The Recycling of Acid Wastewater with High Concentrations of Organic Matter: Recovery of H₂SO₄ and Preparation of Activated Carbon

Abstract

Little work has been focused on the recycling of hazardous acid waste with high concentrations of organic matter from petroleum refining. This study developed an innovative, effective, and simple method for the recycling of acid waste that can successfully resolve this significant problem in industry. After parameter optimization, the optimal process is as follows. (1) Through heat treatment at 170 °C, liquid acid waste was transformed into solid; (2) by washing the solids, 70% by weight of sulfuric acid was recycled; and (3) the solid residue after washing was activated by alkali (NaOH or KOH) at an alkali and organic carbon ratio of 2:1, at a temperature of 650 °C for 60 min, producing superior-grade activated carbon with a specific surface area of 1378 m²/g, a pore volume of 0.5107 cm²/g, an iodine number of 1800 mg/g, and a methylene blue adsorption capacity of 240 mg/g. Thus, in this way, both waste sulfuric acid and organic impurities are turned into valuable resources, and no hazardous waste gypsum residues are generated. This method both reduces carbon emissions and recycles valuable resources, which is of important environmental and economic significance.

1. Introduction

Large amounts of hazardous acid waste containing high concentrations of sulfuric acid (85–90% by weight) and organic matter (8–14% by weight) are produced in the petrochemical industry. Sulfuric acid is used to remove the unsaturated hydrocarbons and sulfides contained in crude oil, diesel oil, and other petroleum products. According to statistics, in petroleum refining, refining a ton of crude oil consumes an average of 1.08 kg of sulfuric acid. China’s annual crude oil refining capacity has reached 541 million tons, consuming approximately 600,000 tons of sulfuric acid and discharging approximately 700,000 tons of waste sulfuric acid. In alkylation oil production, 80–90 kg of waste sulfuric acid was generated for each ton of product produced. The average annual production capacity of alkylated oil was 6.25 million tons, and the corresponding waste sulfuric acid and carbon discharge were approximately 600,000 tons, and 10,000 ton, respectively [1].

At present, the acid waste is generally neutralized by lime [2], producing massive amounts of hazardous solid waste and wasting valuable sulfuric acid. It is estimated that 1 ton of 85% waste sulfuric acid can generate more than 1.2 tons of gypsum residue, and the petrochemical industry can generate more than approximately 1.6 million tons waste gypsum residues every year. Improper disposal of such large quantities of hazardous gypsum residues will lead to serious secondary environmental risks and carbon emissions. The harmless disposal of large amounts of waste gypsum is a major industry-wide problem, and the illegal dumping of waste sulfuric acid often occurs [3]. Thus, some researchers and factories use ammonium salt as a neutralizer to convert high concentrations of sulfuric acid into ammonium sulfate for agricultural use as fertilizer [4]. However, the high concentration of organic matter contained in acid waste is transferred into fertilizer, which enters the soil during use and causes secondary pollution. High-temperature reduction pyrolysis is another recycling method for acid waste [5]. This method uses natural gas as fuel, converts waste sulfuric acid into SO₂ at 1000–1100 °C, and then uses the purified SO₂ gas to produce 98% H₂SO₄ or fuming sulfuric acid. The organic matter in the acid waste is completely decomposed into CO₂ and H₂O. This may be the ideal way to recycle acid waste. However, the application of this method is limited due to the large equipment and significant investment required, as well as the serious equipment corrosion that takes place. In addition, converting organic matter into CO₂ not only increases greenhouse gas emissions, it also wastes valuable organic matter. Therefore, the simultaneous recycling of sulfuric acid and organic matter is an important problem in acid waste treatment. In practice, at present there is no economical and practical technology for recycling acid waste.

