Cadmium adsorption on acric oxisols from the state of São Paulo

Author(s)

Nivea Maria Piccolomini Dias

• Document type: Master's Dissertation
• Press: Piracicaba.
• Institution: Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC)
• Defense date: 2000-01-24
• Advisor: Luís Reynaldo Ferracciú Alleoni

Abstract

Cadmium can be added to the soils through tyre residues, oils, disposal of city wastes, sewage sludge and phosphatic fertilisers. Although it is not essential to plant nutrition, cadmium is easily adsorbed and translocated by plants and has a great potencial to enter the human food chain. The persistence and mobility of cadmium in soils and, consequently, its bioavailability is largely determined by the extent of adsorption by soils particles. So it is important to determine how adsorption is affected by soil attributes such as pH, organic matter, clays, and iron and aluminium oxides, for predicting the environmental impact that can be caused by the addition of antropogenic cadmium to soils. Some Brazilian researchers studied the behavior of cadmium ín soils, but little is known with respect to its behavior in highly weathered tropical soils, as the acric soils, that represent the extreme conditions in the scale of weathering and can exhibit a positive charge balance in deep layers. The objectives of this study were to obtain the amounts of adsorbed cadmium at different pH values in the surface (0-0,20m) and subsurface horizons (B horizon) of two soils with different textures: Anionic Rhodic Acrudox (RA) and Anionic Xanthic Acrudox (XA) and compare them with the amount adsorbed Cd in a Rhodic Kandiudalf (RK), whose electrochemical behavior is the opposite, as it exhibited negative charge balance all along the profile, the maximum adsorption and the constant of Langmuir's equation and K and nof Freundlich's equation were obtained in natural pH and correlated with the chemical, physical and mineralogical soil attributes. The samples were incubated with either calcium carbonate or chloridric acid with a pH ranging from 4.0 to 7,0, samples were suspended in 20 mL of Ca(NO3)2 0,0025 mol L-1 solution, containing cádmium Cd(NO3)2 at 2 mg dm-3 . After 24-hours, cadmium was determined in the extract using an atomic absorption spectrophotometer. With the objective of obtaining adsorption isotherms and Langmuir and Freundlich parameters, Cd was added to the soil in concentrations ranging from 5 to 200 mg dm-3 , and the sarne procedure as described in the first stage was performed. Cadmium adsorption on the soils studied appeared strongly pH dependent. Its adsorption increases with an increase in pH, either in surface or subsurface layers. When the pH values ranged between 6,0 and 7,0, the amounts of adsorbed Cd were similar for all soils, with almost 100% adsorption. Surface horizons adsorbed greater amounts of cadmium in relation to the subsurface horizons, reinforcing the influence of organic matter. RK adsorbed greater amounts of cadmium, due to the negative net charge along the profile. Organic carbon, cation exchange capacity (CEC), cation retention (CR), specific surface (SS), clay content, variable negative charge and permanent negative charge were positively correlated to Langmuir' s K and b parameters and Freundlichs K. The CEC were the main soil attributes that exhibited a greater effect on cadmium adsorption through multiple regression analysis. It was also observed that CEC and pH controlled adsorption of cadmium on the studied soils. (AU)