Performance of oat hulls activated carbon for COD and color removal from landfill leachate

doi:10.1016/j.jwpe.2019.101040

Journal of Water Process Engineering

Volume 33, February 2020, 101040

https://doi.org/10.1016/j.jwpe.2019.101040 Get rights and content

Highlights

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Oat hulls were effectively used as precursor for activated carbon (OHAC).
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High impregnation ratio and pyrolysis temperature resulted in high BET surface area.
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OHAC successfully removed 90% of COD and 100% of color from landfill leachate.

Abstract

This research aimed to recover oat hulls as activated carbon, using the produced adsorbent to remove the organic matter removal from landfill leachate. The organic matter content in leachate was determined by measurements of chemical oxygen demand (COD) and color. In addition, the maximum adsorptive capacities associated with the adsorbents were also determined. Adsorption batch tests were initially performed with synthetic leachate, and variables such as pH, activated carbon dose, temperature and contact time were evaluated. The effectiveness of the adsorbents was also verified for real landfill leachate. Oat hulls samples were chemically activated with phosphoric acid (impregnation ratios, 60 and 100 %) and pyrolyzed (N₂ atmosphere) at 350 and 500 °C. The 100% impregnation ratio resulted in adsorbents with higher surface areas (1090–3880 m²  g⁻¹) rather than the ratio 60 % (123–731 m²  g⁻¹). It was found that the organic matter removal from synthetic leachate was favored by performing the adsorption tests at 20 °C, pH 4, and using a 20-g L⁻¹ activated carbon dose. Under these experimental conditions, the activated carbon sample impregnated at 100% with phosphoric acid and pyrolyzed at 500 °C completely removed the color from leachate, and COD removal was up to 90%. Therefore, oat hulls comprise a suitable precursor for activated carbon, and its application for leachate treatment should be encouraged.

Introduction

Although municipal solid waste can be safely disposed in sanitary landfills, the operation of these facilities is challenged by emissions of greenhouse gases and leachate [1,2]. Landfill leachate is reported by the literature as a high-strength wastewater containing easily and hardly biodegradable organic matter, ammonia in high concentrations, dissolved solids, and heavy metals [[1], [2], [3], [4], [5]].

Depending on the level of purification required for leachate prior to its released into the environment, it can be treated by stand-alone or combined methods, such as: air stripping [6,7], advanced oxidation processes [3,8], biological processes [4,5], and adsorption [9,10]. Significant removals of heavy metals and organic matter have been attributed to the use of commercial activated carbon for leachate treatment [[9], [10], [11]]. Nonetheless, as part of the efforts to divert organic wastes from landfills, these materials have been tested as precursors for activated carbon production and applied for wastewater treatment [12].

Oat is one the most popular cereals consumed worldwide, and its global production in 2017 was approximately 25.9 million tons, according to the Food and Agriculture Organization (FAO) of the United Nations [13]. While oat is processed for human consumption, massive amounts of hulls are produced. The latter is a lignin-cellulosic waste that corresponds to about 25% of the oat grain weight. Oat hulls can be reused as animal feed, biomass for fuel production, or ultimately disposed in landfills [14,15]. Alternatively, oat hulls can be recovered as activated carbon, being successfully applied for dye [14] and arsenic [16] removals from aqueous solutions.

While limited information is available for oat hulls activated carbon applied for water purification, the literature lacks their application for landfill leachate. Therefore, this study aimed to contribute to the literature by using oat hulls as precursors for activated carbon and testing these adsorbents to remove COD and color from landfill leachate.

Section snippets

Oat hulls activated carbon

Oat hulls were collected from a milling industry located in Emerson-MB, Canada. Following methods described in previous studies reporting on waste-derived adsorbents [14,[16], [17], [18]], oat hulls activated carbon samples were prepared by means of chemical activation with phosphoric acid (impregnation ratios 60 and 100 %), followed by pyrolysis for 1 h, under a N₂ atmosphere. The temperatures tested for pyrolysis were 350 and 500 °C [14,[16], [17], [18]].

Landfill leachate

Synthetic leachate was prepared

Physico-chemical and spectroscopic properties of oat hulls activated carbon

The yields obtained for oat hulls activated carbon were about 38 %, whereas burn-off was around 60 %, and ash content 30 % (Table 2). As reported by Hildago-Oporto et al. [31], oat hulls are lignocellulosic materials composed by nitrogen (0.6 %), carbon (43 %), hydrogen (6 %), and oxygen (50.4 %). During pyrolysis, hydrogen and oxygen atoms are removed from lignocellulosic materials, and the formed porous adsorbent is mainly composed by carbon atoms. Therefore, the maximum yield for activated

Conclusion

The adsorbents produced at different impregnation ratios and pyrolysis temperatures presented average pore diameters varying from 2.5–8.3 nm, indicating that mesoporous activated carbon samples were successfully produced. The BET surface area was dependent on both the tested impregnation ratios and pyrolysis temperatures. Adsorption was favored by running the batch tests at 20 °C, using a 20-g L⁻¹ adsorbent dose, and adjusting the pH of leachate to 4. Due to their high BET surface areas, oat

Acknowledgements

The authors would like to thank the University of Manitoba Graduate Fellowships (UMGF) and the Natural Sciences and Engineering Research Council of Canada (NSERC RGPIN-2014-05510) for financial support, and anonymous reviewers for their valuable comments. We also would like to thank the Nano-Systems Fabrication Laboratory (NSFL) director, Dr. Cyrus Shafai, and the manager Dwayne Chrusch, for their invaluable cooperation to this research.