilibrium MB concentrations, respectively.

The Langmuir isotherm was used to analyze the equilibrium based on the assumption of a monolayer coverage of adsorbate over the adsorbent surface, which has been successfully used to explain the adsorption of dyes from solutions.²⁶

The Langmuir isotherm is shown as:

where q_m (mg g⁻¹) represents the maximum monolayer coverage capacity of the adsorbent and K_l (L mg⁻¹) is the Langmuir isotherm constant. The essential features of the Langmuir isotherm would be expressed in terms of the equilibrium parameter R_l:in which the value of R_l indicates the adsorption nature as either unfavorable (R_l > 1), linear (R_l = 1), favorable (0 < R_l < 1), or irreversible (R_l = 0).

2.4.2 Odor adsorption process. Dynamic NH₃ adsorption was carried out in a fixed bed configuration at 293 K. 1 g of the SBC-NaClO samples was packed into a U shaped glass tube (9 mm internal diameter) as the adsorbent. The input gas consisted of 500 ppmv (mol mol⁻¹) of NH₃ passed through the bed at a flow rate of 100 mL min⁻¹, combined with the carrier gas N₂. The concentration of the input and outlet NH₃ was monitored using a gas detector (pGas200, Cnshsh Ltd.). Adsorption experiments were performed until bed exhaustion, at which 50 ppmv (mol mol⁻¹) NH₃ breakthrough capacities (mg NH₃ g⁻¹ carbon) were calculated by the integration of the breakthrough curves taking into account the input concentration of NH₃, flow rate, breakthrough time and the mass of used carbon.²⁷ The regeneration of the sample was studied after adsorption by heating the bed at 378 K for 2 h. Adsorption and regeneration were repeated for three cycles.

Batch experiments of the H₂S adsorption process were carried out, with an inlet gas mixture of H₂S : N₂ = 50% : 50% v/v. 20 mL of the mixture was injected into a 150 mL conical flask and then reacted in a shake batch at 293 K, with an oscillating rate of 100 rpm. 0.2 mL of the H₂S mixture was taken out at a set interval time and measured using a GC-2010 (Shimadzu Corp. Japan Agilent 6890 (ECD/FPD)).

3. Results and discussion

3.1 Effect on porous structure of SBC

The porous structure and morphology of the SBC are shown in Table 2 and Fig. 1. It was found that the BET of the SBC increased from 38 m² g⁻¹ to 253 m² g⁻¹ and 423 m² g⁻¹ after activation using Fenton’s reagent and NaClO under the test conditions, respectively. The total pore volume increased from 0.066 cm³ g⁻¹ (SBC-control) to 0.184 cm³ g⁻¹ (SBC-Fenton) and 0.513 cm³ g⁻¹ (SBC-NaClO), and the corresponding micropore volume increased from 0.004 cm³ g⁻¹ to 0.078 cm³ g⁻¹ and 0.238 cm³ g⁻¹, respectively. The highest V_micro/V_total ratio of 46% was observed in SBC-NaClO, and the corresponding ratios in the control and Fenton were 6 and 42%, respectively.

Table 2 Surface characteristics of SBC

	BET (m^2 g^−1)	Vtotal (cm^3 g^−1)	Vmicro (cm^3 g^−1)	Vmicro/Vtotal (%)
Control	38.7	0.066	0.004	6
SBC-Fenton	253	0.184	0.078	42
SBC-NaClO	423	0.513	0.238	46.39

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Fig. 1 SEM image of SBC, (a) SBC-control and (b) SBC-NaClO; (c) SBC-control and (d) SBC-Fenton.

The radii of all of the SBC samples were mainly arranged between 15–25 Å. The surface of the control sample (Fig. 1a and c) was smoother than the other SBC samples. SBC-NaClO (Fig. 1b) contained many more small pores, and SBC-NaClO had more uniform pores, while the radius was small but the cumulative volume was large, which resulted in a large BET area. For the SBC-Fenton sample (Fig. 1d), the surface showed more roughness, with the appearance of irregular coral, and some Fe and Ca was also doped into the SBC-Fenton, which will be of benefit for pollutant removal.²³ Micro- and mesopores were predominant in the porous structure.

