BIOCHAR AS A FILTER MEDIA FOR THE ADSORPTION OF PHARMACEUTICALS FROM WASTEWATER EFFLUENT IRRIGATION WATER
Open Access
- Author:
- Ndoun Tangmo, Marlene Carla
- Graduate Program:
- Agricultural and Biological Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- June 27, 2019
- Committee Members:
- Herschel Adams Elliott, Thesis Advisor/Co-Advisor
Heather Elise Gall, Committee Member
John Earl Watson, Committee Member - Keywords:
- Treated wastewater effluent
emerging contaminants
pharmaceuticals
adsorption capacity
percentage removal
pseudo-second-order
breakthrough
biochar-sand filter - Abstract:
- Global water shortages due to urbanization and rapid population growth are placing unprecedented pressure on water supplies. The use of treated wastewater effluent has become a valuable solution to preserve water resources but its reuse for irrigation of agricultural lands has led to the widespread presence of pharmaceuticals and personal care products (PPCPs) in the environment and increased public concern over the potential ecotoxicological effects of these contaminants on aquatic life, animals, plants and even humans. This study explores the use of biochars obtained from cotton gin waste (CG) and guayule bagasse (GB) as potential adsorbents for the removal of the following pharmaceutical compounds: acetaminophen (ACT), ibuprofen (IBP), sulfapyridine (SPY), docusate (DCT), erythromycin (ETM) and pseudoephedrine (PED) from aqueous solution. The biochars were prepared through pyrolysis at 350, 500 and 700 C for 2 h in a stream of N2 gas. CG and GB biochars were characterized for specific surface area (SA), pH, surface functional groups using FTIR and zeta potential to estimate the point of zero charge (pHPZC). An increase in pyrolysis temperature led to an increase in the pH, zeta potential and SA of the biochars. All the biochars showed a net negative charge on their surfaces and an increase in pH was accompanied by an increase in the magnitude of negative charge. Both CG and GB biochars showed strong functionality with the presence of OH, C-H, C=O and C=C groups on their surfaces. The main difference between the biochars was the presence of an additional O-containing functional group on the GB biochar surface, indicating it is less hydrophobic compared to the CG biochars. Batch adsorption experiments were performed to determine the removal efficiency of CG and GB biochars for the selected pharmaceutical compounds from aqueous solution. Additionally, the effects of contact time and biochar properties on the adsorption capacity were investigated. The results showed that the adsorption of ACT, IBP, SPY, DCT, ETM and PED onto the CG and GB biochars was influenced by the contact time, solution pH, specific surface area, net charge and functional groups of the biochars. The extent of pharmaceutical adsorption was, in general, positively related to the SA of the biochars. The mechanisms responsible for removal of pharmaceuticals are hydrophobic bonding, hydrogen bonding, π- π electron donor acceptor interactions and diffusion. The lower hydrophobicity of the GB biochars precluded the adsorption of hydrophobic compounds, hence low removal was achieved by these biochars. ACT showed the least removal (< 20 %) of all the tested biochars due to electrostatic repulsion between the anionic ACT and the negatively charged biochar surfaces. Depending on the pharmaceutical being removed, functional groups of both the biochars and the pharmaceuticals or specific surface areas controlled the adsorption process. ACT, IBP, SPY, DCT, ETM and PED adsorption studies using the CG700 biochar were also carried out at pH 7 and compared to the adsorption behavior at inherent system conditions (pH 10-11). The removal of ACT significantly improved at pH 7 due to the reduction in electrostatic repulsion between CG700 and ACT molecules. IBP removal decreased from 50% to 35% and the adsorption of SPY decreased from 70% to 30 % at pH 7 and the decline is attributed to a decrease in the formation of negative-charge assisted hydrogen bonds. There seem to be no appreciable and consistent differences in the extent of DCT, PED and ETM adsorption for the two pH conditions. The adsorption of pharmaceuticals onto biochar followed pseudo-second order kinetics suggesting the adsorption is controlled by the surface areas of the biochars as well as chemisorption due to electron transfer or sharing. The adsorption data was successfully fitted to both the Langmuir and Freundlich isotherm models although the Langmuir model showed a better fit. The fit of the data to the Langmuir model implies that there was negligible interaction between the pharmaceutical molecules and this favored adsorption by the biochar. A series of small-scale downflow column tests were carried out using biochar-amended sand media to determine the breakthrough curves for four selected pharmaceuticals: ACT, SPY, DCT and ETM at initial concentrations of 10 mg L-1, a constant flow rate of 1 mL min-1 and a 13-cm bed depth. Breakthrough (time when the column effluent-to-influent concentration ratio was 0.05) occurred in the following order: SPY (1 h), ACT (2h), ETM (4 h) and DCT (5 h). The ability of CG700 biochar-amended sand filters to remediate ACT, SPY, DCT and ETM was validated by a reduction in contaminant concentration through the column which was confirmed by the high Kd values and maximum bed capacity (qc). The breakthrough curves demonstrated that the biochar-amended sand filters have a high affinity for the pharmaceutical compounds and this resulted in slow saturation of the bed occurring after 12 h. These results show that CG and GB biochars could be added to sand filters and have the ability to act as environmentally friendly adsorbents for the removal of pharmaceuticals from treated wastewater used for irrigation. Lab-scale column data were used to estimate the parameters for a full-scale effluent filtration column. Through scale-up analysis, it was estimated that a total volume of 446 m3 can be treated before the column breakthrough point (Ce/C0 = 0.05) is attained at 22 h with an estimated 39.32 g of ACT removed per kg of biochar. It should be noted that the removal of pharmaceuticals from irrigation water using a full-scale filtration system depends on many system-specific parameters such as the properties of the compounds targeted for removal, the physicochemical characteristics of the biochar, the kinetics of adsorption and the desired pharmaceutical concentrations in the irrigation water.