Professional Education


  • Doctor of Philosophy, Johns Hopkins University (2017)

All Publications


  • Disinfection byproducts formed during drinking water treatment reveal an export control point for dissolved organic matter in a subalpine headwater stream. Water research X Leonard, L. T., Vanzin, G. F., Garayburu-Caruso, V. A., Lau, S. S., Beutler, C. A., Newman, A. W., Mitch, W. A., Stegen, J. C., Williams, K. H., Sharp, J. O. 2022; 15: 100144

    Abstract

    Changes in climate, season, and vegetation can alter organic export from watersheds. While an accepted tradeoff to protect public health, disinfection processes during drinking water treatment can adversely react with organic compounds to form disinfection byproducts (DBPs). By extension, DBP monitoring can yield insights into hydrobiogeochemical dynamics within watersheds and their implications for water resource management. In this study, we analyzed temporal trends from a water treatment facility that sources water from Coal Creek in Crested Butte, Colorado. These trends revealed a long-term increase in haloacetic acid and trihalomethane formation over the period of 2005-2020. Disproportionate export of dissolved organic carbon and formation of DBPs that exceeded maximum contaminant levels were consistently recorded in association with late spring freshet. Synoptic sampling of the creek in 2020 and 2021 identified a biogeochemical hotspot for organic carbon export in the upper domain of the watershed that contained a prominent fulvic acid-like fluorescent signature. DBP formation potential analyses from this domain yielded similar ratios of regulated DBP classes to those formed at the drinking water facility. Spectrometric qualitative analyses of pre and post-reacted waters with hypochlorite indicated lignin-like and condensed hydrocarbon-like molecules were the major reactive chemical classes during chlorine-based disinfection. This study demonstrates how drinking water quality archives combined with synoptic sampling and targeted analyses can be used to identify and understand export control points for dissolved organic matter. This approach could be applied to identify and characterize analogous watersheds where seasonal or climate-associated organic matter export challenge water treatment disinfection and by extension inform watershed management and drinking water treatment.

    View details for DOI 10.1016/j.wroa.2022.100144

    View details for PubMedID 35542761

  • Disinfection Byproduct Recovery during Extraction and Concentration in Preparation for Chemical Analyses or Toxicity Assays. Environmental science & technology Lau, S. S., Forster, A. L., Richardson, S. D., Mitch, W. A. 2021

    Abstract

    Over 700 disinfection byproducts (DBPs) have been identified, but they account for only 30% of total organic halogen (TOX). Extracting disinfected water is necessary to assess the overall toxicity of both known and unknown DBPs. Commonly used DBP extraction methods include liquid-liquid extraction (LLE) and solid-phase extraction (SPE), which may use either XAD resins or other polymeric sorbents. With few exceptions, DBP recoveries have not been quantified. We compared recoveries by LLE, XAD resins, and a mixture of Phenomenex Sepra SPE sorbents (hereafter SPE) for (semi-)volatile DBPs and nonvolatile model compounds at the 1-L scale. We scaled up the three methods to extract DBPs in 10 L of chlorinated creek waters. For (semi-)volatile DBPs, XAD resulted in lower recoveries than LLE and SPE at both 1- and 10-L scales. At the 10-L scale, recovery of certain trihalomethanes and trihalogenated haloacetic acids by XAD was negligible, while recovery of other (semi-)volatile DBPs extracted by XAD (<30%) was lower than by SPE or LLE (30-60%). TOX recovery at the 10-L scale was generally similar by the three extraction methods. The low TOX recovery (<30%) indicates that the toxicity assessed by bioassays predominantly reflects the contribution of the nonvolatile, hydrophobic fraction of DBPs.

    View details for DOI 10.1021/acs.est.1c04323

    View details for PubMedID 34618438

  • Exotic Electrophiles in Chlorinated and Chloraminated Water: When Conventional Kinetic Models and Reaction Pathways Fall Short ENVIRONMENTAL SCIENCE & TECHNOLOGY LETTERS Rose, M. R., Lau, S. S., Prasse, C., Sivey, J. D. 2020; 7 (6): 360–70
  • Assessing Additivity of Cytotoxicity Associated with Disinfection Byproducts in Potable Reuse and Conventional Drinking Waters. Environmental science & technology Lau, S. S., Wei, X. n., Bokenkamp, K. n., Wagner, E. D., Plewa, M. J., Mitch, W. A. 2020

