Grantee Research Project Results
Final Report: Control of pathogens and pharmaceutical compounds by biochar-supported photocatalysts under solar light irradiation
EPA Grant Number: SU836139Title: Control of pathogens and pharmaceutical compounds by biochar-supported photocatalysts under solar light irradiation
Investigators: Kan, Eunsung
Institution: University of Hawaii at Manoa
EPA Project Officer: Page, Angela
Phase: I
Project Period: September 1, 2015 through August 31, 2016
Project Amount: $14,999
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2015) RFA Text | Recipients Lists
Research Category: P3 Awards , Pollution Prevention/Sustainable Development , Sustainable and Healthy Communities , P3 Challenge Area - Safe and Sustainable Water Resources
Objective:
The goal of this project is to effectively remove microbial pathogens and pharmaceutical compounds from reclaimed wastewater using a novel solar light-assisted photocatalytic process. The specific tasks include: 1) design, synthesize and characterize a biochar-supported solar light-assisted multifunctional photocatalyst; 2) understand adsorption of pharmaceutical compounds and pathogens onto the biochar-supported photocatalyst; and 3) assess the oxidative degradation/inactivation of pharmaceutical compounds and pathogens in terms of the removal efficiency of target pharmaceutical drugs, mineralization, and toxicity.
In an effort to solve the problem of increasing water consumption and water shortages, the reuse of treated wastewater has been suggested as a viable solution for providing water resources for agricultural irrigation/practice in many states in the U.S. However, the reuse of treated wastewater can pose serious risks to human health and the environment mainly from the occurrence of various microbial pathogens and chemical pollutants (i.e., pharmaceutical compounds) in wastewater. Since conventional wastewater treatment cannot effectively remove various microbial pathogens and pharmaceutical compounds, innovative solutions to effectively remove these contaminants from wastewater treatment need to be developed.
Summary/Accomplishments (Outputs/Outcomes):
Design, synthesize and characterize a biochar-supported solar light-assisted multifunctional photocatalyst.
Because solar light includes visible light (most of solar light, > 400 nm) and ultraviolet light (5% of solar light, < 360 nm), we prepared the biochar-supported TiO2 (biochar/TiO2) working with ultraviolet light in solar light and the biochar-supported CuS (biochar/CuS) working with visible light in solar light. We also considered the biochar-supported TiO2/Fe2O3 as the photocatalyst working with visible light in solar light. However, the biochar/TiO2/Fe2O3 showed negligible adsorption and poor photocatalytic oxidation (adsorption efficiency of 1-5%, photocatalytic oxidation efficiency of 3-8% with 1-3 g biochar/Ti/Fe2O3 for L of 10 ppm SMX). Such poor adsorption and oxidation efficiency of biochar/TiO2/Fe2O3 was much lower than those by the biochar/CuS (adsorption and photocatalytic oxidation efficiencies up to 80- 98% with 1-3 g biochar/CuS per L of 10 ppm SMX). Thus, in this project, we designed and synthesized two photocatalysts (a biochar-supported TiO2 and a biochar-supported CuS) which can work under solar light irradiation. These photocatalysts were used for removal of antibiotics and microbial pathogens in water. The biochar/TiO2 was prepared using the sol-gel method modified from that reported by Wang et al [1]. The raw biochar was grounded and sieved to make the 30 x 40 mesh size (425 – 600 µm) similar to the size of commercial granular activated carbon used for water treatment. The acid-treated biochar was prepared by suspending the raw biochar in nitric acid at pH 3 for 4 d. The acid treatment of biochar was to increase the acidic surface oxides which would increase attachment of Ti6+ from titanium isopropoxide, the precursor of Ti. The biochar-supported TiO2 was made when the acid treated-biochar was reacted with titanium isopropoxide in ethanol 5 followed by calcination at 325 °C for 1 h. For the synthesis of biochar/CuS photocatalyst, the CuS was immobilized onto a biochar (30 x 40 mesh size) at room temperature using the method modified from the one reported by Yu et al. [2]. Briefly, 50 mL of biochar solution (~ 10 g/L) were mixed with 100 mmol of Cu(NO3)2, 100 mmol of C2H5NS and 200 mL of DMSO. The mixture was stirred for 24 h at ambient temperature to ensure CuS nanoparticles anchored on the surface of biochar. The products were centrifuged and rinsed with acetone, ethanol and DI water several times to remove the residual. The resulting solid samples were freeze-dried for 2 h under − 50 °C to obtain the final biochar/CuS catalysts. In addition, the biochar-supported TiO2/Fe2O3 was prepared by suspending 2.5 g biochar in 60 mL ethanol, 20 mL titianium isoproxide and 20 mL Fe2+ solution (10 g/L) as modified from Adan et al.‘s method [3] . The reaction was made for 2 h under vigorous stirring at ambient temperature and pH followed by another reaction with addition of 8 mL HCl (37%) and 20 mL ethanol.
