Grantee Research Project Results
2023 Progress Report: Optimizing biochar adsorbent production through semi-gasification
EPA Grant Number: SU840404Title: Optimizing biochar adsorbent production through semi-gasification
Investigators: Shimabuku, Kyle
Institution: Gonzaga University
EPA Project Officer: Spatz, Kyle
Phase: I
Project Period: July 1, 2022 through June 30, 2023 (Extended to June 30, 2024)
Project Period Covered by this Report: July 1, 2022 through June 30,2023
Project Amount: $24,982
RFA: 18th Annual P3 Awards: A National Student Design Competition Focusing on People, Prosperity and the Planet (2021) RFA Text | Recipients Lists
Research Category: P3 Awards , Water
Objective:
Biochar produced in top-lit drum (TLUD) semi-gasification systems hold promise for efficient PFAS removal when applied in filter absorbers in both point-of-use and centralized treatment systems. Previous studies have found adsorption capacities of TLUD biochar for other organic contaminants can approach that of a commercial granular activated carbon (GAC) and also be 10-times greater than that of biochar produced in a kiln, which most studies use to make biochar for organic contaminant treatment. However, most of our previous work examining TLUD performance was done with systems that had limited control over production conditions such as primary air flow rate (PAF). Thus, the objective of this research was to determine the optimum production conditions in a TLUD to produce biochar for PFAS adsorption.
A modular TLUD, which we used previously to identify optimal conditions to limit combustion emissions, is being examined to determine optimal production parameters because it can be finetuned to adjust conditions that may influence adsorption efficiency (e.g., PAF). Thus, this project seeks to use an innovative, interdisciplinary approach combining water quality and combustion engineering as well as a custom-designed experimental apparatus to design efficient, low-cost, and sustainable biochar adsorbents.
Progress Summary:
One key output was identifying optimal production conditions with respect to PAF rate in the TLUD stove used to produced biochar from wood pellets. As PAF increased, the temperature also increased but the heating duration decreased. In general, biochar produced at higher temperatures and PAFs in the TLUD exhibited greater adsorption capacities for PFOA, PFOS, and PFBA. PFAS adsorption capacities were evaluated using batch tests where the PFAS were spiked into laboratory clean deionized water. The greatest improvement in PFAS adsorption capacity occurred when the PAF increased from 10 L/min to 30 L/min. However, at PAFs greater than 30 L/min up to 50 L/min, the biochar performance did not significantly improve. The best performing TLUD biochars in batch tests exhibited about 1/6 the adsorption capacity of a commercial GAC.
When evaluating the performance of the best performing TLUD biochars (identified in batch tests) to GAC in rapid small-scale column tests (RSSCTs), it was found that PFOA and PFOS broke through >10 times faster with TLUD biochar than with GAC. While it is uncertain why the TLUD performance was significantly worse in RSSCTs than in batch tests, one reason may have to do with background competing substances (e.g., dissolved organic matter) fouling biochar adsorbents more than GAC in RSSCTs. Thus, it appears under the conditions tested, TLUD biochar shows a moderate affinity for PFAS relative to GAC. However, it may be possible for moderate capacity TLUD biochar to produce waste heat that could be recovered to produce biochar in a kiln at high temperatures (800 ℃) and long durations (several days). Such biochar was shown to have PFAS removal efficiencies similar to that of GAC in batch and column systems. Therefore, biochar manufacturers could develop a process to simultaneously produce high and moderate capacity PFAS adsorbents for different applications (e.g., drinking water and stormwater treatment).
RSSCTs also involved treating groundwater in the presence and absence of free chlorine to simulate point-of-use treatment of water containing chlorine as secondary disinfectant. Chlorine was more strongly adsorbed than PFAS and broke through after PFOA and PFOS in RSSCTs. Biochar and GAC were pre-exposed to chlorine with the goal of partially exhausting their capacity for chlorine while maintaining their PFAS adsorption capacity. It was hypothesized that adsorbents partially exhausted for chlorine adsorption could cause chlorine to breakthrough prior to PFAS to signal that adsorbents should be changed out before PFAS breakthrough. However, it was found that the PFAS adsorption capacity of biochar and GAC was compromised by preexposing the media to chlorine. As a result, the use of chlorine as an indicator for PFAS breakthrough was ruled to be ineffective, and other proxies for PFAS breakthrough are being explored.
The results have also been disseminated to three K-12 schools in Spokane County in areas that have been impacted by PFAS contamination of groundwater. Courses at GU, such as senior design classes, have also been engaged with this project and the results have been presented at local research symposia helping spread awareness of sustainable alternatives for PFAS treatment.
Future Activities:
One objective of this project was to establish a more readily available, lowcost method to estimate PFAS breakthrough as most PFAS detection methods are costly and are not widely available. Since it was concluded that free chlorine would not be an appropriate indicator for PFAS breakthrough, the remainder of this project will investigate the use of other indicators. Since dissolved organic matter usually breaks through absorbers prior to PFAS, the use of total organic carbon to serve as a proxy for PFAS breakthrough will be investigated.
Supplemental Keywords:
Water purification technologies, resource recovery, organicsThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.