Carbonaceous Adsorbent Derived from Sulfur-Impregnated Heavy Oil Ash and Its Lead Removal Ability from Aqueous Solution

Abstract

A novel carbonaceous adsorbent was prepared from sulfur-impregnated heavy oil ash via pyrolysis using potassium sulfide (K₂S) solution, and its ability to remove lead (Pb²⁺) from aqueous solutions was examined. It was compared with an adsorbent synthesized by conventional pyrolysis using potassium hydroxide (KOH) solution. Specifically, the raw ash was immersed in 1 M K₂S solution or 1 M KOH solution for 1 day and subsequently heated at 100–1000 °C in a nitrogen (N₂) atmosphere. After heating for 1 h, the solid was naturally cooled in N₂ atmosphere, and subsequently washed and dried to yield the product. Regardless of the pyrolysis temperature, the product generated using K₂S (Product-K₂S) has a higher sulfur content than that obtained using KOH (Product-KOH). Moreover, Product-K₂S has a higher lead removal ability than Product-KOH, whereas the specific surface area of the former is smaller than that of the latter. Product-K₂S obtained at 300 °C (Product-K₂S-300) achieves the highest lead adsorption and a high selective lead removal from a ternary Pb²⁺–Cu²⁺–Zn²⁺ solution. The equilibrium capacity of Product-K₂S-300 was found to fit the Langmuir model better than the Freundlich model, and its calculated maximum adsorption capacity is 0.54 mmol/g. From the ternary Pb²⁺–Cu²⁺–Zn²⁺ solution, the order of adsorption by Product-K₂S-300 is Pb²⁺ > Cu²⁺ > Zn²⁺, and the removal of Pb²⁺ and Cu²⁺ increases as the pH of the solution increases.

1. Introduction

In oil-fired power plants, oil-fired fly ash following discharge, prior to passing through a flue, is collected by cyclones or electrostatic precipitators. The ash is mainly composed of unburned carbon (up to 90 wt.%), water-soluble sulphates, and metal oxides, such as iron (1–5 wt.%), vanadium (0.6–3.6 wt.%), and nickel (0.1–1.7 wt.%) oxides. There is concern regarding the environmental problems of ash, such as generation of acidic liquid, release of heavy metals, and dusting. Because oil-fired fly ashes have a high content of vanadium (V) and nickel (Ni), their recovery from heavy oil ash has attracted attention since the 1960s worldwide [1]. In the traditional hydrometallurgical processes [2], in the first leaching stage, the vanadium and nickel compounds in ash are dissolved in acidic or basic solutions. Consequently, the produced unburned carbon (carbonaceous residues) from the ash has to be disposed of to landfill sites, requiring high processing costs [3]. Because they have a high carbon content and heavy metal-free, these residues can be used as precursors for activated carbon [4,5,6] or be burned with or without coal in a coal boiler for energy recovery.

Activated carbon has various uses in wastewater treatment for drinking water production, control of atmospheric pollution, and removal of poisonous gases. Activated carbon is favorable for removing organic chemicals from wastewater, whereas it is not promising for removing inorganic pollutants from aqueous solutions, because of its relatively nonpolar character [7]. This property prevents the attraction between some functional groups on activated carbon surface and charged metal species. The adsorption can be enhanced by introducing functional groups with high affinity for specific substrates by functionalization processes. The choice of functional groups allows defining the specificity of an adsorbing material for a particular substrate. For example, a bio-based substance (BBS) from composited bio-wastes was impregnated in γ-alumina to promote the adsorption of polar pollutants via the several acidic and basic functional groups of the BBS [8,9]. Activated carbon is typically modified by various ligands, which introduce additional heteroatoms so that the sorbent develops a high selectivity toward certain metal ions. Sulfur (S)-containing ligands prove more selective toward some heavy metals than their oxygen and nitrogen analogues [10,11,12]. In previous studies, S-impregnated carbonaceous adsorbents with high heavy metal removal were prepared from an industrial waste—paper sludge—by pyrolysis after immersion in potassium sulfide (K₂S) solution [13]. In the Pearson theory, a soft base, S, had better to interact with soft acids, Zn²⁺, Pb²⁺, and Cd²⁺, instead of with a hard base, oxygen, in activated carbon [14,15]. Therefore, there is a possibility to manufacture a low-cost adsorbent with removal of heavy metals from carbonaceous heavy oil ash by pyrolysis with K₂S solution.

