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Acid Rock Drainage Treatment using Membrane Distillation: Impacts of Chemical-Free Pretreatment on Scale Formation, Pore Wetting, and Product Water Quality Eric J Hull, and Katherine Zodrow Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02957 • Publication Date (Web): 15 Sep 2017 Downloaded from http://pubs.acs.org on September 17, 2017
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Environmental Science & Technology
Acid Rock Drainage Treatment using Membrane Distillation: Impacts of Chemical-Free Pretreatment on Scale Formation, Pore Wetting, and Product Water Quality Environmental Science & Technology September 2017
Eric J. Hull and Katherine R. Zodrow*
Environmental Engineering Department, Montana Tech, Butte Montana 59701 * Corresponding author. E-mail:
[email protected] ACS Paragon Plus Environment
Environmental Science & Technology
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TOC Art
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Abstract
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Acid rock drainage (ARD) is a metal-rich wastewater that forms upon oxidation of sulfidic minerals.
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Although ARD impacts >12,000 miles of rivers in the U.S. and has an estimated cleanup cost of $32 -
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$72 billion, the low pH and high metal concentrations in ARD make rapid, high volume treatment
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without chemical addition difficult. This research focuses on a novel method of ARD treatment,
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membrane distillation (MD). In MD, heated ARD is separated from a cooled distillate by a hydrophobic,
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water-excluding membrane. Because water only passes through the membrane in the vapor phase, non-
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volatile sulfate and heavy metals are retained in the concentrate stream. A preliminary in silico analysis
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using an electrolyte thermodynamic model indicated that MD of 10 different mine wastes yields product
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water containing no contaminants at concentrations >0.2 ppm. MD tests of synthetic ARD used a ~34°C
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temperature difference, operated at 80% recovery, and produced an initial flux of 38.4 ± 1.1 L·m-2·h-1.
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This flux decreased slightly after scaling by iron oxyhydroxide; however, membranes maintained >99%
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dissolved solids rejection. Both flux decline and membrane scale formation decreased after a chemical-
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free, thermal precipitation pretreatment. These results indicate that MD can purify contaminated, acidic
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wastewater using low-grade heat sources, such as geothermal energy, without chemical addition.
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Introduction
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Acid rock drainage (ARD) forms when sulfur-rich minerals are exposed to oxygen and water. The effects
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of ARD have negatively impacted thousands of miles of rivers in the United States.1,2 Although the major
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producer of ARD is the mining industry, ARD can form naturally and when any earth moving activity
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exposes sulfur-rich minerals to oxidative conditions. Iron sulfide, pyrite (FeS2), is one of the most
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common sulfide bearing minerals.3 The oxidation of pyrite, which requires both oxygen and water, is
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generally summarized using the following chemical equation.4 4FeS 15O 14H O → 4Fe OH 8SO 16H
ACS Paragon Plus Environment
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Environmental Science & Technology
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This chemical reaction produces sulfate-rich wastewater that can be acidic (pH 2-4)5 and harmful to the
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environment. Additionally, this acidic water can liberate other metals, including Fe, Mn, Cu, Zn, Pb,
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from surrounding rock.6,7
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Traditionally, ARD is treated actively or passively. Active treatment systems use chemical reagents, such
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as lime, to raise the pH and precipitate metals. The metals that precipitate form a sludge, usually
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composed of amorphous ferric oxyhydroxide6 with other heavy metals at high concentrations. After
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metals are precipitated and the acid is neutralized, the water may be further treated to meet water quality
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standards prior to discharge. Passive treatment systems, including bioreactors and engineered wetlands,
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use natural, minimal maintenance processes to treat ARD. Bioreactors use sulfate and iron reducing
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bacteria to process sulfate into hydrogen sulfide. Hydrogen sulfide then reduces metals ions to metal
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sulfides.8 When the pH of the ARD is
