Acid Rock Drainage Treatment Using Membrane Distillation: Impacts of Chemical-Free Pretreatment on Scale Formation, Pore Wetting, and Product Water Quality

Abstract

Acid rock drainage (ARD) is a metal-rich wastewater that forms upon oxidation of sulfidic minerals. Although ARD impacts >12,000 miles of rivers in the U.S. and has an estimated cleanup cost of $32-$72 billion, the low pH and high metal concentrations in ARD make rapid, high volume treatment without chemical addition difficult. This research focuses on a novel method of ARD treatment, membrane distillation (MD). In MD, heated ARD is separated from a cooled distillate by a hydrophobic, water-excluding membrane. Because water only passes through the membrane in the vapor phase, nonvolatile sulfate and heavy metals are retained in the concentrate stream. A preliminary in silico analysis using an electrolyte thermodynamic model indicated that MD of 10 different mine wastes yields product water containing no contaminants at concentrations >0.2 ppm. MD tests of synthetic ARD used a ∼34 °C temperature difference, operated at 80% recovery, and produced an initial flux of 38.4 ± 1.1 L·m·h. This flux decreased slightly after scaling by iron oxyhydroxide; however, membranes maintained >99% dissolved solids rejection. Both flux decline and membrane scale formation decreased after a chemical-free, thermal precipitation pretreatment. These results indicate that MD can purify contaminated, acidic wastewater using low-grade heat sources, such as geothermal energy, without chemical addition.

Fouling and wetting in the membrane distillation driven wastewater reclamation process – A review

“…precipitate formation in this system [120]. 85% water recovery with only a 13-15% decline in flux and a 99% ion rejection were achieved.…”

“…Additionally, analytical techniques like EDX (energy dispersive x-ray spectroscopy) and/or XPS (x-ray photoelectron spectroscopy) have been used along with SEM to gather elemental composition of the membrane surfaces [5,19,47,104,[112][113][114][115]. Techniques like AFM (atomic force microscopy) and XRD (x-ray diffraction) have been employed to characterize the surface structure, roughness of the membranes [5,113,[116][117][118][119] and the crystal composition of the scales formed on the membrane surface, respectively [91,118,[120][121][122]. FTIR (Fourier transform infrared spectrometer) has been used to assess chemical composition of foulants (like functional groups, biopolymers, polysaccharides or humic acids), which helps in understanding the interaction and bonding between the foulants and the membrane surface [95,121,[123][124][125][126].…”

Membrane Distillation for Wastewater Treatment: A Mini Review

“…Table 3 details the many causes of MD wetting. The accumulation of inorganic sediments on the membrane surface in desalination brine concentration or acid mine drainage treatment can also lead to pore wetting [36,183]. The growth of inorganic scaling (usually mainly composed of calcium carbonate, calcium sulfate, and sodium chloride) on the membrane surface reduces the hydrophobicity of the membrane, eventually leading to complete wetting of the membrane pores [165,184].…”

Highly Hydrophobic Electrospun Reduced Graphene Oxide/Poly(vinylidene fluoride-co-hexafluoropropylene) Membranes for Use in Membrane Distillation

“…Membrane distillation (MD) shows promise for use in turning highly saline water into potable water with negligible operating conditions required (low operating pressure and temperature) while theoretically offering complete (100%) salt rejection. − This process is driven by a vapor pressure difference between porous hydrophobic membrane surfaces, through which only water vapor molecules can pass …”