It has been reported that H₂SO₄ can be used as an activator to prepare activated carbon [6,7]. Under high-temperature conditions, raw carbon can be activated and carbonized by H₂SO₄ to prepare activated carbon with a certain porosity. Inspired by this, we had an idea: since there is a high concentration of sulfuric acid and a large amount of organic matter in black acid waste, perhaps it is possible to convert the organic matter into carbon through heat treatment. This may be a way to recycle organic matter in acid waste. In addition, the raw materials for the preparation of activated carbon are mostly solid materials such as coconut shell, wood–plastic composite waste, petroleum coke, and so on, and there has been no study on the use of liquid acid waste as raw materials [7,8,9,10,11]. Therefore, this study may provide a novel raw material for the preparation of activated carbon. Given the above, an innovative treatment and recycling method for acid waste is proposed and studied in this paper. In this method, both H₂SO₄ and organic matter can be recycled. In this method, the effects of heat treatment temperature on the concentration of regenerated H₂SO₄ and residual concentration of organic matter in regenerated H₂SO₄ were studied, as well as the effects of activator type, alkali–carbon ratio, activation temperature, impregnation time, and washing on the adsorption performance of activated carbon, and the physicochemical properties of the prepared activated carbon were characterized.

2. Materials and Methods

2.1. Materials

A mixed sample of black petroleum refining acid waste across a production cycle was collected from an oil factory in Guangxi Province, China. It contains sulfuric acid (approximately 90% by weight) and organic matter (approximately 10% by weight). Its density is 1.75 g/m³. All the reagents used in the study were analytical grade or better, and were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

2.2. Experimental Methods

Three steps were performed in the treatment and recycling method for acid waste (Figure 1): (1) through heat treatment, liquid acid waste was transformed into solid; (2) by washing the solids, 70% by weight of sulfuric acid was recycled; and (3) the solid residue after washing was prepared as activated carbon.

2.2.1. Heat Treatment and Separation of Sulfuric Acid from Organic Matter

For each batch of experiments, 200 mL acid waste was added into a 1000 mL beaker, which was heated in an oil bath at 170–260 °C, and the acid waste was manually stirred with a glass rod. When the liquid acid waste turned into black solid matter, the heating was stopped, and the solid was naturally cooled to room temperature. The solid sample was rinsed with 10 mL deionized water and the concentration of sulfuric acid in the eluent was determined. Another 10 mL of fresh deionized water was taken, and the process repeated several times until the mass fraction of sulfuric acid in the last eluent was less than 1%. All the eluents were then collected and mixed well, and were named as recycled sulfuric acid.

The proportion of residual solid organic matter (%) = [the mass (g) of solid residue after washing/the mass (g) of organic matter in acid waste] × 100.

The recovery rate of sulfuric acid (%) = [the mass (g) of H₂SO₄ in mixed eluent/the mass (g) of H₂SO₄ in acid waste] × 100.

The removal rate of organic matter in recycled sulfuric acid (%) = [COD (ppm) in recycled sulfuric acid/COD (ppm) in acid waste with equivalent sulfuric acid] × 100.

The solid residue after washing is used for the subsequent preparation of activated carbon.

2.2.2. Preparation of Activated Carbon

The preparation of activated carbon includes the pre-carbonization and activation processes [12], which are carried out in atmosphere and nitrogen, respectively. The equipment used for this process is a ZSK-1400 horizontal tube furnace (CNT Furnace Beijing Science and Technology Co., Ltd., Beijing, China), which is equipped with a cylindrical quartz tube with a length of 120 cm and a diameter of 9 cm. During the pre-carbonization process, a total of 60 g of solid residue was put into the quartz boat and heated to 400 °C at a heating rate of 5 °C/min. The heating was kept at 400 °C for 60 min, and then the heating was stopped, and the sample was cooled naturally to room temperature in air. During the activation process, 5 g of pre-carbonized samples was uniformly mixed with 10 mL of KOH or NaOH solutions of different concentrations and placed for 5–48 h, followed by activation in a nitrogen atmosphere (800 mL/min). The weight ratio of KOH or NaOH to pre-carbonized samples was set to between 0 and 2.5/1. The heating rate was 10 °C/min, and the activation temperature and time were 400–700 °C and 60 min, respectively. The prepared activated carbon was then cooled naturally to room temperature, washed, and dried in a vacuum drying oven at 105 °C for more than 24 h.