3.2 Element distribution

The element distribution could be used to reflect the efficiency of the activation process. The generation rate, ash content and main elements in the SBC-Fenton and SBC-NaClO are shown in Table 3. The C ratio in SBC increased after activation using NaClO, since NaClO was able to disrupt the binding interaction between the extracellular polymeric substances (EPS) and the cell, and the detached EPS could furthermore dissolve into solution under centrifugal force,²⁸ although parts of the element C might be lost during the preparation process. The amount of O content in the SBC-NaClO was evidently higher than in the control group because some water was added with the activation reagent solution during the preparation step, which contributed to the increase in the percentage of O and H, as shown in eqn (5)

4NaClO + 2H₂O → 4NaOH + 2Cl₂ + O₂ (5)

Table 3 The element distribution in SBC

	SBC-control	SBC-NaClO	SBC-Fenton
Yield (%)	32.3	31.2	35.7
Ash content (wt%)	52.8	50.4	60.9
C (wt%)	35.6	37.51	32.3
N (wt%)	1.75	1.65	1.57
S (wt%)	0.44	0.72	0.56
H (wt%)	0.99	3.30	0.83
O (wt%)	14.5	15.77	17.3
Fe (wt% in the ash)	18.9	20.1	49.7

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Then, the intermediate product of NaOH reacted with C during the activation at 600 °C as follows:¹⁷

6NaOH + C → 2Na + 3H₂ + 2Na₂CO₃ (6)

Residues of Na₂CO₃ might contribute to the high O content in SBC-NaClO.

Compared to the control group, the ash content in the SBC-Fenton increased from 52.8% to 60.9%, and the C/H/N decreased apparently, among which the C content decreased from 35.6% to 32.3%, since some carbon was released in the form of CO₂ during the Fenton reaction. It should be pointed out that the oxidation capacity of ˙OH (2.85 mV) was higher than that of ClO⁻ (1.61 mV), which led to the decrease of the C ratio and the increase of the ash ratio in SBC-Fenton, compared to the SBC-NaClO. In addition, S in sludge could convert into SO₄²⁻ in the Fenton reaction system, instead of H₂S, and thus increase the S percentage in SBC. The Fe content in SBC-Fenton increased from 18.9% to 49.7% in the ash content with the introduction of FeSO₄ in the Fenton additives.

3.3 FTIR spectroscopy

The surface functional groups could influence the SBC quality greatly, and the FTIR spectra of dried sludge and the SBC samples are shown in Fig. 2a. All of the samples’ spectra exhibited a prominent peak located at 3300–3500 cm⁻¹, which was associated with the presence of hydroxyl groups. As seen from the spectrum of dried sludge, the transmittance peak at about 2925 cm⁻¹ was assigned to the vibration of O–H stretching. The peak of O–H deformation was found at 1407 cm⁻¹. The peak at 1234 cm⁻¹ was related to the strong infrared vibration of the C–O stretching. Those peaks were missing in the spectrum of SBC, since the carbonization process destroyed the structure of the SBC greatly. The peak at 3289 cm⁻¹ was the vibration of O–H stretching in the broad region of 3700–3200 cm⁻¹, which shifted to 3423 cm⁻¹ in the spectra of SBC.²⁹ C=O stretching in the spectrum of dried sludge was found at 1639 cm⁻¹, whereas it was detected at 1585 cm⁻¹ in the spectrum of SBC. The frequency range of 1100–1000 cm⁻¹ was associated with C–O stretching, which was found at 1036 cm⁻¹ and 1076 cm⁻¹ in the spectrum of dried sludge and SBC respectively. There were two vibrations of C–H deformation at the aromatics in the dried sludge and SBC, which moved slightly from the peak of 798 cm⁻¹ and 773 cm⁻¹ in the dried sludge to 796 cm⁻¹ and 775 cm⁻¹ in the SBC.

Fig. 2 FTIR spectra scan of SBC with and without activators. (a) Dried sludge and SBC-control, (b)SBC-NaClO, and (c)SBC-Fenton.

Fig. 2b is the spectra of SBC-NaClO, and the stretching vibrations of the O–H bond, C=O bond, and C–O bond were shown at 3423 cm⁻¹, 1585 cm⁻¹, and 1076 cm⁻¹, compatible with those in the spectrum of SBC. There was one deformation of the C–H bond in the range of 760–800 cm⁻¹, and NaClO solution might modify a C–H bond of SBC during the activation process. It was also found that more peaks had disappeared in SBC-Fenton (Fig. 2c). The stretching vibration of the O–H bond could be seen at the frequency of 3300–3500 cm⁻¹ in SBC-Fenton, while peaks of –CH₃ and –CH₂ had disappeared at 2920 cm⁻¹, meaning that the polysaccharides, proteins and high molecular polymers etc., in sludge were decomposed into small-molecular weight substances after the Fenton reactions. The transmittance at 1735 cm⁻¹, 1629 cm⁻¹ and 1466 cm⁻¹ decreased in the SBC-Fenton samples, which was related to the stretching vibration of the C=O, O–H and C–O bonds, which is ascribed to the polycyclic organic matter possibly being decomposed.³⁰ The band at 1629 cm⁻¹ was believed to arise from aromatic C–C bonds which were polarized by the oxygen atoms’ bond. This might be related to the oxygen groups incorporated into the carbonaceous phase being attacked by the hydroxyl radicals. The region near 1466 cm⁻¹ was commonly associated with carbonyl (C=O) and alkene (C=C) bonds, which were normally from the vibration of small molecule organics.