The biochar/TiO2/Fe2O3 was rinsed with ethanol and DI water. After being dried in the oven for 24 h, the biochar/TiO2/Fe2O3 went through calcination at 325 oC for 1 h in the furnace. The biochar-supported TiO2 and biochar-supported CuS were characterized by the scanning electron microscopy (SEM), and the energy dispersive X-ray spectroscopy (EDX). The SEM images of biochar, biochar/TiO2 and biochar/CuS are presented in Fig. 1. The images exhibited that tiny TiO2 and CuS granules were well dispersed on the biochar with little agglomeration. The energy dispersive X-ray spectroscopy (EDX) were also measured and confirmed the presence of TiO2 and CuS in the biochar (Table 1).
[FIGURE 1]
[INSERT TABLE 1]
The XRD patterns of the TiO2 and biochar/TiO2 showed that the diffraction peaks at 2θ=25.3, 37.8, 48.1 and 54.1 in the biochar-TiO2 were attributed to anatase-TiO2 (Fig.2) [4], which confirmed that the majority of TiO2 in the biochar/TiO2 sample was anatase-TiO2. The XRD patters of biochar/CuS will be also analyzed before the end of this project. Besides, the BET surface analysis revealed the surface area of biochar and biochar/TiO2 were 7.40 m2 /g and 33.41 m2 /g.
[INSERT FIGURE 2]
Understand adsorption of pharmaceutical compounds onto the biochar-supported photocatalysts.
The adsorption capacities of the biochar/TiO2 and biochar/CuS were investigated before the photocatalytic reactions occurred since many (photo-) catalytic reactions are mass transfer or adsorption limited. First, the adsorption capacity of the biochar/TiO2 and the biochar/CuS were compared at the same conditions (Fig. 3). Both biochar/CuS and biochar/TiO2 adsorption capacities were high enough to attract SMX onto the photocatalysts which would increase the photocatalytic reactions at locally higher concentration of SMX. However, the biochar/CuS showed higher adsorption of SMX than the biochar/TiO2 because the biochar/ CuS had higher π-π interaction for SMX than the biochar/TiO2. Compared with these photocatalyst the commercial TiO2 powder exhibited little adsorption of SMX owing to negligible π-π interaction.
[INSERT FIGURE 3]
The adsorption capacities of SMX onto the biochar/CuS and the biochar/TiO2 were found to be 3.3 – 16.3 mg SMX per g biochar/CuS and the biochar/TiO2 at the selected conditions. The Kd (solid-water distribution coefficient) was 261 L/kg (for biochar/CuS) and 176 L/kg (for biochar/TiO2) which were higher than those for the biochar made from bamboo, sugarcane and hardwood (i.e., 60 – 104 L/kg) reported by Yao et al. [5]. This adsorption capacity of the biochar-supported photocatalysts would enhance the overall photocatalytic oxidation efficiency since the previous studies showed the synergy of adsorption and photocatalytic (also catalytic) reaction to enhance overall reaction efficiency [6-8].