In this study, the preparation of a carbonaceous adsorbent with removal property of heavy metals from heavy oil ash by pyrolysis with K₂S solution was investigated, and the lead removal ability of the S-impregnated adsorbents was examined. This was motivated by Pb²⁺ being one of the representative poisons among heavy metals. In addition, to clarify the effect of S in the raw ash, the adsorbent was prepared by the same procedure using potassium hydroxide (KOH) solution to compare the properties of lead removal.

2. Materials and Methods

2.1. Heavy Oil Ash

A heavy oil ash collected from an electrostatic precipitator of a Japanese electric power plant in Akita was used.

Table 1. Analyses of raw ash.

	Raw Ash
Proximate analysis (wt% dry basis)
Volatile matter 1	19.0
Fixed carbon	73.3
Ash	7.7
Ultimate analysis (wt% dry ash free)
Carbon	79.5
Hydrogen	1.2
Nitrogen	4.4
Sulfur	4.0
Ash composition (wt%)
NiO	29.9
Fe2O3	18.8
SO3	14.1
SiO2	7.7
CaO	6.2
V2O5	5.6
Na2O	5.0
Al2O3	3.8
ZnO	3.2
MgO	1.1
K2O	0.9
Specific surface area (m2/g)	6.2

¹ Value at 800 °C.

lists some of the properties of this heavy oil ash. The proximate and ultimate analysis of the heavy oil ash is reported in Table 1. The raw ash has a high carbonaceous fraction content (75–80%), appropriate for the preparation of a carbonaceous adsorbent.

2.2. Preparation of Adsorbent

The preparation procedure for the adsorbent is shown in Figure 1. The raw ash (10 g) was added to 1 M K₂S solution or 1 M KOH solution (100 mL), stirred for 1 day, and then filtrated and dried in an oven to obtain K₂S-immersed ash (Ash-K₂S) or KOH-immersed ash (Ash-KOH), respectively. These immersed ashes were pyrolyzed, then washed with distilled water, and dried in an oven to prepare the carbonized products (Product-K₂S from Ash-K₂S and Product-KOH from Ash-KOH).

The pyrolysis was performed using a horizontal reactor, similar to a previous study [13]. Ash-K₂S or Ash-KOH was set in the middle of a quartz tube (0.45 mm inside diameter and 1 m length). N₂ gas was injected to purge the air in the tube (1.0 L/min rate) for 30 min. The ashes were heated at 100–1000 °C in an electric furnace, with a N₂ gas continuous flow (1.0 L/min). After 1 h of heating, the product was naturally cooled under the flow of N₂ gas in the tube (1.0 L/min rate), and finally, it was collected from the tube.

2.3. Lead Removal

Pb²⁺ adsorption capacities of the samples were examined using lead nitrate (Pb(NO₃)₂) aqueous solutions. Each sample (0.1 g) was added to 10 mM Pb²⁺ solution (10 mL) in a centrifuge tube and shaken with a reciprocal shaker. After shaking for 1 day, the tube was centrifuged, and the Pb²⁺ concentration in the supernatant and the pH of the supernatant were measured.

The adsorption of Pb²⁺ by the S-impregnated adsorbents was examined using solutions containing various Pb²⁺ contents. Each adsorbent (50 mg) was added to 0–8 mM Pb²⁺ solutions (10 mL) in tubes, which were shaken with a shaker. After shaking for 1 day, the tubes were centrifuged, and the Pb²⁺concentrations in the supernatants and the pH values of the supernatants were measured.

