Professional Education

  • Doctor of Philosophy, Johns Hopkins University (2017)

All Publications

  • 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


    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


    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


    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


    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