For further analysis of adsorption onto the biochar/TiO2 and biochar/CuS, the Freundlich and Langmuir isotherms were applied to investigate the overall adsorption efficiency representing two of the most extensively adopted models for describing adsorption phenomena in aqueous solutions [9]. The isotherm parameters of the Freundlich and Langmuir models were calculated using the nonlinear analysis as shown in the method section, and are listed in Table 2. Table 2 supports that Langmuir isotherm is the better-fitted model than Freundlich model for adsorption of SMX onto the biochar/TiO2 and biochar/CuS indicating a monolayer adsorption.
[INSERT TABLE2]
Assess the oxidative degradation/inactivation of pharmaceutical compounds and pathogens.
Degradation of SMX in water by the biochar/TiO2 (under UV light irradiation) and the biochar/CuS (under visible light irradiation).
Fig. 4 compares the direct photolysis of SMX, the photocatalytic oxidation of SMX using the commercial TiO2 powder (0.1 g and 0.5 g TiO2) and the biochar/TiO2 (0.5 g biochar/TiO2 containing 0.015 g TiO2) under UV-C light irradiation.
[INSERT FIGURE4]
The direct photolysis (UV light alone) and photocatalytic oxidation using commercial TiO2 powders (0.5 and 1 g) led to nearly complete removal of SMX in 3h. On the other hand, the photocatalytic oxidation using the biochar/TiO2 exhibited 75% removal efficiency of SMX (considering the residual SMX in the aqueous phase and the biochar/TiO2 after the photocatalytic reaction). Higher removal of SMX by the direct photolysis was made because the SMX had high absorption of UV-C light at 250-270 nm (maximum absorption at 260 nm [10]) which was quite close to the major wavelength (254 nm) of UV-C light from the UV lamp for this study. On the contrary, the presence of TiO2 resulted in lowering the removal of SMX by the biochar/TiO2. Since TiO2 also absorbs effectively UV-C light at 250-260 nm, SMX and TiO2 would compete for the photons in the UV-C light which caused less efficient removal of SMX. However, the COD removal of SMX (as an indicator for mineralization of SMX) by the biochar/TiO2 was almost four times higher than that by the direct photolysis. It indicated the biochar/TiO2 resulted in effective oxidation of SMX and its oxidation products leading to high mineralization of SMX (~ high removal of COD) because TiO2-mediated photocatalysis was heavily dependent on OH radical-driven oxidation.
The photocatalytic degradation of SMX with the biochar/TiO2 was investigated: the effects of biochar/TiO2 loading (Fig. 5A) and the effects of UV irradiation time (Fig. 5B). It was also interesting that increase of the biochar/TiO2 loading resulted in slight decrease in SMX removal efficiency (Fig. 5A). The actual amount of TiO2 in the biochar/TiO2 were found to be 3 - 3.4% of total mass of the biochar/TiO2. As mentioned in Nasuhoglu et al and Abellán et al. [10, 11], the direct photolysis by UV light is more effective mechanism to remove SMX itself than TiO2-mediated photocatalytic oxidation. The higher loading of biochar/TiO2 led to higher absorption of UV light (photons) onto the TiO2 in the biochar/TiO2 while lowering the contact between UV light and SMX and decreasing the photolysis efficiency of SMX. However, the higher loading of biochar/TiO2 enhanced the COD removal efficiency due to the higher generation of OH radicals.
[INSERT FIG5]
As shown in Fig. 5B, the effects of UV irradiation time on the removal of SMX and COD were examined at the selected conditions (5 g biochar/TiO2 per 1 L SMX solution, initial pH of 4, 0-6 h UV irradiation). The SMX removal efficiency increased up to 91% while the COD removal efficiency as an indication of SMX mineralization also increased to 81% for 6 h UV irradiation. Thus, these results indicated effective decomposition of the SMX and SMX oxidation products with increasing UV irradiation time during the photocatalytic oxidation. The biotoxicity tests using the Daphnia magna and the E. coli expressing β-galactosidase (Toxi-Chromotest™) exhibited minimal inhibition of the SMX oxidation products generated by the photocatalytic oxidation with 6 h UV irradiation. Particularly only 20% immobilization of Daphnia magna was obtained after the 6 h photocatalytic oxidation which is acceptable in drink water as described in US EPA guideline (2002). The negligible biotoxicity from the reaction effluent containing SMX oxidation products was mainly due to the high conversion (91%) and mineralization (81%) of SMX resulting in the accumulation of nontoxic final products such as sulfate, nitrate and small organic acids (i.e., oxalic, maleic, formic, and acetic acids).