The selectivity of Pb²⁺ removal of the S-impregnated adsorbents was examined using a mixed aqueous solution of Pb(NO₃)₂, zinc nitrate, and copper nitrate, each having a concentration of 1 mmol/L in the ternary solution. Each adsorbent (0.1 g) was added to 10 mL of ternary solutions having various pH values, which were adjusted to 1–4 using nitric acid solution, and the tubes were shaken with a shaker. After shaking for 1 day, the tubes were centrifuged, and the concentrations of Pb²⁺, Zn²⁺, and Cu²⁺ in the supernatants and the pH values of the supernatant were measured.

The adsorption amounts (q (mmol/g)) and the removal ratio (R (%)) were calculated using the following equations:

q_M = (C₀ − C) × V/w

(1)

R_M = (C₀ − C)/C₀ × 100

(2)

where M = Pb²⁺, Zn²⁺, or Cu²⁺. C₀ and C are the initial and measured concentrations of Pb²⁺, Zn²⁺, and Cu²⁺ in the solution (mmol/L), respectively. V is the solution volume (L), and w is the weight of the added sample (g).

2.4. Characterization

The chemical contents and mineralogical phases of the samples were analyzed via X-ray fluorescence (XRF-Primini, Rigaku, Tokyo, Japan) and powder X-ray diffraction (monochromated CuKα radiation) (Ultima IV, Rigaku, Tokyo, Japan), respectively. The morphologies of the samples were observed via scanning electron microscopy (SU-70, Hitachi, Tokyo, Japan). The carbon contents in the samples were analyzed using a CHNS/O elemental analysis apparatus (2400II, Perkin Elmer Japan, Yokohama, Japan). The specific surface area of each sample was determined using an automatic gas adsorption apparatus (Belsorp-mini II, MicrotracBel, Osaka, Japan). The heavy metal ion concentrations in the solutions were analyzed using an inductively coupled plasma emission analyzer (SPS 5510, Hitachi, Tokyo, Japan). The pH of the solutions was measured by a pH meter (pH/Ion meter D-53, Horiba, Kyoto, Japan).

3. Results and Discussion

Figure 2 shows the mineral compositions of (a) raw ash, (b) Ash-K₂S, and (c) Ash-KOH. The raw ash mainly contains ammonium sulfate ((NH₄)₂SO₄) and magnesium chloride (MgCl₂), produced by the neutralization of acidic gas by ammonia and magnesium oxide. After the treatment with K₂S and KOH, these mineral phases are diminished, elemental S appears in Ash-K₂S, and potassium sulfate (K₂SO₄) appears in Ash-KOH.

Figure 3 shows the lead removal achieved by the products obtained at various pyrolysis temperatures. Regardless of the pyrolysis temperatures, the removal ability of Product-K₂S is higher than that of Product-KOH. In the case of Product-KOH, as the temperature of pyrolysis is increased to 600 °C, the lead removal ability decreases to zero. The lead removal ability of the product obtained above 700 °C is higher than that of the product obtained at 600 °C, and becomes almost constant (approximately 25%). In the case of Product-K₂S, as the temperature of pyrolysis increases to 300 °C, the Pb²⁺ removal ability of the product increases, followed by a decrease up to 600 °C, and becoming almost constant above 600 °C. Therefore, the maximum lead removal ability achieved by heating Ash-K₂S is at 300 °C.

Figure 4 shows the XRD patterns of (a) Product-K₂S and (b) Product-KOH after pyrolysis at various temperatures. All the obtained products present mainly amorphous phases produced by pyrolysis. It is noted that a metal alloy of Fe and Ni appears in Product-K₂S obtained at 1000 °C and in Product-KOH above 700 °C, owing to the reduction of metal oxides in the ash by the carbon content.

