Date of Award

8-22-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Applied Science

First Advisor

Wei Zhao

Abstract

Reduced graphene oxide (RGO) forward osmosis (FO) membranes have emerged as promising candidates for efficient wastewater management and osmotic energy harvesting, due to their enhanced chemical stability and superior FO performance for low-energy waste brine treatments, such as volume reduction of oxidative chromium brine for cost-effective disposal. This dissertation first examines the oxidation resistance of RGO forward osmosis membranes against chromate Cr (VI). Our findings reveal that both the water flux and the reverse salt flux of RGO membranes are suppressed, accompanying an enhanced reverse flux selectivity after exposure to an acidic medium of Cr (VI) (pH 2.0-7.0). Meanwhile, the highly toxic Cr (VI) is reduced to a less toxic Cr (III). In a basic medium of pH 10.0, an increase in reverse salt flux and a reduction of reverse flux selectivity are observed, which could be due to the synergistic effect of both pH and ionic solutes on the RGO membranes, not due to Cr (VI) oxidation. Remarkably, after removal of chromate with a full deionized water rinse, the RGO membranes show a nearly complete recovery of their suppressed FO performance. Similar results are observed on the oxidation resistance of RGO membranes against hypochlorite. Secondly, in combination of the chemical stability with the robust mechanical strength of the thermally annealed RGO FO membranes, we further demonstrate that one can concentrate waste brine and simultaneously harvest osmotic energy as electricity in the FO processes. We determine the power densities attainable by RGO FO membranes to be 40 mW/m2 and 100 mW/m2 with Ag/AgCl and conductive carbon cloth electrodes for full size RGO membranes, respectively, under a salinity gradient of saturated NaCl/100 mM NaCl. Thirdly, by developing a tape masking method, the power density-active membrane area relationship has been studied, reaching ~400 W/m2 at a membrane area of 4.0 ´10-8 m2 (0.23 mm in diameter). It further reveals a gigantic power density of 1.5 MW/m2 under a typical membrane size of 2 mm. With 18 osmotic cells connected in series, the device can power three colored 5 mm LED lights operating at ~8 V. This work highlights the potential of RGO-based FO membranes in addressing both environmental challenges associated with waste brine disposal and precious material enrichment and the need for renewable and sustainable energy solutions.

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