One-step synthesis of zerovalent-iron–biochar composites to activate persulfate for phenol degradation

The recovery of water from wastewater is increasingly important for the sustainable development of the world due to the reduction of freshwater resources (Lei et al. 2015). The extensive use of phenol in different industrial processes has resulted in major water contamination problems, and phenol has been listed in the US EPA's list of priority pollutants (Lisowski et al. 2017). The activation of persulfate for the oxidation of phenol is regarded as an effective technology for phenol degradation in wastewater (Duan et al. 2018). However, persulfate is not able to degrade phenol without an activator (Lei et al. 2015). Thus it is imperative to develop low-cost, efficient and environmentally friendly activators for the activation of persulfate for phenol degradation. The activation of persulfate by iron (Kim et al. 2018) and related iron oxides (Yan et al. 2011) produces sulfate radicals (SO₄^•−) (Fang et al. 2018) to degrade phenol, which is the most common activation process.

Iron and iron oxides have been extensively examined as reductants for treating various contaminants in water due to their strong redox potential (Hsueh et al. 2017), catalytic effects (Oh et al. 2017), low cost and naturally abundant material (Sun et al. 2012). Recently, research discoveries of iron and iron oxides activating persulfates to degrade phenol by using carbon nanotubes (Chen et al. 2007), nanodiamonds (Duan et al. 2016), and graphene (Lee et al. 2016; Wu et al. 2018) as supporters, have attracted unprecedented interest and inspired a myriad of studies of biochar as a sustainable catalyst for wastewater remediation (Huggins et al. 2016). Biochar derived from waste biomass is increasingly recognized as a multifunctional material for wastewater remediation applications due to biochar having the advantages of a feasible preparation method, low cost and abundant feedstocks (Wang et al. 2017). Several attempts have been made to develop upgraded biochar materials in combination with iron-bearing materials, such as biochar–Fe⁰ composites and biochar–iron-oxide composites (Sun et al. 2012; Devi & Saroha 2014; Epold et al. 2015; Jung et al. 2016; Dong et al. 2017) due to the fact that biochar materials show good performance in dispersing and stabilizing the nanoparticles (Zhou et al. 2014). In addition, biochar can provide a large surface area, porous structure, and abundant functional groups to adsorb organic pollutants on its surface, thus enhancing the performance of iron–biochar composites in environmental applications. However, the synthesis of zerovalent-iron–biochar composites is expensive due to additional chemical reactions (e.g., by NaBH₄ reduction or thermochemical reduction) (Su et al. 2013; Oh et al. 2017; Wang et al. 2017) that may occur after pyrolysis of biochar, which is generally complex and time-consuming.

To avoid the cumbersome preparation and costly additional chemical reactions, one-step pyrolysis of FeCl₃- laden biomass was chosen to synthesize the zerovalent-iron–biochar composites in this work. Atomic absorption spectrophotometry, N₂ adsorption–desorption, SEM and XRD were used to characterize the synthesis activator for analysis of structural information and composition of the zerovalent-iron–biochar composites. The effects of iron content, activator dosage, phenol concentration, pH and inorganic ions on the activation of persulfate for phenol degradation were investigated. The oxidation results of phenol by the zerovalent-iron–biochar composite activators were also studied. Further, the mechanisms involved in the activation of persulfate for phenol degradation over the zerovalent-iron–biochar composites are further identified.

The sorghum straw used in this work was collected locally in Shenyang, Liaoning Province, China. The sorghum straw was crushed with a powder machine to 0.125–0.177 mm and washed with deionized water four times to remove dirt, then dried at 80 °C. Sorghum straw powders (1 g) were firstly immersed into a FeCl₃·6H₂O solution at room temperature and the mixture was stirred for 24 h. After that, the solid residues were separated and put into a drying oven at 80 °C for 72 h. The FeCl₃-laden sorghum straw was pyrolyzed in a tube furnace at 800 °C under a nitrogen atmosphere for 2 h. The final composite products are denoted as nZVI/SBC. The FeCl₃·6H₂O/biomass impregnation mass ratios of the final products used were 0.4 g/g for 1ZVI/SBC, 0.7 g/g for 2ZVI/SBC, 1.35 g/g for 3ZVI/SBC, 2.7 g/g for 4ZVI/SBC, 13.5 g/g for 5ZVI/SBC and 27 g/g for 6ZVI/SBC, respectively. The sorghum straw biochar (SBC) was produced by the pyrolysis of sorghum straw without the loading of FeCl₃·6H₂O.

