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The Genesis of a Critical Environmental Concern: Cannabinoids in Our Water Systems.

A 192 million people worldwide aged 15−64 (i.e., 3.9% of the global population, per 2016 estimates) regularly use Cannabis, more commonly known as marijuana. The estimated market share of this widely used drug will surpass $22 billion by 2022. On June 28, 2018, the United States Food and Drug Administration approved Epidiolex, a cannabinoid-based drug developed for the treatment of a rare form of epilepsy. Sales of Epidiolex in the U.S. are expected to exceed $1.1 billion by 2022, reflecting a 1100% increase in consumption in less than a decade. The burgeoning demand for cannabinoid-based pharmaceuticals such as Epidiolex has shifted the perception of the “marijuana problem” from the use of an illicit substance (marijuana continues to be a Schedule I controlled substance) to the use of approved pharmaceuticals. Though the movement in public discourse may not necessarily change environmental release of recreationally consumed marijuana, its approval as a pharmaceutical will increase the release of cannabinoid-based drugs; due to predicted expansion of its market share as high as 700% by 2020. The annual development and release of pharmaceuticals and personal care products continue to expand the list of emerging contaminants (EC) every year. Similar to many pharmaceuticals, cannabinoid molecules are known to transform in both natural and engineered systems and thus render a higher-level uncertainty and toxicity. Predicted toxicity indices (i.e., −log LC50 for 48 h exposure to D. magna) for cannabinoids and their halogenated byproducts are orders of magnitude higher than those for the toxicity of brominated and chlorinated disinfection byproducts (DBPs) i.e., regulated trihalomethanes (THMs) and haloacetic acids (HAAs). Therefore, understanding the fate, transformation, and removal of cannabinoids in environmental systems is of great importance. Unfortunately, despite their widespread availability, uncertainty-in-point and mass production projections, adverse effects on the nervous system and increased pharmaceutical use, cannabinoids remain the most understudied class of ECs within aquatic systems. The transformation of these products, which are often more toxic than the parent compounds, encourages understanding the reaction processes that cause their development. Indeed, new organic contaminants or ECs that are created by the transformation processes in water and wastewater systems have been detected in waste and surface waters, which means that treatment processes must evolve accordingly. Halogenated methanesulfonic acid (HMAs), a new class of organic micropollutant produced from an approved drug, is now prevalent in the water cycle and is one of the latest addition to the EC list. As with HMAs, cannabinoids will likely introduce similar compounds during their passage through engineered treatment systems. Δ9-tetrahydrocannabinol (THC) is the primary psychoactive constituent within the cannabinoid class. THC is metabolized and excreted as 11-nor-9-carboxy-Δ9-tetrahydrocannabinol (THC−COOH) as its main urinary metabolite. This metabolite has been detected in untreated, treated, surface waters, and tap water up to 2500, 750, and 500 ng/L, and 1 ng/L, respectively. The molecular structure of THC and its metabolite(s) includes a heteroaromatic structure with an alkane chain. Although THC is the predominant psychoactive chemical in Cannabis sativa, there are other natural and synthetic cannabinoid-like chemicals. Cannabidiol (CBD), the main nonpsychoactive cannabinoid constituent, is such a compound that is biogenerated from the same parent precursor (i.e., cannabigerolic acid) with a very similar molecular structure. THC/THC−COOH and CBD molecules have a strong tendency to undergo chemical transformation when entering our treatment systems. The transformation of cannabinoids is particularly significant because of the presence of a phenolic group that undergoes electrophilic substitution with halogen atoms (e.g., chlorine and bromine) during disinfection. The chain-shortening and ring opening reactions and halogen-substitution during oxidation and disinfection need to be systematically evaluated for these emerging heteroaromatic molecules to assess their fate and removal in the treatment systems. THC/THC−COOH and CBD chemical structures bring unique reaction possibilities, when compared to common drug molecules. The most heavily excreted pharmaceuticals in our treatment systems are antibiotics, anticonvulsants, antidepressants, beta-blockers, analgesics, anti-inflammatory drugs,