2.3. Analysis Methods

Pore structure of the activated carbon was evaluated by measuring nitrogen adsorption isotherm at 77.3 K (BeiShiDe Instrument Technology (Beijing) Co., Ltd., Beijing, China). The specific surface area, S_BET, was calculated from N₂ adsorption data for relative pressures (P/P₀) of 0.0100–0.0600 by employing the Brunauer–Emmett–Teller (BET) equation. Total pore volume and microporous volume were estimated from the amounts of nitrogen adsorbed at relative pressure (P/P₀) of 0.9961 and 0.1272, respectively. The average porous diameter was estimated from BET specific surface area. The morphology of the activated carbon was obtained on a Hitachi SU-8020 field emission scanning electron microscope (SEM) (Hitachi Ltd., Tokyo, Japan). The mass fraction of C, H, N and S of the activated carbon was measured on a Vario EL III elemental analyzer (Elementar Analysensysteme GmbH, Hanau, Germany). The COD in the eluents was measured according to the APHA standard methods [13]. The mass of sulfuric acid in acid waste and recovered dilute acid was determined by sodium hydroxide titration.

Iodine number and methylene blue (MB) adsorption capacity are accepted as the most fundamental parameters used to characterize the performance of activated carbons. According to the State Standard of the People’s Republic of China, the two indicators in wooden activated carbon should be greater than 900 mg/L and 105 mg/g, respectively [14]. The iodine number was measured according to the standard method [15]. This procedure is as follows: (1) 0.5000 g dried activated carbon was put into 250 mL conical flask with cover and 10 mL 5 wt% HCl aqueous solution was added into the conical flask; (2) the mixture was heated to faint boiling for 30 ± 2 s on an electric heating panel and then cool to room temperature; (3) 50.0 mL calibrated iodine standard solution (0.1000 mol/L) was added into above mixture and then shake for 15 min; (4) the suspension was filtered quickly using vacuum filter unit with 0.45 μm hydrophilic membranes; (5) 10 mL filtrate was transferred to 250 mL wide conical flask with 100 mL distilled water and then titrated using sodium thiosulfate standard solution (0.1000 mol/L) with starch indicator. The iodine number was calculated from Equation (1).

$$\mathrm{A}=\frac{5\left(10{\mathrm{c}}_1-1.2{\mathrm{c}}_2{\mathrm{V}}_2\right)\times 126.93}{\mathrm{m}}·\mathrm{D}$$

(1)

where A—iodine number, mg/g; m—mass of sample, g; c₁—concentration of iodine standard solution, mol/L; c₂—concentration of sodium thiosulfate standard solution, mol/L; V₂—volume of sodium thiosulfate standard solution, mL; D—correction factor.

The MB adsorption was measured according to the standard method [16]. This procedure is as follows: (1) 0.100 g dried activated carbon powder (≤71 μm) was put into 100 mL conical flask with cover; (2) a certain volume (mL) MB test solution (1.5 g/L) was added and the suspension was shake for 20 min at room temperature; (3) the suspension was filtered using medium speed qualitative filter paper; (4) and the concentration of MB in the filtrate was quantified by UV-visible spectrophotometer (HACH DR6000, HACH, Loveland, Colorado, USA) at 665 nm. Repeat above process 2–4 until the absorbance of MB equal to the absorbance of 4 g/L Cu₂SO₄·5H₂O aqueous solution. The MB adsorption value was calculated from Equation (2).

$$\mathrm{A}=\mathrm{B}\times 15$$

(2)

where A—MB adsorption value, mg/g; B—MB adsorption value, mL/0.1 g.

