ISSUE 20

Managing micropollutant contamination

Using thresholds to inform water treatment risks and needs

The Water Institute

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Omar

Omar Chowdhury
Department of Civil and Environmental Engineering

Philip

Philip Schmidt
Department of Civil and Environmental Engineering

Bill

William Anderson
Department of Civil and Environmental Engineering

Monica

Monica Emelko
Department of Civil and Environmental Engineering

Introduction

Microplastics have emerged as contaminants of concern due to their ubiquitous presence in many environments, including in marine and freshwater systems. Although human health impacts of microplastics are not well understood, concern regarding chemical contaminants retained on or within them is growing. Drinking water providers are increasingly faced with managing microplastic risks, but strategies for evaluating the risk, and the extent of treatment potentially required, are lacking.

By knowing the adsorption capacities of contaminants to various types of plastic, it is possible to use existing drinking water guidelines to develop preliminary guidance for managing microplastics in drinking water. This paper presents and applies the concept of Threshold Microplastic Concentration (TMC) to support the management of health risks attributable to waterborne microplastics in the water industry. Recognizing that a quantitative health risk assessment of microplastics themselves is not yet possible due to a lack of data, the TMC framework identifies (1) microplastic concentrations that may result in the intake of regulated contaminants on/in microplastics at levels of human health concern and (2) treatment targets for managing potential risks.

Methodology

To evaluate the TMC, six key data were collected, summarized and integrated: (1) microplastic size, (2) microplastic shape, (3) microplastic polymer type, (4) health guidelines defined by Maximum Contaminant Levels (MCLs) or Maximum Acceptable Concentrations (MACs), (5) contaminant adsorption capacity on plastics and (6) the extent of microplastic removal during drinking water treatment.

High density polyethylene (HDPE), low density polyethylene (LDPE), polyethylene terephthalate (PET), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC) and polycarbonate (PC) were considered in the TMC framework as they are the most common waterborne microplastics. The highest available adsorption capacity value for each contaminant was used and applied to all microplastics/polymer types. Drinking water guidelines or standards were used to identify health-based thresholds for the chemical contaminants adsorbed to the microplastics. A TMC was calculated for each contaminant, shape and a range of longest dimensions (Table 1).

Table 1. Lowest Drinking Water Guideline and Highest Adsorption Capacity on Plastics for Contaminants Used in the TMC Framework

contaminant	adsorption capacity (mg/g)	adsorption capacity (mg/m²)	microplastic type	microplastic size (μm)	MCL or MAC (mg/L)^a
aluminum	0.27^b	185	PET	3000	2.9
antimony	27.8	5720	PC	1000	0.006
arsenic	1.12	20	PS	100	0.003
bromine	13	2330	PS	1000	0.04
cadmium	0.03	0.748	HDPE	154	0.005
chromium	0.000454^b	0.31	PET	3000	0.03
copper	1.32	275	PVC	900	2
lead	0.00187^b	1.28	PET	3000	0.005
manganese	0.13^b	89	PET	3000	0.12
mercury	0.00125	0.0009	HDPE	4.5	0.002
polychlorinated biphenyls	0.35	247	PP	5000	0.0005

^aMCLs and MACs were obtained from USEPA, Canadian, Australian and WHO guidelines.
^bAmount desorbed after exposure to contaminant in the environment.

The TMC framework can then be used to evaluate if a given microplastics concentration, size, and shape of concern in a particular water supply, combined with the adsorption capacity for contaminants of concern, constitutes a potential risk. The potential health risk considers the worst-case scenario, where the maximum amount of a single contaminant is sorbed to the microplastics and released in the human gut upon ingestion of water containing the microplastics. Unlike MCLs and MACs, the TMC only indicates a microplastics concentration that may indicate a potential health risk if exceeded as the amount of contaminants actually present may be less than the adsorption capacity. If the microplastics concentration exceeds the TMC, this does not necessarily indicate an immediate health risk and the need for additional treatment; rather, it highlights that a potentially concerning concentration of microplastics is present and warrants further assessment of water quality and health risk. When TMC values are compared for different scenarios, a lower TMC corresponds to higher potential for risk because fewer microplastic particles are needed for contaminant exposure at concentrations associated with health impacts.

Outcomes

The major drivers of TMC were found to be microplastic size and contaminant type. TMCs decreased as particle size and the resulting surface area per particle increased. The lowest, potentially most concerning, TMCs were calculated for cube-shaped microplastics having a size of 750 μm that are contaminated with antimony (Figure 1).

Figure 1. TMC for combinations of microplastics size, shape and contaminants. For each particle size, TMC values are horizontally jittered to facilitate visualization.

Figure 2 shows the curve of TMC values as a function of size by using the lowest TMCs from among all combinations of factors across the size range analyzed and indicates how more particles are needed to adsorb a given amount of contaminant as the size of the microplastics gets smaller.

Figure 2. Curve with the lowest TMC values (generated for antimony sorbed on cube-shaped microplastics)

With an increase in surface area per particle due to increasing size or difference in shape, the TMC decreases because more area is available for contaminant adsorption meaning that fewer particles with the contaminant adsorbed at full capacity can carry the contaminant at concentrations exceeding the MCL/MAC compared with particles with a lower surface area per particle.

The effect of different contaminants on the TMC was driven by both the toxicity and adsorption capacity on microplastics. For all cases, a combination of higher adsorption capacity and higher toxicity of a contaminant resulted in a lower TMC when all other factors were constant (Figure 3).

Figure 3. Relationship between TMC and adsorption capacity for cube-shaped 750 μm microplastics. Values in square brackets are contaminant MCLs/MACs (mg/L).

For a given shape and size, the TMC had a range of about 6 orders of magnitude, highlighting contaminant type as a major driver of the TMC. Based on the TMCs calculated, the types of contaminants sorbed on microplastics were therefore found to be a key driver in evaluating potential health concerns from microplastics.

While the TMC results can provide an estimate of whether any given concentration of microplastics in source water constitutes potential health risks, it can also be used to calculate either the source or treated water microplastics concentration above which consumption of drinking water may result in the ingestion of regulated contaminants sorbed on microplastics at levels of human health concern or the extent of treatment required to produce potable water from a source water with a known microplastics concentration (Figure 4).

Figure 4. Scenarios where the TMC calculation framework can be used to evaluate unknowns in the drinking water pathway.

Conclusions

The study demonstrated that TMC is useful to indicate the total number of microplastic particles per liter of untreated source water or treated water that may constitute exposure to potentially harmful concentrations of chemical contaminants retained on microplastics if ingested. Water providers and regulators can use the science-based, easily updatable TMC approach to inform their risk management needs as it indicates if existing treatment is sufficient for managing potential health risks attributable to contaminants adsorbed to waterborne microplastics or if more detailed risk assessment is needed.

The TMC framework uses available microplastics and contaminant data, and it has been modularized such that it can be updated as more information regarding toxicity and sorption capacities of various combinations of contaminants and microplastics becomes available and can also incorporate expected treatment efficiency. Thus, it can be applied for system-specific risk management to calculate (1) the source water microplastics concentration above which consumption of downstream treated water may result in the ingestion of regulated contaminants sorbed on microplastics at levels of human health concern or (2) the extent of treatment required to produce potable water that does not result in the potential ingestion of regulated contaminants sorbed on microplastics at levels of human health concern.

While the present framework focused on evaluating TMCs for combinations of a single particle shape and contaminant type, it can be expanded to reflect new insights regarding key assumptions, water contaminants of health relevance, additives released from microplastics, health guidelines, heterogeneous mixtures of contaminants in water, and health impacts of microplastics themselves as more toxicological data become available.