Atomic absorption spectrophotometry (AA-6880) was used to detect the composition of nZVI/SBC and the concentration of iron. N₂ physical adsorption was carried out on a Micromeritics SSA-6000 volumetric adsorption analyzer to evaluate the Brunauer–Emmett–Teller (BET) surface area, the total pore volume, and pore diameters. A scanning electron microscope (SEM, Hitachi S-4800, Japan) was employed to analyze the morphology of SBC and 4ZVI/SBC. XRD (XRD-7000, Japan) analysis of nZVI/SBC was performed using a Rigaku X-ray diffractometer with Cu K_α radiation over 2 h with a collection range of 10°–80°. The absorbance of the samples was analyzed with an 1800PC spectrophotometer. The degradation of phenol was monitored by measuring the maximum absorbance at λ = 510 nm as a function of irradiation time.

All experiments were conducted in solutions made from analytical grade chemicals and deionized water. Before degradation, 0.5 g·L⁻¹ of SBC was added into 0.2 L of 0.025 g·L⁻¹ phenol to examine the adsorption affinity of phenol without persulfate. Then a batch experiment of phenol degradation was carried out by adding persulfate under the previous conditions. To investigate the effect of FeCl₃·6H₂O/biomass impregnation mass ratios on the activation of persulfate for phenol degradation, 0.5 g·L⁻¹ of activators (nZVI/SBC with different FeCl₃·6H₂O/biomass impregnation mass ratios of 0, 0.4, 0.7, 1.35, 2.7, 13.5 and 27 g/g) were added into 0.2 L of 0.025 g·L⁻¹ phenol and 3.17 g·L⁻¹ Na₂S₂O₈ aqueous solution (pH = 6.86) at 25 °C for 30 min. The effect of 4ZVI/SBC dosage (0.1, 0.2, 0.3, 0.4, 0.5 and 1.0 g·L⁻¹) on the activation of persulfate for phenol degradation was observed in 0.2 L of 0.025 g·L⁻¹ phenol and 3.17 g·L⁻¹ Na₂S₂O₈ aqueous solution (pH = 6.86) at 25 °C for 30 min. The effect of inorganic ions (Cl⁻, SO₄²⁻ and NO₃⁻) on the activation of persulfate for phenol degradation was conducted by adding 0.5 g·L⁻¹ 4ZVI/SBC activator in 0.2 L of 0.025 g·L⁻¹ phenol and 3.17 g·L⁻¹ Na₂S₂O₈ aqueous solution (pH = 6.86) at 25 °C for 30 min. The effect of phenol concentration (0.025, 0.05, 0.1 and 0.2 g·L⁻¹) on the activation of persulfate for phenol degradation was done by adding 0.5 g·L⁻¹ 4ZVI/SBC activator in 3.17 g·L⁻¹ Na₂S₂O₈ aqueous solution (pH = 6.86) at 25 °C for 30 min. The effect of pH (3.09, 5.04, 6.86, 9.06 and 11.03) on the activation of persulfate for phenol degradation was conducted by adding 0.5 g·L⁻¹ 4ZVI/SBC activator in 0.2 L of 0.025 g·L⁻¹ phenol and 3.17 g·L⁻¹ Na₂S₂O₈ aqueous solution at 25 °C for 30 min. The chemical oxygen demand (COD_Cr) of the aqueous phase was confirmed in 0.025 g·L⁻¹ phenol and 3.17 g·L⁻¹ Na₂S₂O₈ aqueous solution (pH = 6.86) by adding 0.5 g·L⁻¹ 4ZVI/SBC activator at 25 °C for 30 min.

Specific surface area and pore distribution of nZVI/SBC were determined by N₂ adsorption–desorption (Table 1). As can be observed, 1ZVI/SBC had a specific surface area of 220.93 m²·g⁻¹ and a total pore volume of 156.95 × 10⁻³ cm³·g⁻¹. The pore diameter measured was 1.42 nm for 1ZVI/SBC (below 2.0 nm) thus indicating micro-pores (Leng et al. 2015), and hence could limit the incorporation of the phenol molecules into the pores. In contrast, 4ZVI/SBC and 6ZVI/SBC were mesopore materials according to the classification method recommended by IUPAC (Wang et al. 2017). It is worthwhile to note that the specific surface area (78.67 m²·g⁻¹), pore diameter (5.89 nm) and total pore volume (231.64 × 10⁻³ cm³·g⁻¹) of 4ZVI/SBC are larger than these of 6ZVI/SBC, benefiting the adsorption of phenol on the surface. The result is consistent with the phenol degradation results that 4ZVI/SBC was the best activator of persulfate for phenol degradation (versus 5ZVI/SBC and 6ZVI/SBC).

