Uptake of Contaminants of Emerging Concerns (CECs) by Plants Irrigated With Recycled Waters

The increasing demand for water in arid and semi-arid regions necessitates the use of recycled water (RW) for irrigation. RW is a viable alternative to conventional irrigation water at most locations, but it can be a major route for contaminants of emerging concerns (CECs) to be taken up by the tissues of food crops, thereby leading to their subsequent entry into the food chain. CECs include pharmaceuticals, personal care products, and industrial chemicals that are generally present at low concentrations in environmental waters, hence their common designation as trace organic compounds (TOrCs), and CECs also include microbiological constituents such as antibiotic resistance genes (ARGs). The presence of these contaminants in the edible parts of plants could pose a human health risk. Few studies have conducted comparative analysis of the uptake of TOrCs and ARGs in both soil and hydroponics systems for the same plants and contaminants under similar conditions. This study aims to address this gap. In this research, the uptake and accumulation of 13 TOrCs and 6 ARGs into plants irrigated with advanced treated RW, specifically after ultrafiltration (UF) and ozonation (OZ) was assessed. The uptake was evaluated as a function of plant type (spinach vs. tomato), tissue type (soil, roots, stems, leaves, and fruit), water type (tap, RW, and RW spiked with increasingly elevated concentrations of target compounds), level of treatment (non-ozonated UF water vs. ozonated UF water), and growing environment (soil pots vs. hydroponics). The edible and non-edible portions of plants were analyzed using microwave digestion, solid-phase extraction, and liquid chromatography-tandem mass spectrometry (LC-MS/MS) for detection and quantification of TOrCs. Also, DNA extraction, quantification and analysis by quantitative polymerase chain reaction (qPCR) were performed for ARGs analysis.For the soil system, pots irrigated with ozonated RW and sequential irrigation (i.e., tomato plants only; RW initially and then tap water upon flowering) exhibited lower TOrC concentrations in plant tissues compared to non-ozonated UF filtrate (with and without target compound spiking). Moreover, the accumulation of TOrCs was less in fruit compared to the leaves (p<0.05). Spinach leaves contained higher concentrations of TOrCs compared to tomato leaves, leading to higher risk of exposure. The log10BCFs (L/kg) (at least 80%) of the CECCs across all the plant structures for a particular irrigation water fall within the following intervals: tap water = 92% between 2 and 5, OZ = 91% between 0 and 5, UF = 85% between 1 and 4, low spike = 87% between -1 and 3, high spike = 81% between -2 and 2. For hydroponics experiments, UF irrigation water was found to be significantly (p < 0.05) beneficial to prevent the uptake of four contaminants (atenolol, benzotriazole, carbamazepine, and trimethoprim) in the fruits of tomato plants compared to OZ irrigation due to improved biomass and subsequent enhanced transpiration in OZ-grown plants. The OZ-grown spinach contained four contaminants [N, N-diethyl-meta-toluamide (DEET), lidocaine, sucralose, and sulfamethoxazole] in the leaves. Positive relationships were found between BCFs and the octanol-water partition coefficients [OZ: r2=0.72, UF: r2=0.79, (p<0.05)]. It should be noted that no relationships were found for soil grown plants due to the possible degradation, adsorption and complex chemistry of TOrCs in the soil. The results from both soil and hydroponics systems show de-minimis risk associated with the consumption of edible tissues of the plants. For ARG detection in both hydroponics and soil systems, all ARGs except tetO were detected in multiple plant tissue samples, with concentrations up to 105 gene copies per gram (gc/g). Ozone treatment showed lower effectiveness in reducing ARG concentrations, with log reduction values (LRVs) ranging from 0.23 to 0.94. In contrast, ultrafiltration (UF) effectively decreased ARG concentrations, achieving LRVs above 3. The concentration order (gc/g), based on average concentrations for all plant type, structure type, and irrigation water type, observed in plants grown in soil system was: vanA > ampC > sul1 > sul2 > tetW. Comparisons between irrigation techniques revealed that hydroponics could be preferable to avoid the uptake of vanA, tetO, and tetW ARGs. The results indicated less clarity in the patterns of ARG uptake concerning irrigation water quality. Data showed that ARG accumulation was most pronounced (1000 times) in roots. Despite the lack of established frameworks for quantitative microbial risk assessment in RW, the data from this study provides valuable insights into the risks associated with ARGs. The results from this study provide valuable insights into the safety and viability of RW irrigation including the assessment of potential human health risks and benefits of advanced water treatment processes. Additionally, this research enhances the understanding of in-plant transport mechanism of CECs and evaluates the viability of hydroponic systems. Wider impacts of this study help investigate how water scarcity, sustainability, and economic efficiency can be addressed in arid regions using nutrient containing recycled water in hydroponics. The study's conclusions also provide important information to help create evidence-based policies, guidelines, and regulations that will improve water resource management by encouraging recycled water reuse for agricultural irrigation.