3.4 The adsorption capacity of MB

The adsorption balance between the agent and the adsorbent, affinity, adsorption mechanism and adsorption capacity could be measured using adsorption isotherms, and used to test the adsorption capacity of SBC. The Langmuir and Freundlich models were applied to simulate the adsorption capacity of the SBC obtained, and the parameters of the Langmuir and Freundlich models are listed in Table 4. The Langmuir model has a good fitting performance to simulate the SBC adsorption process, compared to the Freundlich model, meaning that the adsorption process of SBC belongs to monolayer adsorption. Both the SBC-NaClO and SBC-Fenton have a good monolayer adsorption capacity, with values of 67.83 mg g⁻¹ and 71.50 mg g⁻¹, while that of the SBC-control was only 33.10 mg g⁻¹. With the introduction of Fenton’s reagent and NaClO activation agents, the EPS and cell wall in the sludge were destroyed, which contributed to reducing the volatilization during the pyrolysis of the mesopores and macropores. It was found that around 95% and 99% of MB could be adsorbed by the SBC-Fenton and SBC-NaClO under the dosage range of 0.5–2.5 g L⁻¹, respectively, with an initial MB concentration of 20–40 mg L⁻¹ at 25 °C. The physical adsorption process might be carried out in SBC according to the n value in Freundlich model, and the rapid adsorption process was observed.

Table 4 Parameters of adsorption models of MB on SBC

Isotherms	Parameters	SBC-Fenton	SBC-NaClO	SBC-control
Langmuir	qm (mg g^−1)	71.50	67.83	33.10
	Kl (L mg^−1)	0.0148	2.01	0.03
	R^2	0.995	0.980	0.990
Freundlich	Kf (mg g^−1)	56.6	38.23	2.67
	1/n	0.0534	0.20	0.50
	R^2	0.661	0.92	0.98

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3.5 The odorous removal capacity of SBC

Based on the relative properties of SBC-NaClO and SBC-Fenton, NH₃ and H₂S were chosen as the target odors from landfill to test the relative carbon property. Three cycles of NH₃ adsorption breakthrough curves are shown in Fig. 3, and all samples presented a sharp adsorption profile, indicating that fast kinetics of the interactions between SBC and NH₃ were happening.

Fig. 3 The isothermal equation of NH₃ adsorbent using SBC-NaClO.

The regeneration was implemented using thermal treatment at 378 K. In an ideal situation, NH₃ is strongly retained on the adsorbent surface and does not desorb during thermal treatment,³¹ leading to zero adsorption in the next cycle. A small decrease was observed in the second round, while NH₃ uptake in NaClO-SBC was 52% of the original adsorption in the third cycle. A clear diminution in the retention capacities of the NaClO sample was observed in the third cycle since the small pore accessibility is limited by the adsorbate substance.³² The NH₃ breakthrough capacity for SBC-NaClO was around 2.1 mg g⁻¹, 2.0 mg g⁻¹ and 1.1 mg g⁻¹ in the first 3 cycles, respectively. The effect of heating on regeneration reduced as the cycle times increased, suggesting that the preferential interactions between NH₃ and the oxygen surface groups on the carbon surface, especially the acidic groups, were the key factor for the adsorption capacity. Generally, the adsorption capacity of SBC-NaClO for NH₃ was weak, and the amount of less stable oxygen surface groups played a crucial role in the adsorption–regeneration cycles, which resulted in a good regeneration effect due to the ammonia desorption ability.

