Occurrence, eco-toxicological risk assessment, sorption and degradation of selected emerging organic chemical pollutants in water and sediments of Lake Victoria, Uganda

Abstract

The occurrence of emerging chemical pollutants (ECPs) including; 25 pharmaceuticals, 11 personal care products, 6 plasticizers and 8 organophosphorus flame retardants was investigated in water and sediment samples collected from five bays in the Ugandan part of Lake Victoria. In addition, the potential environmental risk of the target emerging organic pollutant compounds to aquatic organisms in the lake ecosystem as well as sorption and degradation kinetics of the predominant ECPs in the lake sediment were evaluated. Moreover, the environment fate of the predominant ECPs was simulated using a Quantitative Water Air Sediment Interaction (QWASI) model. Samples were extracted for pharmaceuticals using solid phase method and analyzed using high-performance liquid chromatography coupled with triple quadrupole mass spectrometry (LC/MS/MS). Personal care products, plasticizers and flame retardants were extracted from water and sediment samples using liquid-liquid and soxhlet extraction respectively and analysed using gas chromatography mass spectrometry (GC/MS). The potential ecotoxic risk of the target ECPs to aquatic organisms in the lake ecosystem was assessed based on risk quotients (RQs), following the European Commission’s Technical Guidance Document on risk assessment. Sorption kinetics parameters and equilibrium sorption coefficient (KD) of selected pharmaceuticals on sediments from Lake Victoria were determined using batch sorption experiments. A two-site nonequilibria (TSNE) batch sorption kinetic model was used to describe the sorption kinetics data. Degradation studies were performed according to OECD Guideline 308, followed by solid phase extraction and LC-MS/MS analysis. Eighteen pharmaceuticals occurred in Lake Victoria waters at quantifiable concentrations, with sulfamethoxazole (1-5600 ng L-1), trimethoprim (1-89 ng L-1), tetracycline (3-70 ng L-1), sulfacetamide (1-13 ng L-1), and ibuprofen (6-780 ng L-1) occurring in all water samples. Murchison Bay, the main recipient of sewage effluents, industrial and municipal waste from Kampala city recorded the highest concentrations of pharmaceuticals. All the target pharmaceuticals were detected in sediment samples with ciprofloxacin (2.8-116 ng g-1), levofloxacin (2.2-133 ng g-1) and ibuprofen (6-50 ng g-1) being the most predominant. Most of the target PCPs were found in all the water samples with Triclosan (89-1400 ng L-1), benzophenone (36-1300 ng L-1), and 4-MBC (21-1500 ng L-1) being the most predominant PCPs. Similarly, all the target plasticizers were found in all the water samples with dibutyl phthalate (350-16 000 ng L-1), and bis-(2-ethylhexyl) phthalate (210-23000 ng L-1) being detected in highest concentrations. With respect to OPFRs, TCP (34-9657 ng g-1) was the most predominant followed by TEHP (39-5652 ng g-1), TPP (66-5218 ng g-1) and tris-(2-chloroethyl) phosphate (8.7-946 ng g-1). Likewise, with the exception of DBP and BEHA (98 % detection frequency), all target plasticizers were quantifiable in all the sediment samples. Generally, the concentrations of all the target ECPs in the lake sediments were several orders of magnitude higher than their respective concentrations in the aqueous phase (water). Ecotoxicological risk assessment for pharmaceuticals showed that sulfamethoxazole, oxytetracycline, erythromycin, and diclofenac pose a high toxic risk to aquatic organisms in both the lake´s water and sediment. A similar assessment showed that; PCPs (triclosan, musk ketone, 4-MBC, BP, TBHA, BHT, 3-MI), plasticizers [dibutyl phthalate, bis-(2-ethylhexyl) adipate, bis-(2-ethylhexyl) phthalate] and OPFRs (TPP, TCEP, EHDPP, TCP, EHDPP, TEHP) pose a high ecotoxic risk to lives of aquatic organisms in the lake (risk quotients, RQ >1). The sorption behaviour of pharmaceuticals on the sediments was found to depend on the amount of organic carbon with corresponding sorption coefficients being higher for sediment with higher organic content. Diclofenac showed the highest sorption coefficient whereas sulfamethoxazole showed the lowest sorption capacity and is therefore likely to be highly mobile in the aquatic environment. Degradation of selected pharmaceuticals in the lake sediments followed first-order, zero-order and zero-bicameral order kinetics. Carbamazepine showed the longest half-life in both Nakivubo (91 days) and Luzira sediments (61 days). Carbamazepine was therefore found to be persistent in Lake Victoria sediments since its half-life exceeds the U.S EPA persistence criterion of more than 60 days. Sulfamethoxazole showed the lowest half-life ranging between 2.31 days for biotic degradation to 5.77 days for abiotic degradation. The fate of the selected ECPs within Lake Victoria was simulated using a Quantitative Water Air Sediment Interaction (QWASI) model. The model predicted the residence times (years) of sulfamethoxazole, trimethoprim, ciprofloxacin, diclofenac, carbamazepine, triclosan, dibutyl phthalate, bis-(2-ethylhexyl) phthalate, tris-(2-chloroethyl) phosphate and tricresyl Phosphate within the lake system as 2.62x10-4, 3.04x10-4, 2.12x10-4, 5.36x10-4, 4.79x10-3, 2.29x10-3, 3.91x10-4, 1.34x10-3, 3.15x10-4 and 7.8x10-4 respectively.