3. Results and Discussion

3.1. Separation of Sulfuric Acid from Organic Matter

The separation of sulfuric acid and organic matter is based on the principle that concentrated sulfuric acid can carbonize organic matter under heating conditions. Temperature is one of the most important factors affecting the separation of sulfuric acid from organic matter. The quality of solid organic matter, the concentration of recycled sulfuric acid, and the residual concentration of organic matter in recycled sulfuric acid are important indexes for evaluating the separation performance of sulfuric acid and organic matter. Therefore, we studied and discussed the influence of temperature on the above three indexes in order to obtain the best separation conditions.

From Figure 2, with the increase of heating temperature from 170 °C to 260 °C, the proportion of residual solid organic matter in acid waste decreased from 85% to 40% (Figure 2a), and the recovery rate of sulfuric acid correspondingly decreased from 45% to 20% (Figure 2b), indicating that with the increase of reaction temperature, the consumption of organic matter and sulfuric acid increased, meaning the amount of recycled organic matter and sulfuric acid is decreasing. When the heating temperature is 170 °C and 180 °C, the proportion of residual organic matter (85%) and the recovery rate of sulfuric acid (45%) are the highest, and the concentration of recovered sulfuric acid can reach 70% by weight under these conditions. From Figure 2c, in the range of 170–260 °C, the removal rate of organic matter in recycled sulfuric acid is as high as 92–95%, indicating that most organic matter has been transformed into solid form in the heat treatment step and separated from sulfuric acid. In light of this, the separation temperature of sulfuric acid and organic matter is set at 170–180 °C. The mass fractions of the main elements (C, H, O, N, S) in solid organic matter obtained under the optimum temperature are 49.9%, 4.3%, 17.1%, 15.1% and 13.6%, respectively. The high carbon content in the residual solid organic matter enables the preparation of activated carbon.

3.2. Preparation of Activated Carbon from Residual Solid Organic Matter

The KOH or NaOH were selected as activation agent for the preparation of activated carbon due to their excellent pore-forming ability [17,18,19]. Among many characteristic parameters of the activated carbon, iodine adsorption value and MB adsorption value are two important indicators, indicating the relative number of micropores and mesopores respectively. In the State Standard [14], the index values of iodine number and MB adsorption capacity for first-grade activated carbon are >1000 mg/g and >135 mg/g, respectively. In this section, the effects of alkali-carbon ratio, activation temperature, impregnation time, washing process on the iodine value and MB adsorption capacity of activated carbon were investigated, and the elemental composition, surface morphology, specific surface area and pore structure of the prepared activated carbon were characterized.

3.2.1. Effect of Alkali/Carbon Ratio

The results (Figure 3) show that alkali/carbon ratio has a significant influence on the adsorption ability of activated carbon. When the alkali/carbon ratio increases from 0 to 2/1, the iodine value of activated carbon activated by KOH increased from 350 to 1800 mg/g, and the adsorption capacity of MB increases from 5 to 240 mg/g. The corresponding values for activated carbon activated by NaOH are from 350 to 1200 mg/g, and from 5 to 200 mg/g. Additionally, under the same conditions, the adsorption ability of activated carbon prepared by KOH activation is better than that prepared by NaOH activation (Figure 3). When KOH/carbon ratio is greater than 2/1 or NaOH/carbon ratio is greater than 1.5/1, the adsorption capacity of iodine and MB on activated carbon is basically unchanged. These results indicate that KOH activation is more beneficial to the formation of micropores than NaOH activation. Above results can be interpreted from the reaction process between KOH (Equations (3)–(14)) or NaOH and carbon. According to the reaction mechanism in previous studies [20,21,22,23,24,25,26,27], H₂, CH₄, CO and other gases are generated in the reaction process, so that the pre-activated carbon form micropores and mesopores. The resulting K or Na can also be inserted into the layers between carbon and carbon to form micropores. Therefore, with increasing alkali-carbon ratio, the production of gas, K, and Na increases, and the corresponding volumes of micropores and mesopores increase. When the ratio of KOH and carbon increases from 1/1 to 2.5/1, the micropore volume increases from 0.3495 to 0.4365 mL/g, and mesopore volume increases from 0.2392 to 0.2798 mL/g, respectively. However, when the NaOH/C ratio is higher than 1.5/1 or the KOH/C ratio is higher than 2/1, excess gas, K, or Na leads to the connection of micropores and mesopores to macropores, causing the micropore and mesopore volume to increase at the growth rate of the alkali–carbon ratio.

