COTTON GIN WASTE AND WALNUT SHELLS DERIVED BIOCHAR FOR THE REMOVAL OF PHARMACEUTICALS AND HUMIC ACIDS FROM AQUEOUS SOLUTIONS
Open Access
- Author:
- Ndoun Tangmo, Marlene Carla
- Graduate Program:
- Agricultural and Biological Engineering (PHD)
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 10, 2022
- Committee Members:
- Suat Irmak, Program Head/Chair
Heather Preisendanz, Chair & Dissertation Advisor
Tameria Veith, Major Field Member
Michael Mashtare, Major Field Member
Stephanie Velegol, Outside Unit & Field Member
Herschel A. Elliott, Special Member
Clinton Williams, Special Member - Keywords:
- Pharmaceuticals
cotton gin waste
walnut shells
agricultural waste
biochar
adsorption
fixed-bed columns
humic acids
wastewater treatment
binary system - Abstract:
- Acetaminophen (ACT), sulfapyridine (SPY), ibuprofen (IBP) and docusate (DCT) are pharmaceuticals with widespread usage and incomplete removal in wastewater treatment systems. As the beneficial reuse of treated wastewater increases, there is a need to remove contaminants that persist in the effluent to mitigate potential impacts on the agroecosystems to which they are land-applied. Additional treatment technologies are often expensive and energy intensive and interactions between natural organic matter (NOM) and pharmaceuticals in water can make it even more difficult for conventional treatment processes to remove these contaminants. Moreover, the sustainable management of agricultural and municipal waste has gained increasing attention worldwide, especially regarding the production and recycling of value-added products that are renewable and carbon-rich. The goal of this study was to explore the production and characterization of biochar from the pyrolysis of two agricultural waste products: cotton gin (CG) waste pyrolyzed for 2 h at 700 C (CG700) and walnut shells (WS) pyrolyzed for 2 h at 800 C (WS800) to better understand their potential to remove pharmaceuticals and humic acids (HA) from aqueous solution. The CG waste feedstock was pretreated with an acid and base to obtain CG-A and CG-B biochars and the same procedure was applied to WS, producing WS-A and WS-B biochars. Feedstock pretreatment was compared to a simpler procedure that involved washing the untreated biochars (CG700 and WS800) for 24 h prior to utilization to obtain CG700-24 and WS800-24. Using fixed-bed column adsorption experiments, the following three objectives were addressed: (1) Determining the physicochemical characteristics of the biochars and investigating their ability to remove pharmaceuticals and HA from aqueous solution; (2) comparing the removal of HA and pharmaceuticals using a single fixed-bed column versus two fixed-bed columns operated in series; and (3) assessing the effects of acid and alkaline pretreatment and deionized (DI) water wash on biochar’s ability to remove the four pharmaceuticals and HA from aqueous solutions. Each biochar was characterized by Brunauer-Emmett-Teller (BET), Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and zeta potential analysis. Physicochemical characterization was used to elucidate the differences in specific surface area, change in morphology, elemental composition and functional groups between the treated and untreated biochars. For both the CG waste and WS feedstocks, FT-IR revealed that pyrolysis led to destruction of the acidic functional groups and an increase in ash content, resulting in the production of hydrophobic alkaline biochars with pH values between 8.72-10.93. Results from the FT-IR analysis showed the presence of the O-H, C-O, C-H, C=C, C=O, C-O-C functional groups on the surfaces of the biochars. The FT-IR spectra demonstrated the presence of carbonyl (C=O) group in the WS800 biochar and this group was absent from the CG700 sample. WS800 exhibited a loss of the O-H group due to increased pyrolysis temperature, which led to dehydration of the biochar. According to the XPS data, carbon is the most abundant surface element and the removal of minerals and impurities as a result of chemical pretreatment and DI water wash led to an increase in total carbon content. XPS data demonstrated that both the CG700 and WS800 contained the C=O group on their surfaces. The difference in results between the FT-IR and XPS data was due to XPS and FT-IR identifying the surface and bulk properties of the biochar samples, respectively. This signified that the surface of the biochar differed from the bulk biochar sample. Results from the BET analysis demonstrated that the specific surface areas (SSA) of the biochars were low (14.07 m2 g-1 for CG700 and 64.95 m2 g-1 for WS800) compared to other biochars reported in literature. An increase in SSA was demonstrated for the acid and base pretreated CG waste biochars (CG-A = 85.16 m2 g-1 and CG-B = 88.88 m2 g-1) as well as the DI water washed biochar (CG700-24 = 150.97 m2 g-1) due to the removal of dissolved materials and biomass fragments that were inside the pores. The SSA of the acid and base pretreated WS biochars (WS-A and WS-B) could not be determined as a result of pore blockage during the pretreatment step. Zeta potential measurements demonstrated that the biochars have a net negative charge on their surfaces from pH 4-10 range and this suggests that the pH point of zero charge (pHpzc) will be attained when the solution pH < 2. Fixed-bed column experiments were performed to determine the difference in removal efficiency between the biochars and elucidate the effects of biochar properties on adsorption capacity. Results showed that a single column of CG700 had a greater affinity for removing DCT (99%; qc = 3.58 mg g-1), IBP (56%; qc = 2.35 mg g-1) and HA (99%; qc = 14.34 mg