Abstract

Humic acid was isolated from three contrasting organicrich soils and acid-base titrations performed over a range of ionic strengths. Results obtained were unlike most humic acid data sets; they showed a greater ionic strength dependency at low pH than at high pH. Forward- and backtitrations with the base and acid revealed hysteresis, particularly at low pH. Previous authors attributed this type of hysteresis to humic acid aggregates;created during the isolation procedure;being redissolved during titration as the pH increased and regarded the results as artificial. However, forward- and back-titrations with organic-rich soils also demonstrated a similar hysteretic behavior. These observations indicate (i) that titrations of humic acid in aggregated form (as opposed to the more usual dissolved form) are more representative of the acid-base properties of humic acid in soil and (ii) that the ionic strength dependency of proton binding in humic acid is related to its degree of aggregation. Thus, the current use of models based on data from dissolved humic substances to predict the acidbase properties of humic acid in soil under environmental conditions may be flawed and could substantially overestimate their acid buffering capacity.

Introduction

The interactions of humic substances with cations exert a powerful chemical influence in the environment, being important in processes such as pH buffering, metal binding and transport, and nutrient control. The interactions involve both solid-phase material in soils and sediments and dissolved material in natural waters. Humic acid and fulvic acid are usually considered to be the major humic substances involved in cation binding. Broadly speaking, humic acid occurs mostly in soils and sediments as part of the solid phase, while fulvic acid, being more soluble, accounts for a major part of the dissolved organic matter in natural waters (1).