SBC-Fenton could be a good adsorbent for H₂S removal.³³ It was found that the H₂S removal rate increased as the SBC-Fenton dosage increased. Around 29.2%, 34.9%, 37.1%, 42.5% and 44.2% of H₂S were removed, with the SBC-Fenton additive of 2 mL, 4 mL, 6 mL, 8 mL and 10 g and 10 mL H₂S in the initial stage, and 2.25 mg g⁻¹, 1.34 mg g⁻¹, 0.95 mg g⁻¹, 0.81 mg g⁻¹ and 0.68 mg g⁻¹ H₂S were uptaken in these processes. The high micropore rate of 47% in SBC-Fenton contributed to the H₂S removal, and the presence of Fe was also helpful to form the crystal as Ca₂Fe₂O₅ during the carbonization process. The formation of Ca₂Fe₂O₅ and other Fe forms in SBC-Fenton was useful for H₂S reduction.

The pollutant uptake capacities of the adsorbents derived from sewage sludge were not only governed by the textural properties of the adsorbents, but also by their surface properties, such as the functional groups and the surface charge and thus it is proposed that both of these crucial factors should be considered concurrently.³⁴ Sewage sludge is the aggregation of microorganisms, in which the cell wall is wrapped with EPS, and the binding water is present between the extracellular polymer and the cell wall. Polysaccharide and binding water were decomposed and evaporated during the carbonization process between 550 °C and 650 °C, which resulted in the formation of large pores, and a low BET value without any activation process. However, the activation process using Fenton’s reagent could improve the SBC properties greatly, since the generation of ˙OH destroyed the microorganism structure, and decomposed the large molecular weight organic matter into small and medium organic matter. This intermediate organic matter was helpful for the formation of CO₂ and H₂O during the carbonization process. The SBC properties benefit from the introduction of Fe, since the Fe₂O₃ and Fe₃O₄ in the precursor react with C at a high temperature as follows:

6Fe₂O₃ + C → 4Fe₃O₄ + CO₂↑ (7)

Fe₃O₄ + 2C → 3Fe⁰ + 2CO₂↑ (8)

4Fe₃O₄ + O₂ → 6γ − Fe₂O₃ (9)

3Fe⁰ + C → Fe₃C (10)

All of these reactions produced CO₂, with the molecular diameter of 0.33 nm, which contributed to forming the micropores in the obtained SBC.

For the NaClO additive, it could produce Cl₂ and NaOH during the activation process, and contributed to the generation of more porosity as shown in eqn (5). The ClO destroyed the C–C bond in EPS and thus dewatered the binding water between the polymer EPS and cell wall. The intermediate product of NaOH could react with C during the carbonization process (550–650 °C), and thus produce micropores, as shown in eqn (6). The adsorption of NH₃ involves acid–base reactions on the carbon surface.^31,32 These reactions could take place between NH₃ and carboxylic groups or carbonyl and epoxy groups, leading to the formation of ammonium salts, amides and amines. With the introduction of NaClO, more active functional groups were produced in SBC, and thus increased the NH₃ uptake.

In summary, the application of SBC in landfill could reduce the volume occupied by sludge, provide a suitable base for plant growth in the final soil cover, and enhance the odor adsorption process at the same time. The greenhouse gases (GHG) CO₂ and CH₄ could be reduced greatly through the carbon storage in sludge derived carbon. Most importantly, the introduction of SBC into the landfill might contribute to the landfill stabilization process through neutral pH values, and the adsorption of acidic compounds and toxic compounds. Further studies should be implemented to optimize the physicochemical and adsorption properties of SBC, to improve the odor uptake, and substitute the soil cover in landfill.

4. Conclusions

Higher quality SBC was prepared with the activators Fenton’s reagent and NaClO. Both of them contributed to the increase of BET and V_micro/V_total, which were 6.7 and 11 times higher than the SBC-control. The activation process was necessary to improve the SBC quality by destroying the cell wall barrier and decompose the complex macrocompounds. The intermediate products of Fe and NaOH contributed to the SBC-activated structure. The saturation adsorption capacity could reach around 71.5, 67.8 and 33.1 mg g⁻¹ in the SBC-Fenton, SBC-NaClO and SBC-control using MB. SBC could be a promising substitute soil cover for landfill, instead of occupying volume, and further research should be carried out for dynamic continuous tests on the odor emission by the mixed soil cover of SBC.

Acknowledgements

This work was financially supported by the National Key Technology R&D Program (No. 2014BAL02B03-4), the National Natural Science Foundation of China (No. 41173108) (No. 51278350), the Shanghai Rising-Star Program (14QA1402400), TÁMOP 4.2.4.A/1-11-1-2012-0001 and the Key project of Science and Technology Commission of Shanghai Municipality (No. 13DZ0511600).

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