Superheated water pretreatment combined with CO₂ activation/regeneration of the exhausted activated carbon used in the treatment of industrial wastewater

Industrial wastewater refers to the production of waste water, sewage and waste that contains industrial materials, intermediates, products and by-products, some of which can be pollutants (Hu et al. 2015). Industrial wastewater mainly comes from the chemical, textile, paper, steel, and electricity industries as well as others. The types of industrial wastewater are complex. An industrial process can discharge different types of wastewater, which may contain different pollutants with different polluting effects (Reungoat et al. 2012).

Wastewater from a single production plant may contain several contaminants. Similarly, in different industrial enterprises, even if the product, raw materials and processing are different, they may discharge a similar nature of wastewater. For example, refineries, chemical plants, coking plants, etc., are all likely to discharge wastewater containing oil, phenol and other substances (Gadipelly et al. 2014).

Activated carbon (AC) is a porous material that has a high specific surface area and strong adsorption capacity. AC is extensively used in effluent treatment and is especially suitable for domestic water, industrial wastewater, sewage depth treatment, water pollution emergency treatment, and other fields (Stoquart et al. 2012; Kulkarni 2013; Hu et al. 2015). However, exhausted AC absorbs amounts of organic compounds, setting up the adsorption–desorption equilibrium, and therefore cannot be continuously used. The exhausted AC is usually discarded, buried or burned, which results in wasted resources and pollution. Therefore, the regeneration of exhausted AC has important economic and environmental significance.

Exhausted AC used in the treatment of industrial wastewater has mainly adsorbed a large number of complex and volatile oils, and phenolic, polycyclic aromatic hydrocarbon. Complex organic matter was adsorbed in the pores of AC through physical and chemical adsorption; the molecules are closely connected by van der Waals forces, plugging the pores, which makes it difficult to regenerate by the traditional regeneration method.

The conventional regeneration techniques of AC include thermal and chemical reagents as well as biological regeneration (Sabio et al. 2004; Guo et al. 2011; Toh et al. 2013). Thermal regeneration is the most extensively used in industrial applications because of its high efficiency and versatility. However, thermal regeneration typically requires heating to 800–1,100 °C, leading to high energy consumption and carbon loss, the change of the AC chemical surface structure and multiple cycle utilization failure. The chemical reagent regeneration method cannot provide a universal and effective regeneration reagent for all the complex organic compounds in wastewater. In addition, the risk of inducing secondary pollution to the environment due to the utilization of chemical reagents should be considered. The biological regeneration method also has low regeneration efficiency, long regeneration time, and poor versatility. Similarly unconventional techniques including electrochemistry, ultrasound, microwaves, electrodialysis, and plasma discharges have been tried. However, they have proven neither to be technically feasible for large scale continuous operation, nor to be economically viable. As a result, these regenerative technologies are not extensively recognized by the industry.

Water is known to be an effective organic reaction medium, as a reactant, solvent, and a catalyst in organic reactions at certain temperatures and pressures. Superheated water presents properties that are similar to strong acid and alkali (Khanjari et al. 2015, 2016). In the supercritical state, water can rapidly oxidize and decompose macromolecules, but hardly reacts with AC. Charinpanitkul (Charinpanitkul & Tanthapanichakoon 2011) regenerated two commercial ACs saturated with pyridine or phenol by using supercritical water heated to over 400 °C enhanced with hydrogen peroxide oxidation. Francisco (Salvador et al. 2013) used 400–500 °C supercritical water regeneration of phenol-containing AC, in which phenol was cracked to CO₂ and H₂O; they obtained a good performance of renewable AC. However, the pressure of supercritical water at 400 °C is 260 bar, which is demanding on the experimental equipment and difficult to popularize in practice. Furthermore, few investigations have been reported using superheated water to regenerate saturated AC used for adsorption of complex phenolic, polycyclic aromatic hydrocarbons.

In this paper, coal-exhausted AC (CEAC) first undergoes regenerated pretreatment by using superheated water with a low temperature, then the thermal regeneration method for CO₂ activation is applied. Lastly, the effects of the experimental conditions, such as the temperature of the superheated water (Tw), liquid–solid ratio (LSR), soaking time (ts), activation temperature (Ta), activation time (ta), and CO₂ flow rate (CFR) on regeneration adsorption performance and recovery rate of regenerated AC (RAC) were studied. The adsorption model, the kinetics of adsorption on methylene blue (MB), and the texture characteristics of RAC were also studied.

CEAC was provided by a domestic industrial wastewater treatment plant in Hunan. The ash element analysis is presented in Table 1, in which the main adsorbent in CEAC is complex phenolic, polycyclic aromatic hydrocarbon and contains trace metal compounds.

The AC adsorption capacity of the iodine values of the detection method refers to GB/T 7702.7-2008. The absorbance of the MB solution was measured at a wavelength of 665 cm⁻¹ using a 721-visible spectrophotometer (Tianjin Guanze Technology Co., Ltd, China). The elemental contents of AC ash were characterized by X-ray fluorescence (XRF) (Shimadzu 1800, Japan). The infrared absorption Fourier transform infrared spectroscopy (FT-IR) (Nicolet 6700, Thermo Fisher Scientific, USA) was used to measure absorption peaks ranging from 4,000 to 400 cm⁻¹ and the chemical functional groups. The specific surface area and pore size distribution were measured by the Quantachrome (QuadraSorb SI, USA) specific surface area tester. Brunauer–Emmett–Teller (BET) specific surface area and average pore size were calculated. The total pore volume was determined by N₂ adsorption and desorption experiments at a relative pressure of 0.99. Density functional theory (DFT) and the Barrett–Joyner–Halenda (BJH) method were used to analyze the pore size distributions from micropores to mesopores.

