[Characterizing the interaction between roxarsone and humic acid by fluorescence quenching experiment]

Area of Science:

• Environmental Chemistry
• Analytical Chemistry
• Spectroscopy

Background:

• Humic acid (HA) is a complex mixture of organic matter found in soil and aquatic environments.
• Roxarsone (ROX) is an organoarsenic compound used as a feed additive, raising environmental concerns.
• Understanding the interaction between HA and ROX is crucial for assessing the environmental fate and transport of arsenic.

Purpose of the Study:

• To investigate the binding interactions between humic acid (HA) and roxarsone (ROX).
• To determine the effects of HA concentration, pH, and temperature on the HA-ROX bonding strength.
• To elucidate the quenching mechanisms involved in the interaction.

Main Methods:

• Fluorescence spectroscopy was employed to monitor changes in HA fluorescence upon addition of ROX.
• Fluorescence quenching titration technique was used to quantify the binding constants.
• Analysis of fluorescence peak intensities and association constants (lg K) under varying conditions (pH, temperature, HA concentration).
• Transient-fluorescence spectrum analyses and linear model simulations were performed to identify quenching mechanisms.

Main Results:

• Roxarsone (ROX) quenched the fluorescence of humic acid (HA) at four distinct peaks (A, B, C, D), with peak C showing the greatest intensity decrease.
• Static quenching was confirmed as the primary mechanism, particularly involving carboxyl and carboxide groups of HA.
• Binding strength (lg K) was influenced by pH, with a maximum at pH 6.00, and decreased with increasing temperature, further supporting static quenching.
• Combined dynamic and static quenching was identified as the main mechanism for peak C, while single static quenching dominated for peaks A, B, and D.

Conclusions:

• Roxarsone (ROX) interacts with humic acid (HA) through static quenching, primarily via its functional groups.
• The binding affinity and quenching mechanism are sensitive to environmental factors like pH and temperature.
• These findings provide insights into the environmental behavior and potential sequestration of ROX by natural organic matter.