Based on the comprehensive consideration of the adsorption capacity of activated carbon and the cost of used NaOH or KOH, the optimal NaOH/carbon ratio and KOH/carbon ratio are selected as 2/1.

3.2.2. Effect of Activation Temperature

From Figure 4a, when the activation temperature increases from 400 to 700 °C, the iodine number of activated carbons by KOH and NaOH activation obviously increase from 410 to 1450 mg/g, and from 410 to 1200 mg/g, respectively. From Figure 4b, the corresponding MB adsorption capacity of activated carbons by KOH and NaOH activation increased from 30 to 212 mg/g, and from 60 to 190 mg/g, respectively. The above results illustrate that the increase in activation temperature is beneficial to the formation of holes. In addition, 650–700 °C is the ideal activation temperature range. Considering that the activation reaction process of KOH and carbon is similar to that of NaOH, the activated carbon prepared by KOH activation is only taken as an example to explain the influence of activation temperature on the adsorption capacity of activated carbon. At 400–600 °C, KOH reacts with –COH, –COOH, –CH₂–, or C to generate H₂, CH₄ and CO (Equations (3)–(10)), and the activated carbon forms a preliminary pore structure. When the activation temperature exceeds 600 °C, KOH is completely transformed into K₂CO₃ or K₂O in the preliminary pores. Then K₂CO₃ or K₂O is reacted with –CH– or –CH₂– to generate H₂ and CO (Equations (11)–(14)), further enlarging the preliminary pore structure of activated carbon. Moreover, the K metal formed at >600 °C is continuously inserted into the carbon inter-layer, which enlarges the layer spacing and forming more micropores. [22,23,24,25,26,27]

$$2\mathrm{KOH}\to {\mathrm{K}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{O}$$

(3)

$$\mathrm{KOH}+-\mathrm{COH}\to -\mathrm{COK}+{\mathrm{H}}_2\mathrm{O}$$

(4)

$$\mathrm{KOH}+-\mathrm{COOH}\to -\mathrm{COOK}+{\mathrm{H}}_2\mathrm{O}$$

(5)

$$4\mathrm{KOH}+-{\mathrm{CH}}_2-\to {\mathrm{K}}_2{\mathrm{CO}}_3+{\mathrm{K}}_2\mathrm{O}+3{\mathrm{H}}_2$$

(6)

$$6\mathrm{KOH}+2\mathrm{C}\to 2{\mathrm{K}}_2{\mathrm{CO}}_3+2\mathrm{K}+3{\mathrm{H}}_2$$

(7)

$$2\mathrm{KOH}+2-\mathrm{COK}+2{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{K}}_2{\mathrm{CO}}_3+3{\mathrm{H}}_2$$

(8)

$$2\mathrm{KOH}+2-\mathrm{COOK}\to 2{\mathrm{K}}_2{\mathrm{CO}}_3+{\mathrm{H}}_2$$

(9)

$$\mathrm{KOH}+-{\mathrm{CHCO}}_2\mathrm{K}+{\mathrm{H}}_2\to {\mathrm{K}}_2{\mathrm{CO}}_3+{\mathrm{CH}}_4$$

(10)

$${\mathrm{K}}_2{\mathrm{CO}}_3+-{\mathrm{CH}}_2-\to {\mathrm{K}}_2\mathrm{O}+{\mathrm{H}}_2+2\mathrm{CO}$$

