Improvement of dyes degradation using hydrofluoric acid modified biochar as persulfate activator

ABSTRACT The hydrofluoric acid modified biochar (HF-BC) was obtained, aiming to improve its performance for activating persulfate (PS) to make acid orange 7 (AO7) degraded in water solution. Results showed that the surface area, micropore volume, carboxyl and phenolic hydroxyl group content of HF-BC increased by 171%, 172%, 23.8% and 50%, respectively, compared to unmodified biochar (BC). The decolorization of AO7 in HF-BC/PS and BC/PS systemwas much more rapid and efficient than that in HF-BC, BC and PS alone system. For the HF-BC/PS system, the AO7 was removal of 99.8%, much higher than that of BC/PS system, probably because of its better surface characteristics. Moreover, for the HF-BC/PS system, it was the higher PS concentration and HF-BC dosage that caused greater AO7 degradation rates, and lower pH was favorable for the degradation. Our results provided a novel method to design biochar as an activator for persulfate-based remediation of dye wastewater. Abbreviations: HF-BC, hydrofluoric acid modified biochar; BC, unmodified biochar; AO7, Acid orange 7; PS, persulfate


Introduction
In recent years, the dyes wastewater pollution, generated from the textile, printing, paper, dyeing as well as food-stuff industry discharge, has become an extrusive environmental issue and drawn more and more concern [1][2][3]. Azo dyes account for nearly 50% among global production of commercial dyes, the molecular structure of which includes the chromophore of -N=Nunit, making it difficult to degrade with biological treatment methods [4][5][6]. The conventional treatment processes like flocculation and adsorption are inadequate for treatment of azo dye wastewater with high concentration and chroma. Moreover, they also cause solid waste, which creates additional environmental problems that require further treatment [7,8]. Therefore, it is urgent to find an appropriate and efficient way to remove azo dye from wastewater.
With the development of advanced oxidation processes (AOPs), the organic pollutants with toxicity and bio-refractory features in wastewater can be degraded [9]. Activation persulfate (PS) oxidation has been viewed as a burgeoning AOP for degradation of the organic pollutants [10]. Although PS has limited capacity to make organics oxidized in an independent manner, it could be activated by transition metal ions (mainly iron), heat or UV-light to produce free radicals SO 4 − • [11][12][13][14][15], with oxidation-reducted potential ranging from +2.5 to +3.1 V vs. NHE [16], making a great majority of the organics oxidized and decomposed in water [17]. As a stable carbon-rich by-product, biochar (BC) is derived from biomass pyrolysis under the conditions of limited oxygen and low temperature. It attracted more and more attention owing to multi-functionality including soil improvement, waste management, climate change, energy production and pollution remediation [18]. Furthermore, most studies showed that biochar was an extremely effective and cost-efficient sorbent in removal of contaminant [19,20]; moreover, biochar like other carbon materials could also use as a catalyst to activate H 2 O 2 or PS for organic pollutant degradation [21][22][23][24][25]. Fang, et al. pointed out that persulfate was activated by biochar for production of sulfate radicals to make polychlorinated biphenyls degraded [24]. Huang et al. also showed that biochar could influence hydroxyl radical generation and sulfamethazine degradation [26]. Kemmoua, et al. reported that persulfate activation seems to occur on biochar surface through interaction with the surface functional groups [25]. All of them pointed that the catalytic effect and mechanism depend on biochar characteristics including specific surface area and functional group. And that biochar with high specific surface area and abundant functional group is considered as a good catalyst.
In order to design high-performance biochar for catalyzing PS, rice-hull was used as a biomass material to prepare biochar sampler with or without HF solution pretreatment, which is used for the removal of mineral matter from biomass beneficial to the development of porous structure for biochar production [27][28][29]. Azo-dye acid orange 7 (AO7) usually serves as a kind of model compound used for dyes degradation [4,9,30,31]. AO7 degradation into an aqueous solution by using those two biochars as persulfate activators were studied in this paper. The aims of this research included: (1) to check the improvement of AO7 degradation using modified biochar as persulfate activator and its mechanism; (2) to testify the factors that influence modified biochar activate PS for AO7 degradation in aqueous solution.

Materials
Purchase of materials was made from Sinopharm Chemical Reagent Company, such as sodium thiosulfate (Na 2 S 2 O 3 ) and AO7 (4-(2-hydroxy-1-naphthylazo) benzenesulfonic acid). All chemicals adopted in this study are analytical grade.
Collection of rice hull was made from a rice manufactory in the suburb of Bengbu, China, air-dried to make the water content below 5% and then ground to <0.15 mm. Biochar was produced in a laboratoryscale pyrolysis system under an N 2 atmosphere with a heating rate of 15°C min −1 to reach the temperatures at 500°C and the heating time was kept for 2 h [32]. Residues in the solid state left within the reactor were referred to as the biochar. In order to prepare HF-modified biochar, 50 g of rice hull powder was mixed with 1000 mL of HF solution (2 mol L −1 ) in a plastic container. After stirred uniformly, the container was under ambient conditions (25°C) for 24 h and then the treated rice hull was separated and washed with deionized water until the Fcannot be detected, after that it was dried at the temperature of 80°C to a constant weight. The dried rice hull was used to produce HF-modified biochar. What's more, for simplicity, biochar with and without HF pretreatment was referred to as BC and HF-BC, respectively.