On the other hand, the photocatalytic degradation of SMX by the biochar/CuS was investigated as the biochar/CuS increased (Fig. 6A).
[INSERT FIG6A-B]
Surprisingly there were 92-97% removal of SMX in 4 h visible light irradiation when 1-5 g biochar/CuS per 1 L SMX solution were used. Compared with the SMX removal by the biochar/TiO2 under UV light irradiation, the biochar/CuS showed the excellent degradation of SMX even under the visible light irradiation with much lower light intensity and energy consumption. It indicated high potential for the visible light-assisted photocatalytic process using the biochar/CuS. Such excellent performance was also proved by Yu et al. [2] for highly effective degradation of dyes and E. coli disinfection. Besides, the effects of visible light irradiation time on the removal of SMX using the biochar/CuS at the selected conditions (5 g biochar/TiO2 per 1 L SMX solution, 0-4 h visible light irradiation) as shown in Fig. 6B. The SMX removal efficiency increased from 67% to 92% for 4 h visible light irradiation. Thus, these (A) (B) 0 10 20 30 40 50 60 70 80 90 100 0 1 2 3 4 5 SMX removal % BC/CuS loading (g/L) 0 10 20 30 40 50 60 70 80 90 100 0 1 2 3 4 SMX removal % Visible light irradiation time (h) 10 results supported effective decomposition of the SMX with increasing visible light irradiation time during the photocatalytic oxidation.
Finally, the photocatalytic degradation of SMX by the biochar/TiO2 and biochar/CuS under solar light irradiation were comparatively investigated. These experiments were conducted on the roof of College of Tropical Agriculture and Human Resources at the University of Hawaii (using natural sunlight).
[INSERT FIG7]
The results in Fig. 7 emphasized that the biochar/TiO2 effectively degraded the SMX with natural solar light. Compared with biochar/TiO2, the biochar/CuS demonstrated slower and less effective degradation of SMX with the natural solar light probably because of relatively low loading of photocatalyst and lack of photons from visible lights in the solar light.
Inactivation of E. coli in water by the biochar/TiO2 (under UV light) and the biochar/CuS (under visible light).
Various methods to control microbial pathogens have been also investigated, including advanced oxidation, physical separation and electrochemical disinfection [12-15]. Among various methods, the photocatalytic oxidation have shown highly efficient inactivation of various microbial pathogens under UV or visible light irradiation [16]. OH radicals generated from lightexcited photocatalysts (i.e., TiO2, CuS) can effectively inactivate various pathogens via direct or indirect routes [17, 18]. Recently the visible light-assisted photocatalysts have also shown excellent removal of microbial pathogens in wastewater and water. The antibacterial activity of biochar/TiO2 and the biochar/CuS photocatalysts were evaluated with ultraviolet light (for biochar/TiO2) and visible light (for biochar/CuS) using E. coli as the bacteria model (Fig. 8). The biochar/CuS revealed the excellent inactivation of E. coli (92-94% removal) for 2-4 h visible light irradiation while the biochar/TiO2 showed moderate inactivation of E. coli (51% removal) for 6 h (Fig. 8). It indicated more effective inactivation/control of microbial pathogens by the biochar/CuS under visible light irradiation with lower light intensity than the biochar/TiO2 using high light intensity.
The biochar/CuS revealed the excellent inactivation of E. coli (92-94% removal) for 2-4 h visible light irradiation while the biochar/TiO2 showed moderate inactivation of E. coli (51% removal) for 6 h (Fig. 8). It indicated more effective inactivation/control of microbial pathogens by the biochar/CuS under visible light irradiation with lower light intensity than the biochar/TiO2 using high light intensity.
