Sorption and Release of Nickel and Zinc Using a Mixed-Algae Community Collected from a Mine Drainage Passive Treatment System

Mine water from upwellings in Commerce, Oklahoma is treated by the Mayer Ranch Passive Treatment System (MRPTS) to remove contaminants. Nickel [Ni(II)] and zinc [Zn(II)], which are toxic to both plants and animals when present in elevated concentrations, are still detectable at the effluent of the system out of the polishing pond (also known as Cell 6). Research has shown that living algae are capable of both adsorption and absorption of metals, whereas dead algae biomass can only adsorb metals due to the absence of metabolic processes. Previous exposure to metal contaminants influences the metal uptake efficiency of algae, as well as future growth rates when contaminants are present. Researchers have hypothesized that sorbed metals will be released from algae detritus as the algae decomposes. This research examined Ni(II) and Zn(II) sorption and release by a community of mixed algae species collected from MRPTS. Effluent water from MRPTS containing detectable levels of both Ni(II) ($>0.015\mathrm{mg}/\mathrm{L}$) and Zn(II) ($>0.25\mathrm{mg}/\mathrm{L}$) was used to create elevated concentration solutions (0.5, 2.0, 5.0, 10.0, and $20.0\mathrm{mg}/\mathrm{L}$) of both Ni(II) and Zn(II). A solution of MRPTS final cell effluent water with no addition of Ni(II) or Zn(II) also was included, along with a no-algae control solution with Ni(II) and Zn(II), for comparison of results. All analyses were performed in triplicate. Samples were incubated in the laboratory to mimic changing seasonal environmental conditions to which the native algae community in a passive treatment system (PTS) would be exposed, thus causing growth, death, and decomposition of that algae. Algae and associated solutions for each sample at the end of each phase (growth, chilled, and decomposition) were processed using microwave-assisted acid digestion to extract the metals present in the samples. The data obtained from these experiments showed sorption of both Ni(II) and Zn(II) by the algae community during the growth phase. The algae released a portion of the previously sorbed metals during the chilled phase. Samples showed continued sorption during the decomposition phase. Greater-concentration solutions had greater levels of sorption by the algae. The data followed the Langmuir isotherm model ($R^2>0.90$), indicating that the principal metal removal mechanism was adsorption. These data indicate that algae, and ensuing decomposing algal biomass, both are capable of removing and retaining Ni(II) and Zn(II) from contaminated waters. Hence, natural algae populations within PTS can provide additional water treatment.