Figure 5 shows the contents of carbon and S of Ash-K₂S, Ash-KOH, Product-K₂S, and Product-KOH as functions of the pyrolysis temperature. It is noted that the results of Ash-K₂S and Ash-KOH are plotted at the origin. The carbon contents of Ash-K₂S and Ash-KOH are approximately 50% each, which is lower than that of raw ash. After pyrolysis, the carbon contents of all the products are approximately 65–70%. The S content of Ash-K₂S (12%) is higher than those of Ash-KOH (4%) and raw ash (4%). After pyrolysis, the S contents of all the Product-K₂S are higher than those of all the Product-KOH. The S content of Product-K₂S obtained at 100 °C is almost the same as that of Ash-K₂S, and that of Product-K₂S obtained above 200 °C is almost constant (5–7.5%), which are lower than those of Ash-K₂S. The S content of Product-KOH is almost constant, which is almost same as that of Ash-KOH.

Figure 6 shows the specific surface areas of Ash-K₂S, Ash-KOH, Product-K₂S, and Product-KOH as functions of the pyrolysis temperature. It is also noted that the results of Ash-K₂S and Ash-KOH are plotted at the origin. The specific surface areas of raw ash, Ash-K₂S, and Ash-KOH are almost the same (approximately 6%). After pyrolysis, regardless of the pyrolysis temperature, the specific surface area of Product-K₂S is lower than that of Product-KOH, and both are almost constant (5–10% and 15–20 %, respectively).

The S contents of Ash-K₂S and Ash-KOH comprise elemental S and K₂SO₄, respectively. Volatile matters in these ashes are released at 100–200 °C to form carbonaceous materials. The melting and boiling points of elemental S are 188 °C and 445 °C, respectively, whereas those of potassium sulfate are 1069 °C and 1689 °C, respectively. For Ash-K₂S, melting S was rated with the surface of carbonaceous ash to form S functional groups for heavy metal ion removal at 200–400 °C. Moreover, S was vaporized without reacting with the carbonaceous surface above 400 °C. Therefore, the lead removal ability of the product pyrolyzed at approximately 300 °C from Ash-K₂S is the highest. For Ash-KOH, K₂SO₄ does not react with the carbonaceous surface without the melting S content forming the S functional group. Therefore, the lead removal ability of the product pyrolyzed from Ash-KOH is lower than of that from Ash-K₂S.

The SEM images of (a) the raw ash, (b) Product-K₂S obtained at 300 °C, and (c) Product-KOH at 300 °C are shown in Figure 7. A collection of spongy particles and some aggregates are observed in the raw ash, and the morphologies of both the obtained products appear very similar to that of the starting ash, which suggests that the spongy structure of the heavy oil ash is maintained in the obtained adsorbents.

The lead removal isotherm of Product-K₂S obtained at 300 °C is shown in Figure 8. With the increase in the equilibrium lead concentration, the lead adsorption increases to approximately 0.4 mmol/g, and then becomes almost constant. The isotherm was analyzed using two typical isotherm models: Langmuir and Freundlich models. The liner forms of the Langmuir and Freundlich models are given by:

C_e/q_e = 1/(Q_max × K_L) + C_e/Q_max

(3)

ln(q_e) = ln(K_F) + (1/n) × ln(C_e)

(4)

where q_e is the lead concentration on the adsorbent at equilibrium (mmol/g), and Q_max (mmol/g) and K_L (L/mmol) are the maximum adsorption capacity and Langmuir constant (equilibrium adsorption constant), respectively. K_F and n are the Freundlich constants.

The calculated parameters for each isotherm model are listed in

Table 2. Parameters and correlation regression for two models, Langmuir and Freundlich.

Langmuir			Freundlich
Qmax	KL	R2	N	KF	R2
0.54	8.48	0.999	4.06	0.41	0.813

. The correlation regression coefficient (R²) as a measure of the goodness-of-fit using the Langmuir and Freundlich models are 0.999 and 0.813, respectively, indicating that the Langmuir model is fitted better than the Freundlich model. This suggests that the lead adsorption occurs uniformly and follows monolayer adsorption, instead of multilayer adsorption. Moreover, the adsorption occurs at equivalent adsorption energies and homogeneous binding sites with no interaction between the adsorbates [16]. The calculated maximum adsorption capacity, Q_max, is 0.38 mmol/g. The lead adsorbents, such as natural materials [17,18,19], synthetic carbon materials [20,21,22], and activated carbon prepared from wastes [23,24,25,26,27,28,29,30] were reported, and the maximum adsorption capacity of 0.38 mmol/g is comparable to those of other reported adsorbents (

Table 3. Comparison of lead ion adsorption with some reported data.