As the results shown in Fig. 8, the CuS in the biochar showed high antibacterial activity, indicating CuS would play an important role in the toxicity. A possible mechanism is that Cu2+ leached from CuS during the photocatalytic process can form precipitation with biomolecules or binding onto some metallo-proteins causing cell inactivation. The improved inactivation efficiency is attributable to the synergistic effect in photocatalysis process by combination of adsorption and photo-oxidation occurred at the surface of biochar/CuS as illustrated in Fig. 9. The ROS (reactive oxygen species) and radicals would attach the bacterial cells at the surface of biochar/CuS when visible light excites the surface of biochar/CuS.
[INSERT FIG9]
Outcome/Outputs
This project demonstrated high potential of the biochar/CuS as an effective photocatalyst working under solar light irradiation (specifically, visible light regime). The biochar/CuS was easily prepared at ambient conditions, and showed high performance of adsorption and photocatalytic oxidation of antibiotics and microbial pathogen in wastewater. The biocharsupported TiO2 working with the ultraviolet light in solar light also showed the good adsorption and photocatalytic oxidation of SMX and pathogen, however, its overall performance was lower than that of biochar-supported CuS. The project resulted in several outputs including: i) Synthesis of novel functional biochar-supported photocatalysts, ii) understanding of adsorption capacity to eliminate pharmaceutical compounds and pathogens from wastewater, and iii) photocatalytic oxidation/inactivation of pharmaceutical compounds and pathogens.
The results from this project will contribute to sustainable management of water resource for agricultural use by allowing the beneficial reuse of reclaimed wastewater and helping to protect human health and environments by providing a secure water resource. It will lead to enhancement of water quality, protection of human health and environment, and secure conservation of water resources for agriculture and manufacturing industries. Since the undergraduate students (two female native Hawaiian students) mainly contributed to this project, this project also enhanced the recruitment of student team from the minority (Native Hawaiian, female) and education of future environmental engineers through this project.
Conclusions:
In this project two types of photocatalysts (biochar-supported TiO2 and the biocharsupported CuS) were prepared and used to degrade SMX (as a model antibiotics) and inactivate E. coli (as a model microbial pathogen). The biochar/CuS was activated under the visible light irradiation using the LED lamps while the biochar/TiO2 was activated under the ultraviolet light irradiation using the UV lamps. The biochar-supported TiO2/Fe2O3 prior to using the biochar/CuS was considered as the photocatalyst under visible light irradiation, but several trial showed its poor adsorption and degradation of SMX under visible light.
The biochar/TiO2 showed higher mineralization of SMX than the photolysis (by UV irradiation only) and the photocatalysis using the commercial TiO2 powder. The higher loading of biochar-TiO2 also led to enhancement in mineralization of SMX due to the higher generation of OH radicals. When the UV irradiation time increased up to 6 h, both SMX removal efficiency and mineralization enhanced (91 %, 81%) at the selected conditions (5 g biochar/TiO2 per L, pH 4). It led to high decomposition of SMX and its oxidation products, negligible toxicity and accumulation of non-toxic products.
Very interesting findings from this study was that the biochar/CuS under visible light irradiation (low light and energy consumption) demonstrated the higher adsorption and photodegradation of SMX than those by the biochar/TiO2. The adsorption capacity of biochar/CuS was 3.3-8.1 mg of SMX adsorbed per g of biochar/CuS while 1.5-2.4 mg of SMX adsorbed per g of biochar/TiO2 at the selected conditions. The biochar/CuS also showed higher photodegradation of SMX which was 92% removal of SMX for 2-4 h visible light irradiation (91% removal of SMX for 6 h using the biochar/TiO2). The biochar/TiO2 demonstrated high removal of SMX (90-92%) for 6-8 h solar light irradiation indicating practical application of these processes under solar light.
The biochar/CuS revealed the excellent inactivation of E. coli (92-94% removal) for 2-4 h visible light irradiation while the biochar/TiO2 showed moderate inactivation of E. coli (51% removal) for 6 h UV light irradiation. It indicated more effective inactivation/control of microbial pathogens by the biochar/CuS under visible light irradiation with lower light intensity. and energy consumption than the biochar/TiO2 using UV lights having high light intensity and energy consumption than the biochar/TiO2 using UV lights having high light intensity.
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