Water Environment Research • 1–8, 2021 Research Article 1 Department of Geography & Geological Sciences, University of Idaho, Moscow, ID, USA 2 Department of Chemistry, University of Idaho, Moscow, ID, USA 3 Environmental Science Program, University of Idaho, Moscow, ID, USA 4 Department of Chemical & Biological Engineering, University of Idaho, Moscow, ID, USA Received 12 January 2021; Revised 9 March 2021; Accepted 10 March 2021 Office of Surface Mining, Reclamation and Enforcement, Grant/Award Number: S17AC20000 Correspondence to: Jeff B. Langman, Department of Geography & Geological Sciences, University of Idaho, Moscow, ID, USA. Email: jlangman@uidaho.edu DOI: 10.1002/wer.1557 © 2021 Water Environment Federation Clinoptilolite and iron sorption/desorption under multiple pH conditions: Testing a substrate for passive treatment of acidic, iron-rich solutions Jeff B. Langman , 1 Wes R. Sandlin , 1 Kris Waynant , 2 Michael Traver-Greene , 3 James G. Moberly 4 • Abstract Equilibrium sorption and desorption experiments were conducted with clinoptilo- lite to evaluate the potential sorption/desorption of iron during different pH con- ditions. Sorption experiments indicated a partitioning of 0% to 17% of the iron in solution given pH of 2 to 4. The pH 2 solution was able to desorb 70% of the iron that was captured from a pH 3 solution. The largest desorption and sorption of iron and corresponding pH represent the end points of iron capture primarily by sorption/ exchange. These endpoints are the estimated pH pzc of 2.5 and the initial precipitation point of iron(II) at pH ~3.5. This acidity range is where clinoptilolite is able to capture iron without precipitation or the occurrence of full surface protonation. The inability of the highest acidity to remove all sorbed iron represents the greater bound iron that will not readily desorb with a change in pH. This retained iron creates a metastable state of the clinoptilolite that has a lower sorption capacity but reflects the ability of clinoptilolite to retain a sorbed transition metal with changes in pH. As pH varies, clinoptilolite may evolve in a sequence of metastable states reflective of its ability to capture or retain metals. © 2021 Water Environment Federation • Practitioner points • Clinoptilolite is a capable reactive substrate, but its sorption/exchange effectiveness at low and variable pH and ability to retain captured metals was unknown. • Clinoptilolite retains its metal capture properties to a pH of 2.5 where surface proto- nation and mineral degradation likely occurs. • The ability of clinoptilolite to retain captured iron under greater acidity reflects an evolution of its sorption/retention capacity. • Key words acidity; clinoptilolite; desorption; metal retention; passive treatment; sorption; zeolite Introduction The oxidative weathering of sulfide minerals (e.g., pyrite) can result in the genera- tion of acid rock drainage (ARD) (Akcil & Koldas, 2006; Johnson & Hallberg, 2005; Nordstrom, 2009; Nordstrom & Alpers, 1997), which continues to significantly impact water resources across the United States and around the globe (Blowes et al., 2014; Johnson & Hallberg, 2005; Nordstrom & Nicholson, 2017). Iron is a primary metal associated with ARD because of the role of pyrite [FeS 2 ] and pyrrhotite [Fe (1- x) S] in acid generation (Belzile et al., 2004; Nordstrom et al., 2015). Passive treatment systems were developed to reduce contaminant concentrations, minimize health risks, and lower treatment costs by relying on natural physical, chemical, and bio- logical processes to treat drainage without regular maintenance, power inputs, and personnel requirements (Egiebor & Oni, 2007; Johnson & Hallberg, 2005; Kefeni et