Adsorption and volatilization of mercury intropical soils

Abstract

Mercury (Hg) has a biogeochemical cycle complex, as a result of several physical, chemical and biochemical processes. These processes dictate the dominant chemical species of Hg and also their distribution in the compartments that compose the environment (air, water, soil, biota, sediments), producing complex patterns of distribution, some yet unknown, particularly in tropical environments. In this context, the objective was to evaluate the adsorption and volatilization of Hg in tropical soils. The thesis was divided into two chapters: Chapter 1- Hg adsorption capacity was evaluated. The adsorption experiment was conducted in a completely randomized design in a 5 × 8 factorial scheme, with five soils with contrasting texture and organic matter content and eight Hg concentrations (0, 0.05, 0.1, 0.2, 0, 4, 0.8, 1.2 and 2.4 mg L-1). A zeolite material was also added to the sorption study for comparison purposes. Chapter 2- this chapter evaluated the volatilization of Hg from four tropical soils in dry and humid conditions and the Hg adsorption capacity of four adsorbent materials: hopcalite; zeolite-NaP; two biochars (W and P). As main results we have that the soils sand and sandy clay loam texture, with low contents of organic matter, presented less capacity of adsorption of Hg when compared to the soils of clay loam, clay and an organic soil. The analysis by means of infrared spectrometry showed that the oxygenated functional groups (Al-O, Si-O, Fe-O) of the clay minerals and oxides present in the soil are responsible for the high Hg adsorption capacity. Clinoptilolite zeolite has the ability to adsorb Hg and has the potential to remedy soils contaminated with Hg. The volatilized Hg content under dry soil condition was 3.7, 3.4, 1.6, and 3.8 mg kg-1 d-1 of Hg and under moist soil condition was 5.5, 4.6, 2.5, and 4.2 mg kg-1 d-1 of Hg in Typic Rhodustult, Humic Rhodic Acrustox, Typic Ustifolist and Oxic Dystrudept, respectively. Hopcalite was the material with the highest Hg adsorption capacity with 97, 53, 93 and 7% of the Hg value volatilized by Typic Rhodustult, Humic Rhodic, Typic Ustifolist and Oxic Dystrudept, in dry soil condition, respectively. Zeolite NaP adsorbed 1.0, 0.3, 0.2, 0.4 mg kg-1 of Hg, these values represent an adsorption of 27, 8, 14 and 9% of the total volatilized value by Typic Rhodustult, Humic Rhodic, Typic Ustifolist and Oxic Dystrudept, respectively, in the dry soil condition. Biochars W and P showed low Hg removal capacity (0.5 to 9%), when compared to hopcalite and zeolite NaP. Soils with low levels of organic matter have less capacity to adsorb Hg and greater capacity to volatilize Hg.