Molecular foundations for shear-induced dynamics of natural organic matter

Abstract

The overall objective of the present work was to quantify how shear, coupled with varying salt concentration, affected the particle size distribution and relaxation/aggregation behavior for various organic sources of nonliving natural organic matter (NNOM) in surface water. NNOM has been implicated as a conditioning agent leading to the formation of biofilms such as algae. NNOM is also a responsible in surface waters for facilitated transport of a variety of anthropogenic pollutants. These are NNOM surface-related phenomena, yet the variable surface area and surface composition of NNOM, which can change dependent on shear rate, is not discussed in the literature. NNOM polymer-like dynamics can interact with stream water velocity differences to determine the process and result of aggregation. The fundamental role of post-shear NNOM molecular structure and dynamic aggregation (self-assembly) is examined here alongside fresh (hydrological) versus mined (terrestrial) NNOM. Shear rate can be seen as a change in the velocities of streamlines in hydrology. In this early work, the response to shear rate for three types of NNOM was measured using a stress-controlled rheometer under varying conditions of ionic strength. Samples were studied for rheological response after a variety of pre-shear conditions, and data then coupled with surface composition data from previously reported fluorescence studies. Interestingly, a size class of 5 μm aggregates disappeared when Aldrich humic acid samples were treated with 0.3 M Ca²⁺. Evidence is also presented that the environmental samples flocculated at shear rates up to 400 s⁻¹, rather than exhibiting particle breakup, with implications for reducing NNOM surface area. Dynamic response of different NNOM sources was not the same, some sources showing evidence of self-assembly. The molecular response to shear may play an important role in understanding the surface area and composition of NNOM responsible for facilitated transport of pollutants and initiation of biofilms.