Behavior of Cd, Cr, Cu, Ni and zinc in latosols repeated amended with biosolids: sequential extraction, phytoavailability and humic substances characterization

Author(s)

Edna Ivani Bertoncini

• Document type: Doctoral Thesis
• Press: Piracicaba.
• Institution: Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC)
• Defense date: 2002-04-11
• Advisor: Maria Emília Mattiazzo

Abstract

The agricultural use of biosolid is recent in Brazil and some studies have been shown that this practice improves not only the soil fertility but also the crop production, However, the presence of heavy metals in sludges can represent a risk to the environmental quality, since these elements can be absorbed by plants and/or leached through the soil profile. The present work aimed at studying some aspects related to the consequences of biosolid disposal in Brazilian Oxisols. Specifically, the following topics were investigated: (1) plant availability of Cd, Cr, Cu, Ni and Zn in Oxisols treated with biosolid, (2) distribution of these metals on the soil phases after the biosolid application and (3) changes in the soil humic substances as a consequence of biosolid addition. The soils employed in this study, classified according to Brazilian classification in Latossolo Vermelho-Amarelo Distrófico (Typic Hapludox) and Latossolo Vermelho Distrófico (Rhodic Hapudox), were put in 0,5 m3 - vases disposed outdoor, and treated with biosolid in a quantity equivalent to 388 Mg ha-1 (dry basis), divided into five applications. The biosolid was proceeding from SABESP sewage treatment plant situated in Barueri (SP) where the sewage is submitted to aerobic and anaerobic disgestions and the sludges are treated with Ca(OH)2 and FeCl3. Seven months after the last application of biosolid, the soil samples were collect in two layers: 0 – 0.25 m and 0.25 – 0.50 m. These samples were characterized in relation to their chemical, electrochemical and mineralogical properties. Besides, it was carried out the characterization of humic substances and the sequential extraction of Cd, Cr, Cu, Ni and Zn. Finally sugarcane (RB72454 variety) was grow in the vases in order to evaluate the availability of those metals for this species and the effects of biosolid additions on the sugarcane production. The use of biosolid resulted in the increase of sugarcane production and there were no alterations in quality parameters such as: BRIX, POL%CA and total reducer sugar. However the purity and the greater contents of reducer sugar were observed in plants grown in treated soils and the smaller fiber contents were associated to the canes produced in soils that received biosolid. The Cr, Cu, Ni and Zn leaf contents increased in soils treated with sludge while the Cd ones were not detected. It was not verified significant contents of Ni and Cd in the sugarcane juice obtained from treatments that received biosolid. In relation to Cr, it was detected high contents of this element in the sugarcane juice only for plants grow in typic Hapludox treated with biosolid. There was no effect of treatments on Cu and Zn leaf contents, which were considered adequate. The characterization of humic acids isolated from biosolid showed that this sludge present a small degree of humification (low C/N and high H/C ratios, small quantities of free radicals). The addition of biosolid in soils reduced the humification degree of native humic adids. The presence of a great number of phenolic groups in biosolid, whose pKa are higher than soil pH values, probably explain the low effectiveness of this sludge in increasing the cation exchange capacities of treated soils. For the humic acids isolated from biosolid and treated soils, the low values for E4/E6 ratio and the presence C=0 bands in FTIR spectra indicated the addition of aromatic compounds to soil humic acids. However, the fluorescence spectra showed that these aromatic compounds are lesser condensed than ones present in soils that did not received biosolid. In relation to interactions between humic substances and metals, the use of spectroscopic tools, specifically FTIR and fluorescence analyses, revealed the existence of bonds between metals and humic substances. Additionally fluorescence analyses revealed differences between treated soils in relation to stability of humic substances – metals bonds. The sequential extraction of heavy metals showed that the Cd coming from biosolid was mainly associated to carbonates. It was also observed significant amounts of Ni and Zn in carbonate fraction. Thus, it is probable that the decrease in pH values of the studied soils can result in desorption of these metals. In the treated soils, the metals Cr, Cu and Ni were mainly associated to organic fraction. This indicates that in spite of the low humification degree of humic substances present in the biosolid, this fraction has an important role in metal retention. It was observed for both studied soils the possibility of Cr, Cu, Ni and Zn leaching to 0.25 – 0.50 m layer, probably as carbonates and/or associated with humic substances. In the treatments that didn’t receive biosolid, it was observed that the metals were associated mainly to oxide fraction in Typic Hapludox and to residual fraction in Rhodic Hapludox. (AU)