Enhancement effect of water associated with natural organic matter (NOM) on organic compound-NOM interactions: A case study with carbamazepine

Natural organic matter (NOM) in soils and sediments is recognized to strongly affect environmental distributions of organic compounds. Water associated with NOM may have a significant impact on NOM-organic compound interactions. The objectives of this research were (1) to determine the effect of hydration of a model NOM sorbent on interactions with a probe organic compound, carbamazepine (CBZ), and (2) based on the comparison with the literature data, to evaluate the effect of organic compound structure on the cooperative participation of water molecules in organic sorbate-NOM interactions. CBZ is one of the most widely reported water pollutants from the pharmaceutical and personal care products family. Therefore, CBZ sorption on Pahokee peat was compared from water and from n-hexadecane, using solubility-normalized solute concentrations. CBZ-NOM interactions were enhanced by one to two orders of magnitudes when NOM became fully hydrated. This enhancement is associated with the distinct ability of CBZ to undergo strong, specific interactions with NOM which was revealed by comparing the transfer of CBZ and another model sorbate, phenanthrene, from solution in n-hexadecane to the hydrated NOM sorbent. The enhancing effect of NOM hydration on CBZ-NOM interactions was also observed when CBZ sorption was examined on partially hydrated NOM. In comparison with a smaller-size organic sorbate such as phenol, CBZ needs more NOM-associated water in order to demonstrate the strengthening of interactions with NOM. Therefore, for penetration of the larger sorbate molecules into the NOM interior, a greater number of water molecules are needed to compensate for the local NOM disintegration thus suggesting the greater extent of the cooperativity in an involvement of water molecules in the CBZ-NOM interactions.