Grantee Research Project Results

Final Report: Optimizing biochar adsorbent production through semi-gasification

EPA Grant Number: SU840404
Title: 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 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: Water , P3 Awards

Objective:

Biochar produced in top-lit drum (TLUD) semi-gasification systems holds promise for efficient PFAS removal when applied in filter absorbers in both point-of-use and centralized treatment systems. Biochar can be less expensive to produce than GAC, a byproduct of energy production, and can sequester carbon unlike GAC production, which typically consumes energy and emits carbon. Previous studies have primarily examined the organic contaminant adsorption capacity of biochar produced in furnace or kiln systems. Our previous work found adsorption capacities of TLUD biochar for organic contaminants can approach that of a commercial granular activated carbon (GAC) and be 10-times greater than that of biochar produced in a kiln at a similar peak temperature and heating duration (<2 hrs). 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 with a focus on PAF in a TLUD to produce biochar for PFAS treatment.

A modular TLUD that was able to identify optimal conditions to limit combustion emissions in a previous study was also used here because it can be fine-tuned to adjust conditions (e.g., PAF) that were hypothesized to influence biochar physicochemical properties and PFAS adsorption efficiency. Thus, this project used 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.

Summary/Accomplishments (Outputs/Outcomes):

One key output was identifying optimal production conditions with respect to PAF in the TLUD stove used to produce 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 Perfluorooctanoic acid (PFOA) and Perfluorooctanesulfonic acid (PFOS). PFAS adsorption capacities were evaluated using 7-day batch tests where the PFASs were spiked into deionized water. The greatest improvement in PFAS adsorption capacity occurred when the PAF increased from 10 L/min to 30 L/min. However, when increasing PAFs from 30 L/min up to 50 L/min, the biochar performance did not significantly improve. Halting TLUD operation with water or dry by preventing oxygen from entering the TLUD also did not appear to impact the adsorption capacity of the resulting biochar. In addition, the potential for steam activation was explored by introducing steam into the TLUD, but this inhibited the TLUD’s operation. Thus, the best performing biochars that could be produced in the TLUD with PAFs ≥30 L/min exhibited about 1/6 the adsorption capacity of a commercial GAC.

The performance of biochar produced in a furnace with the same wood pellets used to produced TLUD biochar was also explored. Consistent with previous studies, furnace biochar produced at temperatures of 700 and 800 ℃ in the furnace at shorter heating durations (<4 hrs) performed worse than TLUD biochar that achieved the same peak temperature. These results suggest that the flow of air through a gasifier may have an activating effect. However, furnace biochar produced at durations for 12 hours or greater exceeded the performance of the top performing TLUD biochars for both PFOA and PFOS. The top performing biochar was produced in the furnace for 48 hrs at 800 °C and it was also able to slightly outperform GAC for both PFOA and PFOS removal.

When evaluating the surface area of these biochars measured by N2 adsorption, the surface area was found to increase as the PAF increased from 10 to 30 L/min and then decline as PAF rate increased further. In addition, surface areas were greater for the TLUD biochar than furnace biochar produced at shorter durations (<4 hr) when they were produced at the same peak temperature, suggesting that the flow of air through the TLUD expanded the surface area of the biochar that translated to additional adsorption sites for PFAS. However, at longer heating durations, furnace biochar surface areas substantially increased, and started to approach that of GAC. These results suggest that surface area is only partially related to biochar performance because biochar produced at higher PAFs showed good adsorption capacity for PFAS even though the surface areas declined at higher PAFs. In addition, the top performing furnace biochar for PFAS removal that outperformed GAC exhibited surface areas that were less than that of GAC.

The furnace biochar produced at 48hrs at 800 °C was selected for further testing to compare its PFAS removal efficiency with that of GAC in flow-through column adsorption tests using the rapid small-scale column test (RSSCT). RSSCTs assuming constant diffusivity were performed using empty bed contact times (EBCT) of 10 min and 20 min in a low total organic carbon (TOC) (<0.5 mg/L) groundwater. For both PFOA and PFOS, there appeared to be a minimal effect of the EBCT on the performance of both adsorbent medias. The relative performance of the GAC and furnace biochar produced at 48hrs at 800 °C for PFOA removal in the RSSCTs was similar to that in the batch adsorption tests. Though the furnace biochar produced for 48hrs at 800 °C slightly outperformed the GAC in batch tests, these adsorbent’s performance was similar in the RSSCTs. The biochar appeared to slightly outperform the GAC for PFOS removal as was the case in the batch tests. One of the differences between the batch tests and the RSSCTs that could cause differences in their relative performance was the background water that the PFAS were spiked into. In the batch tests, the background water was deionized water whereas in the RSSCTs the water was groundwater with a low concentration of TOC (<0.5 mg/L). Organic matter, which is quantified by TOC analysis, can compete with target organic contaminants, like PFAS, for adsorption sites. Therefore, it is possible that the furnace biochar was fouled more by the background organic matter with PFOA than for PFOS.

Conclusions:

In summary, TLUD biochar showed a moderate affinity for PFAS relative to GAC, but biochar in a kiln at high temperatures (800 ℃) and long durations (several days) can rival the performance of GAC in both batch adsorption conditions and in filter adsorbers. Though TLUD biochar produced here may be less appealing to implement in drinking water applications as the media would need to be changed out frequently, it may be possible for moderate capacity TLUD biochar to be used in different applications where PFAS and control of other organic contaminants is needed (e.g., stormwater treatment). Moreover, TLUDs produce excess heat that could be recovered to produce biochar in a kiln at high temperatures (800 ℃) and long durations (several days). 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) with minimal energy inputs. The results of this project could enable biochar manufactures to produce more environmentally friendly PFAS adsorbents and at a lower cost that could increase access to PFAS treatment and improve human health. Such biochar adsorbents could also support the removal of other organic contaminants in different environmental media such as stormwater, industrial and municipal wastewater, and polluted soils and sediment remediation.

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 pre-exposing the media to chlorine. As a result, the use of chlorine as an indicator for PFAS breakthrough was determined to be infeasible.

The results from this project 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 and national conferences helping spread awareness of sustainable alternatives for PFAS treatment.

Journal Articles:

No journal articles submitted with this report: View all 3 publications for this project

Supplemental Keywords:

Water purification technologies, resource recovery, organics

Progress and Final Reports:

Original Abstract

2023 Progress Report