Adsorbent	Adsorption Capacity (mmol/g)	Reference
Siderite	0.06	[17]
Kaolinite clay	0.09	[18]
Rice husk ash	0.06	[19]
Alumina supported iron oxide	0.14	[20]
Carbon nanotube	0.01	[21]
Sulfur-functionalized ordered mesoporous carbon	0.14	[22]
Hazelnut husk activated carbon	0.06	[23]
Date pits activated carbon	0.15	[24]
Peanut hull activated carbon	0.15	[25]
Rice straw activated carbon	0.17	[26]
Algal waste activated carbon	0.21	[27]
Pecan shell activated carbon	0.31	[28]
Coconut shell activated carbon	0.37	[29]
Palm shell activated carbon	0.4	[30]

). It can be concluded that the adsorbent obtained from oil ash by S-impregnation is a promising material for treatment of wastewater containing lead ion.

Lead removal ability of Product-K₂S prepared at 300 °C from ternary solutions was examined. Figure 9 presents the heavy metal removal ability of the product from a Pb²⁺–Zn²⁺–Cu²⁺ solution. The product can remove Pb²⁺ and Cu²⁺ ions from the ternary solution, whereas Zn²⁺ ions cannot be removed in this experiment. With the increase in the equilibrium pH, the removal of Pb²⁺ first increases from pH 3 to pH 6, and then that of Cu²⁺ increases from pH 4.5 to pH 6. The increase in the removal of Pb²⁺ and Cu²⁺ when the pH increases may be attributed to two reasons. First, the number of negatively charged active sites increases and the number of positively charged sites decreases at high pH values, which increase the electrostatic adsorption between the metal ions and the S-impregnated product. Second, at a low pH, the competitive interaction between the metal ions and protons with the active sites increases, resulting in a low adsorption capacity of the metal ions.

The order of adsorption using Product-K₂S-300 is Pb²⁺ > Cu²⁺ > Zn²⁺, indicating that the product has a high affinity for Pb²⁺ among the heavy metal ions. A functionalized carbon nanotube with S functional groups (=C=S, -SH) presented increased adsorption of Cu²⁺ with an increase of the solution pH from 2 to 6 and was more favorable to Cu²⁺ than Zn²⁺ [31]. For a resin with sulfur functional groups, the adsorption of Pb²⁺ and Cu²⁺ increases with an increase of the solution pH, and the adsorption amount of Pb²⁺ is slightly higher than that of Cu²⁺ [32]. There is good agreement with our results.

From these results, a carbonaceous product can be prepared by both the procedures, and Product-K₂S has a higher lead removal ability than Product-KOH. Product-K₂S has a higher S content and lower specific surface area than Product-KOH, which suggests that chemical adsorption with S content has a higher dependence on lead removal from an aqueous solution than physical adsorption.

4. Conclusions

Preparation of a carbonaceous adsorbent with removal of heavy metals from heavy oil ash by S impregnation was investigated, and its lead removal ability from an aqueous solution was estimated. K₂S solution can immerse S into the ash, and K₂S-immersed ash can be converted into a carbonaceous product with a high S content by pyrolysis. Product-K₂S pyrolyzed at 300 °C presents the highest lead adsorption from an aqueous solution, and there is high selective removal for lead ions from a ternary Pb²⁺–Cu²⁺–Zn²⁺ solution. The lead removal is found to fit the Langmuir adsorption model more than the Freundlich model. In the ternary Pb²⁺–Cu²⁺–Zn²⁺ solution, with an increase in the solution pH, the removal of Pb²⁺ and Cu²⁺ increased and the order of adsorption was Pb²⁺ > Cu²⁺ > Zn²⁺. The carbonaceous adsorbent with heavy metal removal can be prepared from heavy oil ash by S impregnation, and it can be applied to water treatment for heavy metal removal, particularly Pb²⁺.