Metals Treatment and Removal Mechanisms

1
Neutral Mine Drainage Treatment with Constructed
Wetlands Mark W. Fitch, Joel G. Burken, Chang Ye
Department of Civil, Architectural and
Environmental Engineering University of
Missouri-Rolla, Rolla, Missouri
Metals Treatment and Removal Mechanisms
Abstract Heavy metals at low levels in mine
drainage, mine tailings leachate, and industrial
wastewater is a national issue. In Missouri, the
legacy of lead mining during the last 130 years
includes several Superfund sites. Constructed
wetlands pose a passive treatment system that may
provide efficient, sustainable treatment for long
periods at such sites. Laboratory scale wetlands
have been operating for 6 years, sustaining
effective lead and zinc removal and reducing
toxicity to acceptable levels. Removal
mechanisms include sorption to the wetlands
media, precipitation as metal sulfides, and
co-precipitation with iron oxy/hydroxides. Any
of the three mechanisms can dominate under
different design and operating conditions. Tests
on the wetland media following years of metals
accumulation show that small fractions of the
metals can be remobilized under certain
conditions. Physical disturbance of the media
resulted in elevated metals being released in a
colloidal fraction, whereas a drying period
resulted in an increase in the soluble metals,
likely do to oxidation of metals sulfides.
Findings overall show that constructed wetlands
can efficiently and effectively treat alkaline
mine drainage and process waters, and design and
operations are important in optimizing treatment
and long-term stability.
Overview Treatment wetlands were prepared in
the laboratory, Figure 1. Media used in the
wetland were partially based on local
availability and low cost and included chip
bark, alfalfa, composted manure, silica sand, and
peat moss. Removal has averaged 90 for Pb and
75 for Zn overall and been steady over 6 years.
Individual removal mechanisms were investigated
in laboratory experiments, including sulfide
generation and precipitation of metal-sulfide
precipitates, adsorption isotherm determinations
and sequential extraction and microscopic
analyses of media from operating wetlands.
Adsorption Greater sorption potential than
previously thought was discovered. Individually,
wetland components had higher sorption capacities
than had been shown, Figure 2. Recent kinetics
testing suggests that sorption may be more rapid,
reaching apparent equilibrium in as little as 12
hours. Sulfide Precipitation Sulfide
generation and metal-sulfide precipitation
potential was tested using a dialysis membrane to
isolate media and surface phenomenon. Sulfide
was actively generated by the wetland media, and
lead was repeatedly removed from solution, Figure
3. Sulfate reducing bacteria (SRB) were
identified in the laboratory wetlands. Using
Scanning Electron Microscope-Energy Diffraction
Spectrometry (SEM-EDS) zinc sulfide and lead
sulfide, Figure 4, were positively identified in
wetland media. Iron Coprecipitation Lead and
zinc rapidly coprecipitated with iron
oxy/hydroxides. Iron oxides were clearly present
in the laboratory wetlands. Sequential
extraction showed that up to 30 to 70 of Pb and
Zn were removed in the gravel lens and the
initial portion of the wetland media.
Coprecipitation was highly variable on iron
content and aeration time. Summary Overall,
removal of zinc and lead was consistent over 6
years in laboratory wetlands. The effluent had a
negligible toxicity to fathead minnows and C.
dubia. Estimates on relative fates were
generated, Figure 5. One remaining question is
the potential precipitation of metals following
desorption from the wetland media if the local
metals concentration is decreased in the presence
of sulfide. Kinetics suggest that sorption is
much more rapid, yet sulfides were readily
located.
Figure 1 Schematic of laboratory wetlands

• Objectives
• Investigate the relative fates of Pb and Zn in
  constructed wetlands..
• Elucidate removal capacity and failure modes of
  constructed wetlands treating Pb and Zn.
• Determine the impact of disturbances on the
  sequestered metals.
• Develop design and operational information on
  wetlands treatment of mine drainage and process
  waters.

Figure 2 Sorption to wetland media constituents

• Disturbance Testing Release of Metals
• Rationale Various forms have differing stability
  and are stability is environmentally dependent.
  When environmental parameters such as pH or redox
  change, different forms may not remain
  sequestered. Some of these environmental changes
  such as reconstruction work, bioturbation by
  rooting animals, or drying from drought
  conditions or drainage can be abrupt and could
  potentially result in a high concentration, short
  duration release of the sequestered metals. The
  uncertainty of the effects of disturbances on the
  fates of fixed metals in constructed wetlands
  could stifle application.
• Testing Wetland media was taken from the 6 year
  old wetland, separated and placed into two new
  wetlands, similar to Figure 1. The two were
  subjected to different tests 1. Physical
  disturbance (i.e. repeated mixing) and 2. Drying.
  Following the perturbations, influent was
  returned and the effluent was tested for both
  filtered and unfiltered Pb and Zn levels.
• Findings Effluent metals concentrations in the
  physical disturbance test were elevated in the
  unfiltered samples, Figure 6. The particulate
  form of the metals suggests that sequestered
  metals were not chemically altered, but rather
  the release of colloidal sized particulates, as
  is likely as colloidal lead sulfides and lead-DOM
  complexes have been identified, Figures 4 5.
  The release was very brief and repeated mixing
  did not result in continued release, showing that
  the sequestered metals were stable. Effluent
  from the drying test was elevated in dissolved
  metals, reaching up to 450 ppb. The release was
  quite brief as in just a few pore volumes, the
  lead levels dropped back below the average
  influent concentration, Figure 7. The brief
  duration of the impacts suggest that wetlands are
  robust in their functionality and can quickly
  recover after anticipated perturbations.

Figure 6 Effluent from physical disturbance test.
Closed symbols are unfiltered samples, open
symbols are filtered
Figure 4 Lead-sulfide Via SEM - EDS
Figure 3 Sulfide precipitation
Figure 5 Composite of metals fate in lab scale
wetlands

• Conclusions Recommendations
• Constructed wetlands can provide sustained
  removal of Pb and Zn and reduce whole effluent
  toxicity (WET) with minimal operations and
  maintenance.
• Metals sequestration occurs via three primary
  mechanisms Adsorption, Sulfide precipitation
  Iron oxy/hydroxide coprecipitation. Multiple,
  complimentary mechanisms result in the observed
  reliability.
• Adsorption capacity is higher than previously
  thought, related to wetlands composition.
• Sulfide precipitation was observed, but was
  slower than thought and appears to be of lesser
  importance than previously published.
• Remobilization is a minimal concern after the
  wetlands have sequestered the metals and can be
  mitigated by avoiding dry conditions and major
  disturbances of the wetland.

Figure 7 Effluent from drying test for filtered
and unfiltered samples