    Abstract

    Recent studies used the sum of the measured concentrations of individual disinfection byproducts (DBPs) weighted by their Chinese hamster ovary (CHO) cell cytotoxicity LC50 values to estimate the DBP-associated cytotoxicity of disinfected waters. This approach assumed that cytotoxicity was additive rather than synergistic or antagonistic. In this study, we evaluated whether this assumption was valid for mixtures containing DBPs at the concentration ratios measured in authentic disinfected waters. We examined the CHO cell cytotoxicity of defined DBP mixtures based on the concentrations of 43 regulated and unregulated DBPs measured in eight drinking and potable reuse waters. The hypothesis for additivity was supported using three experimental approaches. First, we demonstrated that the calculated additive toxicity (CAT) and bioassay-based calculated additive toxicity (BCAT) of the DBP mixtures agree within 12% on a median basis. We also found an additive toxicity response (CAT ≈ BCAT) between the regulated and unregulated DBP classes. Finally, the empirical biological cytotoxicity of the DBP subset mixtures, independent of the calculated toxicity, was additive. These results support the validity of using the sum of cytotoxic potency-weighted DBP concentrations as an estimate of the CHO cell cytotoxicity associated with known DBPs in real disinfected waters.

    View details for DOI 10.1021/acs.est.0c00958

    View details for PubMedID 32275830

  • Aqueous Chlorination Kinetics of Cyclic Alkenes-Is HOCI the Only Chlorinating Agent that Matters? ENVIRONMENTAL SCIENCE & TECHNOLOGY Lau, S. S., Reber, K. P., Roberts, A. 2019; 53 (19): 11133–41

    Abstract

    Although Cl2 and Cl2O have been recognized as highly reactive constituents of free available chlorine (FAC), robust rate constants for Cl2 and Cl2O remain scarce in the environmental literature. In this work, we explored the chlorination kinetics of three structurally related alkenes (α-ionone, β-ionone, and dehydro-β-ionone), a class of compounds whose reactivities with Cl2 and Cl2O have not been previously investigated. Second-order rate constants for Cl2, Cl2O, and HOCl were computed from experimental rate constants obtained at various pH values, [Cl-], and [FAC]. Our results show that while HOCl is the predominant chlorinating agent for the most reactive alkene, Cl2 and Cl2O can dominate the chlorination kinetics of the less reactive alkenes at high [Cl-] and high [FAC], respectively. The tradeoff between overall reactivity with FAC and selectivity for Cl2 and Cl2O previously observed for aromatic compounds also applies to the alkenes examined. In laboratory experiments in which high [FAC] may be used, omission of Cl2O in data modeling could yield second-order rate constants of dubious validity. In chlorinating real waters with elevated [Cl-], formation of Cl2 may enhance the formation kinetics of chlorinated disinfection byproducts (DBPs) and exacerbate the burden of DBP control for water utilities.

    View details for DOI 10.1021/acs.est.9b01171

    View details for Web of Science ID 000488993500010

    View details for PubMedID 31478649

  • 1,3,5-Trimethoxybenzene (TMB) as a new quencher for preserving redox-labile disinfection byproducts and for quantifying free chlorine and free bromine ENVIRONMENTAL SCIENCE-WATER RESEARCH & TECHNOLOGY Lau, S. S., Dias, R. P., Martin-Culet, K. R., Race, N. A., Schammel, M. H., Reber, K. P., Roberts, A., Sivey, J. D. 2018; 4 (7): 926–41

    View details for DOI 10.1039/c8ew00062j

    View details for Web of Science ID 000436569400003

  • Chlorination Revisited: Does Cl- Serve as a Catalyst in the Chlorination of Phenols? ENVIRONMENTAL SCIENCE & TECHNOLOGY Lau, S. S., Abraham, S. M., Roberts, A. 2016; 50 (24): 13291–98

    Abstract

    The aqueous chlorination of (chloro)phenols is one of the best-studied reactions in the environmental literature. Previous researchers have attributed these reactions to two chlorine species: HOCl (at circum-neutral and high pH) and H2OCl+ (at low pH). In this study, we seek to examine the roles that two largely overlooked chlorine species, Cl2 and Cl2O, may play in the chlorination of (chloro)phenols. Solution pH, chloride concentration, and chlorine dose were systematically varied in order to assess the importance of different chlorine species as chlorinating agents. Our findings indicate that chlorination rates at pH < 6 increase substantially when chloride is present, attributed to the formation of Cl2. At pH 6.0 and a chlorine dose representative of drinking water treatment, Cl2O is predicted to have at best a minor impact on chlorination reactions, whereas Cl2 may contribute more than 80% to the overall chlorination rate depending on the (chloro)phenol identity and chloride concentration. While it is not possible to preclude H2OCl+ as a chlorinating agent, we were able to model our low-pH data by considering Cl2 only. Even traces of chloride can generate sufficient Cl2 to influence chlorination kinetics, highlighting the role of chloride as a catalyst in chlorination reactions.

    View details for DOI 10.1021/acs.est.6b03539

    View details for Web of Science ID 000390620900015

    View details for PubMedID 27993072