Degradation experiments
Experiments had been performed in batches within 500 mL beaker and shaken by using the thermal oscillator tank, with the agitator rotation at the speed of 150 r min −1 under the temperature of 25°C. AO7 solution (200 mL, 20 mg L −1 ), PS as well as biochar were placed in a simultaneous manner into a beaker at the beginning of every experiment. Through using 5 mL injection syringe for analysis, withdrawal of those samples was performed within preset time intervals from the beaker. And the molar ratio of PS/AO7 was 100: 1, with 5.0 g L −1 of biochar dosage. The reaction time was 24 h.
Effect of initial pH from 3.5 to 10.5 adjusted by 0.1 mol L −1 NaOH or 0.1 mol L −1 H 2 SO 4 , on the decolourization of AO7 was studied. Moreover, effects of biochar dosage (1 to 10 g L −1 ) and PS concentration (molar ratio of PS/AO7 ranging from 50/1 to 700/1) on the decolourization of AO7 were also studied.

Analysis
The monitoring of AO7 concentration was performed through measurement of the maximum absorbance at λ = 484 nm by using an ultraviolet spectrophotometer (UV1800, Shimadzu, Japan). With application of an element analyzer, total C and H content of biochar was analyzed (Vario ELIII, Elementar, Germany). In addition, certain surface area as well as distribution of biochar pore size was also determined by utilizing the BET-N 2 SA analyzer (JW-BK222, China). Acidic groups of biochar surfaces were characterized via the Boehm titration [33]. Fourier transform infrared (FTIR) spectroscopy was used to make a determination of the distributions of surface functional group of biochar (IR Prestige 21 FTIR, Shimadzu). The morphology as well as surface atomic composition of biochar was analyzed by adoption of a scan electron microscope with energy-dispersive spectra (SEM-EDS, Zeiss EVO LS-185, England).

Characterization of the biochar and the HF modified biochar
The physical-chemical properties of both BC and HF-BC are shown in Table 1. The pH of BC was alkaline, the similar results were observed in many former researches [34,35]. Conversely, hydrofluoric acid treatment led to the pH value of HF-BC was reduced to 6.21. Compared to BC, the yield of HF-BC was reduced by 36.4%, however, the total C and H content of HF-BC increased by 72.9% and 24.7%, respectively. The H/C of HF-BC was much lower than that of BC indicated that the stability of HF-BC improved compared to BC [32]. Moreover, both the surface area (SA) and micropore volume (MV) of HF-BC were also increased by up to 2.7 times except that the average pore diameter (APD) was reduced about 20%, compared to BC. Phenolic hydroxyl of the two biochar was low. Compared to BC, the carboxy groups of HF-BC increased by 23.8%, on the contrary, the lactonic group contents reduced by 45.8%. The SEM images of BC and HF-BC were shown in Figure 1. The rice hull cell morphology was still clearly observed after pyrolysis and it clearly showed that biochar had a porous structure. The quantitative surface chemical compositions were presented in Table 2. The C content of HF-BC was higher than that of BC, indicating accordance with the result based on element analysis (Table 1). Moreover, the content of Si and O in HF-BC was obviously lower, indicating that the differences between the two biochars were probably due to the decrease of SiO 2 with HF pretreatment.
It was the FTIR spectra that clearly demonstrated changes in surface functional groups of biochars with/ without HF modifications ( Figure 2). And its absorption peak at 3428 cm −1 represents the O-H stretching vibration, and 2935 cm −1 , 2843 cm −1 and 1407 cm −1 assigned to -CH 2 groups [36], respectively, and there was no obvious difference between the two biochars. Nevertheless, the peak at 1700 cm −1 resulted from the carboxyl C = O stretching vibration [37,38] obviously appeared in HF-BC in accord with the Boehm titration analysis results ( Table 1). The C = C peaks of 1610 [39] intensities remained nearly unaltered. Relatively, the intensities of the Si−O−Si peaks at 1100 cm −1 and 800 cm −1 [27,40] obviously decreased of HF-BC, indicating that removal of Si of HF pretreatment in accord with the SEM-EDS analysis results (Table 2).