g-1), while WS800 removed 72% of SPY (qc = 2.92 mg g-1) and 68% of ACT (qc = 2.99 mg g-1). Data from column experiments and the physicochemical characterization of the biochars suggest that adsorption was influenced by the solution pH, surface area, net charge, and functional groups of the biochars. The mechanisms for removal included pore filling and diffusion, hydrophobic interactions, hydrogen bonding and pi-pi electron donor acceptor interaction. The washed CG700-24 and pretreated CG-A and CG-B showed a significant increase in pharmaceutical removal (more than 95%) due to the increase in SSA and the introduction of new functional groups capable of participating in adsorption. Contradicting results were shown by the WS800-24, WS-A and WS-B biochars where a decline in adsorption of contaminants occurred due to pretreatment and washing which led to the removal of salts and organic matter that contributed to the adsorptive properties of the biochars. A series of double columns consisting of a single column of CG700 biochar connected in series to a second column of WS800 in two configurations; (1) CG700 --> WS800 and (2) WS800 --> CG700 were also tested for the removal of contaminants and compared to the performance of single fixed bed columns. An increase in the removal of ACT (from 68% in a single column of WS800 or CG700 to 95% and 87% in configurations 1 and 2, respectively) and IBP (56% in CG700 and 42% in WS800 to 65% in configuration 1 and 62% in configuration 2) occurred in the double columns due to an increase in SSA and bed depth allowing for more interactions with the contaminants. The double columns for the removal of SPY and DCT achieved similar removal as a single column of WS800 and CG700, respectively. Moreover, DI water washed biochars were connected in series (CG700-24 --> WS800-24 and WS800-24 --> CG700-24) and tested for the removal of the four pharmaceuticals and HA. Columns experiments demonstrated a significant increase in the removal of all contaminants from the unwashed double columns to the washed double columns connected in series (CG700 --> WS800 removed 55% of SPY and adsorption increased to 99% using CG700-24 --> WS800-24). The increase in adsorption was attributed to the larger SSA caused by the removal of ash, salts and organic matter from the pores as a result of washing. Moreover, the effect of NOM was investigated through the addition of HA to the adsorption system. Synergistic effects were demonstrated between HA and ACT and SPY on the surface of the CG700 biochar, where removal of ACT increased from 68% to 92% and SPY from 13% to 70% while similar removals were observed for IBP and DCT in the presence and absence of HA. Increase in removal is attributed to interactions between already adsorbed HA molecules on the surfaces of the biochar and the pharmaceuticals. Using WS800, a decline in the removal of ACT, DCT, SPY and IBP with HA present was shown as a result of competition for active sites, pore blockage by the HA molecules and interactions between free HA and pharmaceuticals in solution. Double columns in series (CG700 --> WS800 and WS800 --> CG700) were also tested for the removal of the four pharmaceuticals in the presence of HA and no significant differences in removal were demonstrated between the presence and absence of HA. However, a single column of CG700 showed better removal of pharmaceuticals in the presence of HA compared to the double columns, despite more active sites being available in the double columns, demonstrating that pore filling was not a limiting factor. To conduct predictive modeling of the column breakthrough curves, the Thomas, Adams-Bohart and Yoon Nelson models were applied to the experimental data. Results demonstrated that these models provided a poor fit for the description of the asymmetrical breakthrough curves obtained in this study due to the high binding site heterogeneity of the biochar surfaces and the existence of multiple adsorption mechanisms. Overall, the results demonstrate that biochars from cotton gin waste and walnut shells could be used as cost-effective, environmentally friendly adsorbents for the removal of ACT, SPY, IBP, DCT, and HA from aqueous solutions and the data can be extrapolated to the removal of a wide variety of other contaminants. Nevertheless, careful selection of the biomass feedstock for biochar production is warranted as the physical and chemical properties of the biochars and the ability for these biochars to be successfully used in several environmental applications depend on the inherent characteristics of the parent feedstock. Chemical pretreatment of the feedstock proved to be a promising method to produce designer biochars with high surfaces areas and functional groups that can interact with contaminants in water. However, for practical applications, especially in communities with limited access to resources, simply washing the biochar with water for a period of time is recommended in order to increase the specific surface areas and the removal efficiencies of the biochars. In addition, synergistic interactions between pharmaceuticals ACT and SPY and HA on the surface of the cotton gin waste biochar suggest that this biochar is better suited for the removal of contaminants in wastewater given the abundance of NOM in these systems. Biochar from walnut shells will be more efficient for use in drinking water systems that typically have low levels of NOM present. Moreover, production of biochar from agro-waste can serve as a waste mitigation strategy by diverting waste away from landfills and the reported potential for the spent biochars to be used as a soil amendment to improve fertility when land-applied.