(11)

$${\mathrm{K}}_2{\mathrm{CO}}_3+2-\mathrm{CH}-\to 2\mathrm{K}+{\mathrm{H}}_2+3\mathrm{CO}$$

(12)

$${\mathrm{K}}_2\mathrm{O}+-{\mathrm{CH}}_2-\to 2\mathrm{K}+{\mathrm{H}}_2+\mathrm{CO}$$

(13)

$$2{\mathrm{K}}_2\mathrm{O}+2-\mathrm{CH}-\to 4\mathrm{K}+{\mathrm{H}}_2+2\mathrm{CO}$$

(14)

Considering the adsorption capacity of activated carbon and heating cost, 650 °C is selected as the optimum temperature for activation of NaOH and KOH.

3.2.3. Effect of Impregnation Time

From Figure 5, the adsorption ability of activated carbon by NaOH activation increases rapidly with increasing impregnation time. When the contact time of pre-activation carbon with NaOH solution increases from 5 h to 48 h, the adsorption capacity of iodine of NaOH-activated activated carbon increases from 625 mg/g to 1150 mg/g, and the adsorption capacity of MB increases from 47 mg/g to 175 mg/g. This is because with the extension of impregnation time, the contact between alkali and carbon atoms is more complete, which is more conducive to the completion of the reaction between NaOH and carbon atoms [28].

Considering the adsorption capacity and time cost of activated carbon, the optimal contact time of alkali solution and pre-activated carbon is selected as 24–48 h.

3.2.4. Effect of Washing

The ash contained in activated carbon will have an adverse effect on its adsorption capacity; therefore, in most cases, it is necessary to remove the ash by washing. The experimental results show that the ash content of pre-activated carbon is 1.5–1.8% before activation, and that of activated carbon is 11.5–12.7% after alkali activation. The ash content increase of 10% following activation may be Na or K and their unreacted compounds, such as K, Na, K₂O, Na₂O, K₂CO₃ and Na₂CO₃. By boiling the activated carbon in boiling water for 2 h, the ash content is reduced to 1.2–1.5%, indicating that almost all of the remaining activator has been removed. After washing, the iodine number and MB adsorption capacity of the activated carbon by NaOH activation (conditions: alkali carbon ratio 1.5:1, activation temperature 650 °C and impregnation time 48 h) has increased from 930 mg/g to 1150 mg/g and from 160 to 175 mg/g, respectively. For KOH activation (conditions: alkali carbon ratio 1.5:1, activation temperature 650 °C and impregnation time 48 h), the values corresponding to the above increase from 1250 to 1400 mg/g and from 170 to 190 mg/g, respectively. The above results indicate that the residual intermediates from the activation process can be dissolved by washing to release the micropores and mesopores occupied by intermediates.

3.3. Characterization of the Activated Carbon

The activated carbon prepared by NaOH activation under the optimum conditions of alkali carbon ratio 1.5:1, activation temperature 650 °C, and impregnation time 48 h was characterized. The iodine number and MB adsorption capacity are only 350 mg/g and 5 mg/g, respectively, for the original organic matter sample, whereas the corresponding values are 1150 mg/g and 175 mg/g for activated carbon sample, which is better than those in the State Standard [14]. Figure 6 shows the macroscopic features before and after activation by NaOH. After activation, the close-grained carbon sample became loosened and porous. From the SEM image (Figure 7), many unopened pores and a plenty of pores can be observed in the unactivated and activated carbon samples, respectively. Moreover, the BET specific surface area, pore volume, average pore size (4 V/A by BET), and micropore volumes are 227 m²/g, 0.1280 mL/g, 2.25 nm and 0.0907 mL/g for un-activated carbon, the corresponding values are 1065 m²/g, 0.5107 mL/g, 2.16 nm and 0.4194 mL/g for activated carbon sample, indicating that activation has been successful.