Decolorization of AO7 under different systems
AO7, as a representative azo dye, is either absorbed by biochar or oxidized by PS as presented in Figure 3. When the initial concentration of AO7 was 20 mg• L −1 , 23.7% to 27.0% of AO7 was adsorbed by the sole biochar (BC or HF-BC) and about12.6% AO7 was oxidized by the sole PS within 11 h. Biochar together with PS can induce decolorization of AO7 in a more significant manner with its removal rate ranging from 53.2% to 99.8%. Hence, an important synergistic effect was seen in the combined system of BC/PS and HF-BC/PS. The outcomes were similar to other studies [41,42]. Compared to the BC, HF-BC showed a good catalyst for PS decomposition activation and the AO7 degradation induction at ambient temperature ( Figure 3) probably due to its high surface area and micropore volume (Table 1). Good development of surface area may make it very convenient for the existence of substantial un-saturated carbon atoms as well as the functional group, which results in great concentration of the unpaired electron and active site [26]. Moreover, the higher surface area stood for more chances for S 2 O 8 2ions to become capable of interacting with active spots of biochars in this study. Furthermore, HF-BC has much more carboxyl and phenolic hydroxyl   Figure 2) that were beneficial for activating S 2 O 8 2to produce more SO 4 − • [22,43] led to more AO7 degradation. Some studies reported that the longlasting free radical was found within biochars to make persulfates activated for contaminant degradation [23,24]. Whether the kinds or/and quantity of the persistent free radicals changing in both unmodified biochar and HF-modified biochar in this study need to explore in the future. All of which offered a new idea that the designated biochar could be explored and produced with great catalysis ability. Figure 4 shows that the increase in the concentration of PS may accelerate AO7 degradation in the combined system of HF-BC/PS. 46.1-92.8% of AO7 was degraded in 3 h, with the molar ratio of PS/AO7 increasing from 50/1 to 700/1. After 11 h, almost 100% of AO7 was degraded under different PS/AO7 molar ratios except for the ratio of 50/1 with 90.2% removal. Nevertheless, no obvious raise was found for the removal of AO7 as the oxidant increased, when the ratio of PS/AO7 was >300: 1. These similar outcomes had been revealed in prior researches [44,45]. It was found that a slightly slow increment was seen in the degradation rate of organics, with the concentration of PS rising above a certain numerical value. According to the report, SO 4 − • that was produced by PS catalysis could react with superfluous PS for the formation of SO 4 2− [46]. Figure 5 demonstrates the effect of HF-BC dosage on AO7 decolorization. Within 3 h, 18.0% to 98.6% of AO7 was degraded by the HF-BC/PS-combined system with the HF-BC dosage of 1 g L −1 to 10 g L −1 , respectively. It was clear that with the increment in HF-BC dosage, a quicker AO7 bleaching rate got enhanced in this HF-BC/PS combined system. As for its reason, higher HF-BC surface would increase the number of active spots for the purposes of adsorption and catalytic reaction.

Effect of initial pH
Additionally, study on the initial pH effectiveness was performed at the HF-BC dosage of 5.0 g/L, PS/AO7 molar ratio 100/1. As the aqueous phase pH raised from 3.5 to 10.5, the decolorization efficiency of AO7 was obviously decreased from 100% to 27.2% ( Figure  6). Under the conditions of acidic and neutral pH, SO 4 − • was the primary radical in the solution. When the solution pH was under alkaline, SO 4 − • react with OHto generate •OH, and the lifetime of the •OH is shorter than that of SO 4 − • led to low decolorization efficiency of AO7 [15,45]. Moreover, the pH PZC of HF-BC in our researches was about 5.5, when the solution pH <5.5, the surface of HF-BC was positive charge which was beneficial to attract S 2 O 8 2ion (negative charge) on its surface where persulfate activation seems to occur on through interaction with the surface functional groups   generating radicals to degrade AO7 [25]; in contrast, when the solution pH ＞ 5.5, the surface of HF-BC was negative charge and with pH increased, the negative charge strength enhanced, which repelled S 2 O 8 2ions easily led to the free radicals decrease from HF-BC surface and decolorization efficiency of AO7 was obviously declined. Therefore, the pH of the aqueous solution had great influence in organic degradation and lower pH was favorable for organic degradation in the combined system of HF-BC/PS.

Conclusions
In summary, in contrast with the process of simple use of PS or biochar, the process of combined use of PS and biochar played a more effective role in removing AO7. The HF-BC had obviously advantage for catalyzing PS to degrade the AO7 indicated that it was a good way for exploration and production of specified biochar featured with great catalysis ability by HF pretreatment. For the HF-BC/PS system, with the increase of PS concentration and the HF-BC dosage, an improvement was seen in the degradation efficiency. The pH of an aqueous solution had an important impact on organic degradation. Moreover, lower pH was favorable for its degradation. HF-BC can be easily gained through this easy-to-operate combined system, which demonstrates that HF-BC/PS has a potential practical application. It is important to note that this research only adopted a biomass of rice hull; tests on various biomass feedstocks ought to be carried out and further study should be made in the future in terms of HF-pretreatment effectiveness and the persistence of catalytic ability.

Disclosure statement
No potential conflict of interest was reported by the authors.  Figure 6. Effect of initial pH on the decolorization of AO7 by HF-BC/PS system. PS/AO7 molar ratio = 100/1, HF-BC dosage = 5 g/L.