(746b) Mechanisms and Kinetics of Biochar Mediated Sorption of Linuron, Diuron and Atrazine from Aqueous Media

The continued intensive use of pesticides for agricultural and non-agricultural purposes has led to their presence detection in various environmental compartments- surface water, groundwater, soil, air. Whereas the maximum permissible concentration of an herbicide such as diuron in the drinking water for instance, is 100 ng/L, research has shown that 70% of water samples collected from rivers around Europe contains concentration as high as 864ng/L. Consequently, it is aimed to facilitate effective removal of pesticides in water. Adsorption - as a surface phenomenon, has been found to hold a great potential for this task. Due to the amount and type of functional groups found on biochars, and their relatively low cost, these materials have shown great potentials for sorption of several organic micropollutants from aqueous solution. To this end, two biochars, B1 and B2 derived from potato peels and livestock feeds residues respectively, were tested as adsorbents for the removal of two phenylurea herbicides (diuron and linuron) and one triazine (atrazine) from aqueous media. The physicochemical properties of the biochars were studied using proximate analysis, CEC, %C, %N, C:N, specific surface area, methylene blue number, Fourier Transform Infrared Spectroscopy, and X-ray diffractometry. Batch adsorption was performed by varying adsorbent dosage, initial solute concentration, and solution pH. The equilibrium data were analysed with Langmuir and Freundlich equations. The physicochemical studies revealed specific surface areas of 1.35 (B1) and 132.60 (B2) m²/g, pH values of 8.95 (B1) and 10.02 (B2), CEC of 3.15 (B1) and 20.64 (B2) mmolc/100g, %C 91.4 (B1) and 79.9 (B2), %N 0.51 (B1) and 0.54 (B2), with C:N ratio 181 (B1) and 148 (B2). The FTIR spectra of the biochars before and after adsorption of the herbicides revealed some similarities in their prominent spectra bands. The following surface functional groups were identified: aromatic and aliphatic C-H stretching, polysaccharide C-O stretching, aromatic C=C vibration, C=O stretching (indicative of esters, aldehyde, ketone, and carboxylic acids), C≡C vibration, O-alkyl groups, and O-H stretching of hydroxyl groups from alcohol, phenols and carboxylic acids. the adsorption of the herbicides onto the biochars, caused some vibration peaks to increase in intensity, some low intensity peaks to appear, and some peaks to become less intense (a decrease in the C=O peak intensity was more pronounced in B2 after adsorption). Also, it can be seen that the polar groups (-COOH, O-alkyl, C=O, and/or phenolic -OH) on the biochars favorably influenced pesticides adsorption. This may be ascribed to strong H-bonding interactions occurring between the herbicides and these polar groups on the biochars. In addition, the primary adsorption mechanisms could involve n-Ï€ interactions, Ï€-Ï€ stacking, pore fillings and some van der Wal’s interactions. For both biochars, isotherm data fitted better to Langmuir equation (R² = 0.94-0.99) than the Fruendlich (0.92-9.97), to deliver the maximum monolayer adsorption capacities, Q_max 0.48, 3.22, and 10.5 mg/g respectively for atrazine, diuron, and linuron on B1; and Q_max 18.1, 71.43, and 333.37 mg/g, respectively for atrazine, diuron and linuron on B2. The study showed that B2 has far much higher adsorption capacities for the three compounds than B1, thus pointing out the great influence of the physicochemical properties (especially the specific surface area and CEC) of the biochars on adsorption. The corresponding adsorption coefficients was in the order linuron > atrazine > diuron, which is the same as the order of their octanol-water partition coefficients (log K_ow) also points to the influence the molecular property of the compounds exerts on their adsorptions. In conclusion, B2, more especially, has shown to be a very promising low-cost adsorbent for these three moderately polar herbicides from two molecular groups, and by extension, for other pesticides from the groups. Available literature data have also shown that B2 competes more favourably than many other adsorbents for the removal of these compounds. However, its effectiveness for compounds outside these groups needs to be investigated.