Razmi et al. (2017) successively removed phenol from wastewater using biochar-La as an activator to activate persulfate for phenol degradation. The specific surface area of biochar-La was 31.2 m²·g⁻¹. The specific surface area of bentonite-supported nanoscale zerovalent iron used by Diao et al. (2016) for the removal of phenol was 39.41 m²·g⁻¹. However, the specific surface area of these reported activators was lower than that of the 4ZVI/SBC activator in the paper, and the higher surface area of 4ZVI/SBC increased the ability to activate persulfate. Rahmani et al. (2018) demonstrated 93.98% removal of phenol with chelating agent Fe⁰/complex as the activated persulfate material. The phenol removal percentage was 91% employing biochar modified with iron support as a catalyst (Liu et al. 2017). Nguyen & Oh (2019) studied the degradation efficiency of phenol maximized up to 97% in an Fe(0)–biochar–persulfate system after 330 min. Relative to the past literature, the one-step synthesis of zerovalent-iron–biochar composites in our work activates persulfate for phenol degradation with high efficiency (95.65% for 30 min at 25 °C).

The effect of 4ZVI/SBC dosage on the activation of persulfate for phenol degradation is shown in Figure 4. The removal efficiency of phenol increased from 37.55% at 0.1 g·L⁻¹ to 95.65% at 0.5 g·L⁻¹ of 4ZVI/SBC. This trend can be attributed to the fact that more species are yielded with the increasing of the dosage of 4ZVI/SBC which then accelerates the decomposition of phenol. At the same time, this could be due to an increase in the 4ZVI/SBC dosage accelerating the presence of additional redox-active centers and the presence of a sufficient quantity of iron species, which serve as electron donors. However, if the 4ZVI/SBC dosage was excessive, and the excessive Fe²⁺ consumed and scavenged through electron transfer reactions, then a reducing reaction efficiency may be observed (Dong et al. 2017). From the above tests, we determined that the reasonable 4ZVI/SBC dosage was 0.5 g·L⁻¹.

The theoretical oxygen demand for the oxidation of phenol to CO₂ is an obligatory (Contreras et al. 2011) parameter that must be measured. The concentration of phenol and COD in the aqueous phase were measured as a function of time to confirm the reliability of the proposed methods. From Table 4 we can observe that the trend of phenol and COD concentration changed in the aqueous phase at different time periods. The removal efficiency of phenol increased from 83.91% at 15 min to 95.65% at 30 min, while the removal efficiency of COD values increased from 72.81% at 15 min to 95.64% at 30 min. The results revealed that COD concentration values decreased with the decrease of phenol concentration in the solution, which indicated that phenol was degraded using the activated sodium persulfate by 4ZVI/SBC into non-toxic CO₂, H₂O, and other small molecular compounds.

The zerovalent-iron–biochar composite was synthesized via a one-step pyrolysis of FeCl₃-laden sorghum straw biomass and activated persulfate for phenol degradation. The structural information and composition of the nZVI/SBC were characterized by atomic absorption spectrophotometry, N₂ adsorption–desorption, SEM and XRD. The 4ZVI/SBC was a mesopore material with a specific surface area of 78.669 m²·g⁻¹, a pore diameter of 5.89 nm and a total pore volume of 231.64 × 10⁻³ cm³·g⁻¹, which benefitted the adsorption of phenol on its surface. The 4ZVI/SBC was mainly composed of Fe⁰ with the best FeCl₃·6H₂O/biomass impregnation mass ratio of 2.7 g/g. The removal efficiency of phenol increased with increased dosage of 4ZVI/SBC from 37.55% at 0.1 g·L⁻¹ to 95.65% at 0.5 g·L⁻¹. The appearance of Cl⁻, SO₄²⁻ and NO₃⁻ affected the removal efficiency of phenol with the addition of 4ZVI/SBC in the following order: Cl⁻ < SO₄²⁻ < NO₃⁻. The optimal removal efficiency of phenol and first-order kinetic rate constant were observed at pH 3.09. The results showed that phenol was successfully transformed into CO₂ and H₂O in the presence of 4ZVI/SBC activator with a small amount of dissolved iron in the water. The one-step preparation method of 4ZVI/SBC exhibited satisfactory activation of persulfate for phenol degradation that may be used as an alternative eco-friendly methodology.

This project was supported by the National Key Research and Development Program of China (2017YFD0800301) and the National Natural Science Foundation of China (Grant Nos. 41373127, 41703129 and 41773136).