3.4. The Activated Carbon Yield and Alkali Consumption

The yield of activated carbon by NaOH and KOH under the optimal activation conditions (alkali/carbon ratio = 2/1, temperature = 650 °C, impregnation time = 48 h) was estimated to be 25% and 23%, respectively. Accordingly, the alkali consumption was estimated to be approximately 3000 Kg NaOH or KOH for 1000 kg final activated carbon product. The current market prices of industrial-grade NaOH (96% by weight) and KOH (95% by weight) are approximately USD 700 and USD 1380 per ton, respectively. It is obvious that the cost of KOH activation is nearly twice as much as that of NaOH activation. However, the iodine number and MB adsorption capacity for the obtained activated carbon under the optimal conditions are 1800 mg/g and 240 mg/g for KOH activation, and 1200 mg/g and 200 mg/g for NaOH activation, respectively. Obviously, the indicators of the former product are superior to the latter, so we can choose the appropriate activator according to the required activated carbon specifications.

3.5. Comparative Adsorption Capacity with Respect to Other Activated Carbon

The iodine number and MB adsorption capacity for the obtained activated carbon under the optimal conditions are 1800 mg/g and 240 mg/g with KOH activation and 1200 mg/g and 200 mg/g with NaOH activation, respectively. In the current work, the maximum adsorption capacity of MB was achieved at 240 mg/g. These indexes meet the excellent quality standard of activated carbon for water treatment [14]. In addition, BET surface area and MB adsorption capacity were compared to those obtained in the literature for different precursors of activated carbon (

Table 1. Comparison of adsorption capacity of MB of various activated carbon precursors.

Activated Carbon Precursor	Experimental Conditions of AC Activation		BET Surface Area (m2/g)	Experimental Conditions of MB Adsorption		MB Adsorption Capacity (mg/g)	References
	Activating Agent	Activation Time (min)		Temperature (°C)	pH
Loofah sponge	NaOH	60	733	30	11	210.97	[19]
Chitosan flakes	NaOH	90	318.40	50	11	143.53	[29]
Oil palm ash	NaOH	90	615.406	50	N.A	285.71	[30]
Acid waste	NaOH	60	1065	25	7	200	This study
Bamboo chip	KOH	60	720.69	40	10	305.3	[31]
Coffee husk	KOH	120	862.2	30	7	418.78	[32]
Acid waste	KOH	60	1230	25	7	240	This study

), and are higher than most other activated carbon. Moreover, the activated carbon in this study is produced from industrial acid waste, providing a practical recycling path for the reuse of acid waste.

4. Conclusions

This study provides an innovative method for the resource recycling of acid waste containing sulfuric acid and organic matter. Firstly, acid waste is heated to a solid at 170 °C, and then the solid sulfuric acid is washed with water. The mass concentration of reclaimed sulfuric acid was able to reach 70% by weight. Secondly, activated carbon with high adsorption performance is prepared by means of a pre-carbonization and alkali activation process from the residual solid after removing most of the sulfuric acid. The optimal activation conditions are as follows: the pre-carbonization process is carried out at 400 °C for 60 min, the alkali activation process is carried out at 650 °C for 60 min, the ratio of NaOH or KOH to organic carbon is 2:1, and the immersion time is 48 h. Under the above conditions, the iodine value and MB adsorption value of activated carbon prepared by KOH activation are 1800 mg/g and 240 mg/g, respectively, and those of activated carbon prepared by NOH activation are 1200 mg/g and 220 mg/g, respectively. These indexes meet the excellent quality standard of activated carbon for water treatment. Under these conditions, the yield of activated carbon is 25% and 23% under NaOH and KOH activation conditions, respectively. Through the above process, sulfuric acid and organic matter are separated and recovered from acid waste, which not only avoids pollution resulting from acid waste discharge to the environment, but also recovers valuable resources, offering significant environmental, economic, and social benefits.