PhD graduate defence seminar for Abhijeet Pathy, Wednesday October 21 at 1:30 pm, 849 General Services Building. “Biochar applications for energy related contaminants: potentially toxic elements in oil sands process water and petroleum hydrocarbons in ground water and peat soil.”
Zoom link: https://zoom.us/j/97726253846?pwd=kfmV4Zzm0lYzijcsBeaPH4Cfpu3Ugn.1
Abstract:
Energy production and processing generate contaminated water and soil with complex chemical compositions that can limit the effectiveness of conventional treatments. Biochar, a carbon rich material produced by biomass pyrolysis, has been widely investigated as a low cost sorbent. However, most evidence is based on single contaminants in clean solutions and does not adequately represent environmental matrices or operating conditions. This thesis evaluated canola straw biochar for removing potentially toxic elements and petroleum hydrocarbons from water and soil within the energy sector, examining how contaminant properties, matrix chemistry, treatment configuration, regeneration, and spent biochar management influence remediation.
A review of wastewater generated across energy value chains established a broad context for this experimental research. It showed that biochar suitability depends on the interactions between contaminant chemistry, wastewater composition, competing constituents, and biochar surface characteristics. Evidence was strongest for selective removal of metals, radionuclides, fluoride, and ammonium, whereas biochar was poorly suited to bulk salinity removal. The review identified major gaps between laboratory adsorption studies and practical application, particularly the limited use of real wastewater, continuous flow systems, economic assessment, and spent biochar management. Subsequent experiments addressed these gaps across water and soil matrices of increasing environmental and operational complexity.
Canola straw biochar was first tested for simultaneous removal of arsenic, cadmium, cobalt, chromium, copper, nickel, selenium, and zinc from alkaline oil sands process water. In batch experiments, removal exceeded 90 % for cadmium, cobalt, copper, nickel, and zinc. Uptake of arsenic, chromium, and selenium was negligible because their anionic forms were repelled by the negatively charged biochar surface. Spectroscopic and textural evidence supported surface complexation and mineral precipitation as main removal mechanisms, with ion exchange making a smaller contribution. Regeneration with 0.5 M hydrochloric acid recovered adsorbed elements, although removal of cadmium, cobalt, nickel, and zinc declined during reuse. Copper was the most stable, decreasing from 98-89 % removal over four cycles.
Fixed bed column experiments assessed whether this batch experiment removal performance could be maintained under continuous flow. Adsorption followed the order cadmium > cobalt > nickel > zinc, with maximum equilibrium capacities of 0.279, 0.210, 0.126, and 0.104 mg g⁻¹, respectively. Copper retention was limited by complexation with dissolved organic matter, demonstrating that high batch removal did not always translate to column performance. Empty bed contact times of 17.2-29.4 min extended cadmium and cobalt service times, while the Thomas and Yoon-Nelson models described the breakthrough of cadmium, cobalt, and nickel, but not poorly retained oxyanions. Economic modelling indicated treatment cost decreased from CAD 17.80-2.65 L⁻¹ as capital costs were distributed across repeated operating cycles. Incorporating spent biochar into cement at 1-2 % by weight reduced leaching of most retained elements. However, increased chromium release from the cement treatments showed that this management option requires further evaluation.
The investigation was extended to petroleum hydrocarbons in ground water and peat soil. In crude oil spiked ground water collected from a northern peatland, canola straw biochar simultaneously adsorbed 12 hydrocarbons, including benzene, toluene, ethylbenzene, xylenes, linear chain hydrocarbons, and polycyclic aromatic hydrocarbons. Removal of benzene, toluene, ethylbenzene, and xylenes exceeded 95 % within 7 days at 1 g L⁻¹ biochar and 99 % at 2 g L⁻¹. Modelling and surface characterization indicated that adsorption involved pore diffusion, hydrophobic and π-π interactions, and biochar surface functional groups.
Biochar dosages of 1-20 % were subsequently evaluated over 1-60 days in non-sterile, legacy crude oil contaminated boreal peat soil. Incubation time did not significantly affect hydrocarbon removal, whereas higher molecular weight polycyclic aromatic hydrocarbons showed consistent concentration reductions, particularly at 10 and 20 % biochar. Although biochar altered enzyme activities and relative abundance of several microbial groups, these responses did not correspond to increased native hydrocarbon degradation. Hydrocarbon removal was therefore attributed primarily to sorption rather than biostimulation.
Overall, canola straw biochar functioned as a selective treatment material rather than a universal sorbent. It showed greatest potential for divalent metals and petroleum hydrocarbons when contaminant properties and matrix chemistry favoured adsorption. The column, regeneration, economic, and spent biochar results provide a basis for further application development. Oxyanion removal, dissolved organic matter interference, reuse performance reuse, high dose requirements in peat, and long-term contaminant stability must be addressed before full scale implementation.