Sorption and desorption of pyridine by Pahokee peat from hexadecane in the presence of organic co-solvents.

Abstract

A study of the interactions of the specifically interacting organic compound pyridine with a model soil organic matter sorbent (Pahokee peat) was carried out from different nonaqueous organic liquid media, including neat n-hexadecane, acetonitrile, acetone and nhexadecane mixtures with either acetone or acetonitrile. Kinetic and equilibrium studies using an activity-based comparison of the organic compounds in solution was used to

study the interactions of soil organic matter (SOM) and pyridine sorption capability in the various non-aqueous organic liquid media. Quantification and qualification of pyridine and the other co-solvents were done using Gas Chromatography (GC).

Sorption of pyridine from neat organic solvents was not masked by sorption of the organic solvent. The apparent sorbed amount calculated from the change in solute concentration and reported on a dry weight basis was considered to represent the true sorbed concentration of pyridine in the sorbent phase. Pyridine sorption was found to be non-linear and distribution coefficients decreased with solute concentration, by

approximately three times in n-hexadecane, more than five times in acetonitrile, and by ten times in acetone over the experimental concentration range. Pyridine sorption from nhexadecane

was also found to be comparable with sorbed amounts from acetone, but

much lower in comparison to sorption from acetonitrile.

Sorption of pyridine from n-hexadecane mixtures with acetonitrile or acetone demonstrated the solvent assisted effect of pyridine sorption. Sorption uptake of pyridine increased as initial acetonitrile concentration increased, this acetonitrile assisted trend for pyridine sorption was found in the presence of a large excess of n-hexadecane. Sorbed concentrations of pyridine measured in the presence of high concentrations of acetonitrile

(close to it's solubility limit) were found to be very similar to pyridine sorption from neat acetonitrile. Sorption behaviour of pyridine in n-hexadecane-acetone mixtures showed that increasing acetone concentrations had no effect on pyridine sorption.

Pyridine sorbed from n-hexadecane, n-hexadecane-acetonitrile, and n-hexadecaneacetone mixtures showed a hysteretic desorption to n-hexadecane. After a series of repeated solvent extractions with solvents of increasing solvating power(1,4-dioxane, ethanol, dimethylsulfoxide), a fraction of pyridine remained bound to the peat. This nonrecoverable

fraction was approximately the same for the different organic media (OA5±

0.09 in n-hexadecane suspensions, 0.57±O.12 in n-hexadecane-acetonitrile mixtures, and OA6±0.07 in n-hexadecane-acetone mixtures). Acetonitrile sorption by peat from nhexadecane was found to be very non-linear and hysteretic. The acetonitrile sorbed was almost fully recoverable, around 90%, for the initial acetonitrile concentration range varying from 0.14-0.7% by volume. However in the presence of pyridine a significant portion of acetonitrile was not recovered even after multiple extractions of polar organic solvents. Pyridine irreversible binding was not induced by acetonitrile additions and was found to occur to the same extent in both neat n-hexadecane and n-hexadecane-acetone mixtures.

The solubilities of acetonitrile and acetone were determined by the flask method at 25°C using GC analysis. Solubility in volume percent for acetonitrile in n-hexadecane, 0.9±0.07, 0.57±0.02 for n-hexadecane in acetonitrile, 24.0±OA for acetone in nhexadecane, and 13 A±O.2 for n-hexadecane in acetone, were found. Log Ostwald coefficient (1.63±O.02) for acetonitrile in n-hexadecane was measured at 25°C using head

space analysis and was found to be constant in the acetonitrile concentration range 0.10.8% by volume. Log Ostwald coefficient for pyridine in hexadecane used was 3.02, for the pyridine concentration range 50 mgIL-500 mg/L, this value was constant even with 0.5% by volume additions of acetonitrile. Analyses of sorption isotherms were reported

on an activity basis to eliminate the effect of differential solute interactions in the solvent, calculated using the solute equilibrium concentration, the concentration of saturated vapour, and the Ostwald coefficient.

Dissolution of peat components into n-hexadecane are known to be negligible. Peat components extracted after 12 hours and 3,5 months acetonitrile and acetone treatment (solid liquid ratio 1: 10) Showed 15 to 20 times less visible absorbance respectively (A. 465, 620, and 665, E4:E6 ratios using DV-Visible Spectroscopy), than the 12 hours aqueous peat extract. Quantification of the dissolved humic materials in the aqueous

extract was followed using a Total Organic Carbon analyser. The study found the degree of humification to be much lower in non-aqueous organic solvent extracts (2.5 for acetone extracts, and 3 for acetonitrile extracts) than in aqueous solution extracts (8.2).