MB (3,7-bis(dimethylamino)-phenothiazin-5-ium chloride) is a widely used organic dye that can be applied to paper and textile colorants. The MB solution is extensively used as a model compound in the process of adsorption and decoloration of organic matter in wastewater (Hameed et al. 2007). The adsorption capacity of MB is used to study the surface properties of AC and its affinity to adsorbents like complex phenolic, polycyclic aromatic hydrocarbon as well as the adsorption of larger molecules of organic matter to determine the size of the pores. In this paper, MB (Sinopharm Group Chemical Reagent Co. Ltd) was used as the model compound, and the Langmuir and Freundlich adsorption model, pseudo-first-order and pseudo-second-order kinetics, and the intraparticle diffusion model were used to quantitatively describe the adsorption process and adsorption mechanism.

Figure 1(d)–1(f) show the results of the fixed thermal pretreatment process (Tw = 180 °C, ts = 3 h, LSR = 8:1), in which the relationships between Ta, ta, CFR, respectively and regeneration efficiency and recovery rate were investigated. The figure shows that the regeneration efficiency increased as the activation temperature and the activation time increased (Figure 1(d) and 1(e)). When Ta = 600 °C and ta = 30 min, the regeneration efficiency reached 101.05%. At this time, Ta and ta continued to increase and the regeneration efficiency did not obviously change; however, the recovery rate significantly decreased. When Ta and ta were further increased, the regeneration efficiency did not improve but the recovery rate significantly decreased because the increase in temperature and time exacerbated the reaction between carbon dioxide and carbon, producing new pores that simultaneously cause the collapse of the original pores. Figure 1(f) shows that an appropriate increase in CFR enhances the regeneration efficiency of AC. However, when the CFR is excessive (more than 0.7 L min⁻¹), the diameter of the primary pores is destroyed, the pores are enlarged, the micropores change into mesopores and even macropores, and the regeneration efficiency and recovery rate decrease further.

The preceding experimental results show that the following are the optimal conditions for AC regeneration from superheated water pretreatment combined with CO₂ thermal regeneration: the temperature of the superheated water, LSR, soaking time, activation temperature, activation time, and CO₂ flow rate are 180 °C, 3 h, 8:1, 600 °C, 30 min, and 0.7 L min⁻¹, respectively. The regeneration efficiency of AC is 102.25%, and the recovery rate is 79.8%.

The Langmuir and Freundlich adsorption models are the most commonly used AC adsorption models (Hameed et al. 2007). The experiments were conducted by adding 200 ml of different concentrations (230, 260, 290, 320, and 350 mg L⁻¹) of standard MB solution and 0.2 g RAC to a 500 ml grinding conical flask and maintaining a constant temperature oscillator for 7 h to reach adsorption equilibrium.

The pseudo-second-order kinetic equation fits precisely because the model is based on chemical reactions or chemical adsorption via electron sharing, gain and loss. This equation includes all processes of adsorption, such as external liquid film diffusion, surface adsorption, and intra-particle diffusion, which can more realistically and comprehensively reflect the adsorption mechanism of organic matter on AC (Chang & Juang 2004). By contrast, the pseudo-first-order equation is less well fitted to the MB adsorption behavior because of the limitations in the calculation of boundary conditions. The pseudo-first-order model should provide the Qe value before fitting. However, reaching the adsorption equilibrium in the actual adsorption process is time-consuming, and equilibrium is a dynamic process; therefore, accurately measuring the equilibrium adsorption amount is difficult.

The BJH method for the characterization of microporous AC is inaccurate and unreliable for an in-depth study (Thommes et al. 2015). Therefore, DFT is required to correctly evaluate RAC micropores and narrow mesopores. The pore size is clearly mainly distributed from 0 to 4 nm with the micropores and the narrower mesopores; this result is consistent with the BJH test result. The peak value at 1.83 nm is 0.10338 cm³ nm⁻¹ g⁻¹. The textural properties of RAC are shown in Table 4, in which the specific surface area, total pore volume, micropores, and mesopores are 763.39 m² g⁻¹, 0.3039 cm³ g⁻¹, 0.2425 cm³ g⁻¹ and 0.0614 cm³ g⁻¹, respectively, which are all higher than the values of CEAC and close to fresh AC. A certain amount of narrow mesopores ensures the adsorption of larger molecules by RAC.

Superheated water pretreatment combined with traditional CO₂ activation can regenerate the CEAC used in the treatment of industrial wastewater. Lower regeneration temperatures reduce energy consumption. The regeneration efficiency and recovery rates are 102.25% and 79.8%, respectively. The repeated regeneration effect is remarkable. The adsorption model of RAC is consistent with the monolayer Langmuir model. The pseudo-second-order kinetics and the intraparticle diffusion model confirm that the RAC strongly absorbs MB. The FT-IR analysis showed that RAC has a wider adsorption peak at the oxygen-containing functional groups than that of CEAC, with more oxygen-containing functional groups and a stronger adsorptive capacity. The RAC mainly comprises micropores and narrow mesopores with a specific surface area of 763.39 m² g⁻¹.

This work was financially supported by the National Natural Science Foundation of China, Projects (51374253 and 51574289) and Graduate students to explore innovative projects (2017zzts452).