Project 4: Designing the next generation of highly selective sorbents for water remediation

Removing metals and other chemicals that can contaminate drinking water is critical for public health, but traditional approaches to filter and remove neurotoxic metals are not sufficiently selective in the presence of other non-toxic ions, which frequently occur at higher concentrations, have similar chemical structures, and compete for sorption sites. 

Our Goals   

This project aims to develop a drinking water filter technology that 

  • selectively removes neurotoxic oxo-anions from drinking water by adsorption
  • can be used for a variety of water treatment systems (e.g., individual households, on well water, small-scale community systems or schools)
  • are more effective, efficient, and sustainable than existing technologies.

Our Approach

Recent advances in polymer- and nano- science allow for unprecedented bottom-up capabilities to thermodynamically model, characterize, and controllably synthesize adsorbents. Project 4 is designing selectively adsorptive polymers based on the waste bio-polymer chitosan and metal oxides particles. By selectively exposing different surfaces of the metal oxide particles, we are able to exploit chemical behavioral differences such as polarity, charge distribution, size, and hydrophobicity between the target oxoanion metal pollutants and other naturally occurring competing ions, to generate highly selective and tunable polymeric and nano-surfaces.    

Project 4 Team

Project 4 News

Project 4 study featured in NIEHS 'Papers of the Month'

Project 4 study featured in NIEHS 'Papers of the Month'

February 8, 2024

Project 4's latest work on filtering arsenic from drinking water is featured in NIEHS's Environmental Factor newsletter. See Culinary-inspired technique removes arsenic from waterThe study, pubished in Environmental Science and Technology, was conducted in Paul Westerhoff's lab at Arizona State University. Authors include MEMCARE trainees Alireza Farsad and Mariana Marcos...

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Recent Publications

Holly E Rudel and Julie B Zimmerman. 2023. “Elucidating the Role of Capping Agents in Facet-Dependent Adsorption Performance of Hematite Nanostructures.” ACS Appl Mater Interfaces, 15, 29, Pp. 34829-34837.Abstract

Organic capping agents are a ubiquitous and crucial part of preparing reproducible and homogeneous batches of nanomaterials, particularly nanocrystals with well-defined facets. Despite studies reporting surface ligands (e.g., capping agents) having a non-negligible role in catalytic behavior, their impact is less understood in contaminant adsorption, an important consideration given their potential to obfuscate facet-dependent trends in performance. To ascribe observed behaviors to the facet or the ligand, this report evaluates the impact of poly(-vinyl-2-pyrrolidone) (PVP), a commonly utilized capping agent, on the adsorption performance of nanohematite particles of varying prevailing facet in the removal of selenite (Se(IV)) as a model system. The PVP capping agent reduces the available surface area for contaminant binding, thus resulting in a reduction in overall Se(IV) adsorbed. However, accounting for the effects of surface area, {012}-faceted nanohematite demonstrates a significantly higher sorption capacity for Se(IV) compared with that of {001}-faceted nanohematite. Notably, chemical treatment is minimally effective in removing strongly bound PVP, indicating that complete removal of surface ligands remains challenging.

Lauren N Pincus, Holly E Rudel, Predrag V Petrović, Srishti Gupta, Paul Westerhoff, Christopher L Muhich, and Julie B Zimmerman. 2020. “Exploring the Mechanisms of Selectivity for Environmentally Significant Oxo-Anion Removal during Water Treatment: A Review of Common Competing Oxo-Anions and Tools for Quantifying Selective Adsorption.” Environ Sci Technol, 54, 16, Pp. 9769-9790.Abstract

Development of novel adsorbents often neglects the competitive adsorption between co-occurring oxo-anions, overestimating realistic pollutant removal potentials, and overlooking the need to improve selectivity of materials. This critical review focuses on adsorptive competition between commonly co-occurring oxo-anions in water and mechanistic approaches for the design and development of selective adsorbents. Six "target" oxo-anion pollutants (arsenate, arsenite, selenate, selenite, chromate, and perchlorate) were selected for study. Five "competing" co-occurring oxo-anions (phosphate, sulfate, bicarbonate, silicate, and nitrate) were selected due to their potential to compete with target oxo-anions for sorption sites resulting in decreased removal of the target oxo-anions. First, a comprehensive review of competition between target and competitor oxo-anions to sorb on commonly used, nonselective, metal (hydr)oxide materials is presented, and the strength of competition between each target and competitive oxo-anion pair is classified. This is followed by a critical discussion of the different equations and models used to quantify selectivity. Next, four mechanisms that have been successfully utilized in the development of selective adsorbents are reviewed: variation in surface complexation, Lewis acid/base hardness, steric hindrance, and electrostatic interactions. For each mechanism, the oxo-anions, both target and competitors, are ranked in terms of adsorptive attraction and technologies that exploit this mechanism are reviewed. Third, given the significant effort to evaluate these systems empirically, the potential to use computational quantum techniques, such as density functional theory (DFT), for modeling and prediction is explored. Finally, areas within the field of selective adsorption requiring further research are detailed with guidance on priorities for screening and defining selective adsorbents.

