Journal of Environmental Science and Engineering A 4 (2015) 367-377 doi:10.17265/2162-5263/2015.07.005 Investigations on the Thermodynamic Stability and Availability of Nutrients for Plants by Humic Substances Extracted from Peat Samples Camila De Almeida Melo, Lilian Karla De Oliveira, Bruno Barboza Cunha, Leonardo Fernandes Fraceto and Andre Henrique Rosa Department of Environmental Engineering, Sao Paulo State University (UNESP), Sorocaba 18087-180, Sao Paulo State, Brazil Abstract: This work evaluated the complexation capacity, exchange constants and availability of micronutrients for plants and humic substances extracted from peat samples. Samples of humic substances extracted from two tropical peats (HS-P1 and HS-P2) were enriched with the micronutrients Cu(II), Co(II), Fe(II), Mn(II), Ni(II) and Zn(II) and the parameters for formation of the complexes (HS-N) were evaluated at different pH. The Scatchard model was used to calculate the maximum complexation capacity and the nutrient availability was studied using exchange capacity experiments based on ultrafiltration procedure. The optimum pH for complexation was 4.5 and the order of affinity was: Fe(II) > Cu(II) > Co(II) > Mn(II) = Ni(II) > Zn(II). The maximum complexation capacity reached 56.8 mg·g -1 Fe of HS-P1 (the highest) and 1.7 mg·g -1 Zn of HS-P2 (the slightest). The exchange experiments showed that HS-P-Fe complexes were formed preferentially. The least stable complex was formed with Zn, which was therefore, more easily available. The results contribute to understand the behavior and availability of some nutrients in soils. Key words: Peat humic substances, complexation, nutrient availability, exchange capacity, soil. 1. Introduction Humic substances have undefined chemical structures and can vary widely depending on the characteristics of the environment in which they are formed [1, 2]. The most widely recognized functions of HS in the environment are the complexation and precipitation of cationic species, such as metals and the adsorption of organic compounds [3-6]. These capabilities are due to the functional groups present in the chemical structure of HS, such as carboxylic groups, phenols and ketones, which are responsible for the interactions [7-9]. The interaction of HS with metal species results in the formation of HS-Metal complexes, hence, influence speciation transport and availability of inorganic species in the environment [10, 11]. The beneficial effects of HS in soils include heat retention (due to its dark color), which contribute in Corresponding author: Andre Henrique Rosa, Ph.D., main research field: environmental chemistry. E-mail: ahrosa@sorocaba.unesp.br. seed germination and water retention, which prevents soil erosion. Also HS can be used as a soil conditioner that increases the stability of aggregates, hence, reduces risks of erosion by runoff and decreasing water infiltration rates, which is beneficial in terms of crop productivity [1, 12]. Successful and sustainable food production depends on the quality of the soil, which needs to contain the organic matter and nutrients required for plant growth [13]. HS enriched with micronutrients responsible by plant growth present themselves as an attractive alternative for increasing agricultural productivity. There are some published studies have only evaluated the complexation capacity of elements present as contaminants without considering the natural competition that occurs between elements in the soil [14-16]. The appropriate soil management requires adequate knowledge of the interactions that involve humic substances, which play a major role in the transport, complexation and bioavailability of D DAVID PUBLISHING