BIO-SORPTION STUDIES OF POLYCYCLIC AROMATIC HYDROCARBONS AND PESTICIDES FROM SELECTED SURFACE WATERS IN NAROK AND BOMET COUNTIES, KENYA USING LOCALLY AVAILABLE BIOCHAR ADSORBENTS

Abstract

Narok and Bomet Counties of Kenya rely on surface waters mainly from rivers for domestic use, livestock and plant farming. Increased anthropogenic activities such as charcoal burning and agrochemical practices near riparian areas in the two Counties have led to environmental and ecotoxicological concerns of people and animals that rely on these waters. This study aimed to determine the presence and levels of polycyclic aromatic hydrocarbons (PAHs) and pesticide residues in surface waters from Narok and Bomet Counties; and further explore the bio-sorption of these pollutants using locally available biochar adsorbents. The surface waters of the two Counties were sampled via a two-factorial design based on geographical distribution and proximity to point-source pollutants during both wet and dry seasons. PAHs and pesticides were extracted by solid-phase extraction and analyzed by gas chromatography hyphenated to mass spectrometry (GC-MS) and GC- time of flight mass spectrometry (GC-ToFMS) respectively. Biochar adsorbents were prepared from cow dung (CDB) and sugarcane bagasse (SBB) and characterized. Adsorption studies of the prevalent PAHs and pesticides were conducted using the two adsorbents. Various adsorption optimization studies were conducted to determine the optimal adsorption conditions. There were 38 different types of pesticides detected in the two Counties. The pesticides chlorpyrifos, cypermethrin, cyfluthrin and cyhalothrin were more prevalent in Narok County while triazine, semicarbazone and epinephrine were more prevalent in Bomet County. There were significantly more pesticides detected during the wet season (p ≤ 0.05). Four of these posed serious ecotoxicology concerns with risk quotients above 1.0 (high risk). Seven United States Environmental Protection Agency (US EPA) priority PAHs were detected in concentrations of up to 31.42 µgL-1 (dibenzo[a,h]anthracene). The surface waters were significantly polluted with dibenzo[a,h]anthracene with risk quotients above 1.0 in the surface waters and were found to be hazardous, with hazard quotients above 10.0, thus indicating potential environmental risks. The biochar findings indicated the SBB sample to have a smoother morphology with a higher C/H/O ratio compared to the CDB which had more mineral oxides composition. The XRD diffractograms indicated the samples were largely amorphous while the TGA and DTG thermograms showed CDB to be more thermal resilient than SBB. The adsorption studies indicated high adsorption efficacies of between 78 to 90 % under optimal conditions of: 10 – 15 °C, pH of 4 – 8, 30 min sorption time, sorbate concentration of 4.0 ppb and adsorbent dosage of 2.0 g/Lsorbate. Temkins adsorption model was most favored model while the adsorption processes were feasible and spontaneous under room temperature conditions. The adsorbents were found to be regenerated with an adsorption capacity of 63 – 80 % for the third adsorption cycle. The study indicated pesticide and PAH pollution in the surface waters of Bomet and Narok Counties thus there is a need for more government policy interventions in deterring farming near riparian lands and in training of famers regarding best practice for pesticide applications. The study also recommends fabrication of biochar-based filters from locally available materials for removal of pesticides and PAHs in waste waters.