Elliot Reid, Thomas Igou, Yangying Zhao, John Crittenden, Ching-Hua Huang, Paul Westerhoff, Bruce Rittmann, Jörg E Drewes, and Yongsheng Chen. 2023. “The Minus Approach Can Redefine the Standard of Practice of Drinking Water Treatment.” Environ Sci Technol, 57, 18, Pp. 7150-7161.Abstract

Chlorine-based disinfection for drinking water treatment (DWT) was one of the 20th century's great public health achievements, as it substantially reduced the risk of acute microbial waterborne disease. However, today's chlorinated drinking water is not unambiguously safe; trace levels of regulated and unregulated disinfection byproducts (DBPs), and other known, unknown, and emerging contaminants (KUECs), present chronic risks that make them essential removal targets. Because conventional chemical-based DWT processes do little to remove DBPs or KUECs, alternative approaches are needed to minimize risks by removing DBP precursors and KUECs that are ubiquitous in water supplies. We present the "Minus Approach" as a toolbox of practices and technologies to mitigate KUECs and DBPs without compromising microbiological safety. The Minus Approach reduces problem-causing chemical addition treatment (i.e., the conventional "Plus Approach") by producing biologically stable water containing pathogens at levels having negligible human health risk and substantially lower concentrations of KUECs and DBPs. Aside from ozonation, the Minus Approach avoids primary chemical-based coagulants, disinfectants, and advanced oxidation processes. The Minus Approach focuses on bank filtration, biofiltration, adsorption, and membranes to biologically and physically remove DBP precursors, KUECs, and pathogens; consequently, water purveyors can use ultraviolet light at key locations in conjunction with smaller dosages of secondary chemical disinfectants to minimize microbial regrowth in distribution systems. We describe how the Minus Approach contrasts with the conventional Plus Approach, integrates with artificial intelligence, and can ultimately improve the sustainability performance of water treatment. Finally, we consider barriers to adoption of the Minus Approach.

Alireza Farsad, Ken Niimi, Mahmut Selim Ersan, Jose Ricardo Gonzalez-Rodriguez, Kiril D Hristovski, and Paul Westerhoff. 2023. “Mechanistic Study of Arsenate Adsorption onto Different Amorphous Grades of Titanium (Hydr)Oxides Impregnated into a Point-of-Use Activated Carbon Block.” ACS ES&T EngineeringACS ES&T Engineering, 3, 7, Pp. 989 - 1000. Publisher's VersionAbstract
Millions of households still rely on drinking water from private wells or municipal systems with arsenic levels approaching or exceeding regulatory limits. Arsenic is a potent carcinogen, and there is no safe level of it in drinking water. Point-of-use (POU) treatment systems are a promising option to mitigate arsenic exposure. However, the most commonly used POU technology, an activated carbon block filter, is ineffective at removing arsenic. Our study aimed to explore the potential of impregnating carbon blocks with amorphous titanium (hydr)oxide (THO) to improve arsenic removal without introducing titanium (Ti) into the treated water. Four synthesis methods achieved 8–16 wt % Ti-loading within the carbon block with a 58–97% amorphous THO content. The THO-modified carbon block could adsorb both oxidation states of arsenic (arsenate and arsenite) in batch or column tests. Modified carbon block with higher Ti and amorphous content always led to better arsenate removal, achieving arsenic loadings up to 31 mg As/mg Ti after 70,000 bed volumes in continuous-flow tests. Impregnating carbon block with amorphous THO consistently outperformed impregnation using crystalline TiO2. The best-performing system (TTIP-EtOH carbon block) was an amorphous THO derived using titanium isopropoxide, ethanol, and acetic acid via the sol–gel technique, aged at 80 °C for 18 h and dried overnight at 60 °C. Comparable pore-size distribution and surface area of the impregnated carbon blocks suggested that chemical properties play a more crucial role than physical and textural properties in removing arsenate via the amorphous Ti-impregnated carbon block. Freundlich isotherms indicated energetically favorable adsorption for amorphous chemically synthesized adsorbents. The mass transport coefficients for the amorphous TTIP-EtOH carbon block were fitted using a pore-surface diffusion model, resulting in Dsurface = 3.1 × 10–12 and Dpore = 3.2 × 10–6 cm2/s. Impregnating the carbon block with THO enabled effective arsenic removal from water without adversely affecting the pressure drop across the unit or the carbon block’s ability to remove polar organic chemical pollutants efficiently.Millions of households still rely on drinking water from private wells or municipal systems with arsenic levels approaching or exceeding regulatory limits. Arsenic is a potent carcinogen, and there is no safe level of it in drinking water. Point-of-use (POU) treatment systems are a promising option to mitigate arsenic exposure. However, the most commonly used POU technology, an activated carbon block filter, is ineffective at removing arsenic. Our study aimed to explore the potential of impregnating carbon blocks with amorphous titanium (hydr)oxide (THO) to improve arsenic removal without introducing titanium (Ti) into the treated water. Four synthesis methods achieved 8–16 wt % Ti-loading within the carbon block with a 58–97% amorphous THO content. The THO-modified carbon block could adsorb both oxidation states of arsenic (arsenate and arsenite) in batch or column tests. Modified carbon block with higher Ti and amorphous content always led to better arsenate removal, achieving arsenic loadings up to 31 mg As/mg Ti after 70,000 bed volumes in continuous-flow tests. Impregnating carbon block with amorphous THO consistently outperformed impregnation using crystalline TiO2. The best-performing system (TTIP-EtOH carbon block) was an amorphous THO derived using titanium isopropoxide, ethanol, and acetic acid via the sol–gel technique, aged at 80 °C for 18 h and dried overnight at 60 °C. Comparable pore-size distribution and surface area of the impregnated carbon blocks suggested that chemical properties play a more crucial role than physical and textural properties in removing arsenate via the amorphous Ti-impregnated carbon block. Freundlich isotherms indicated energetically favorable adsorption for amorphous chemically synthesized adsorbents. The mass transport coefficients for the amorphous TTIP-EtOH carbon block were fitted using a pore-surface diffusion model, resulting in Dsurface = 3.1 × 10–12 and Dpore = 3.2 × 10–6 cm2/s. Impregnating the carbon block with THO enabled effective arsenic removal from water without adversely affecting the pressure drop across the unit or the carbon block’s ability to remove polar organic chemical pollutants efficiently.
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