Assessment of mixtures of residual soil from Eldorado Paulista (SP) with peat for retention of potentially toxic metals

Author(s)

Jéssica Pelinsom Marques

• Document type: Master's Dissertation
• Press: São Carlos.
• Institution: Universidade de São Paulo (USP). Escola de Engenharia de São Carlos (EESC/SBD)
• Defense date: 2018-09-24

Examining board members

Valéria Guimarães Silvestre Rodrigues; Eduardo Anselmo Ferreira da Silva; Eny Maria Vieira

• Advisor: Valéria Guimarães Silvestre Rodrigues

Abstract

In the Ribeira Valley region, in Brazil, mining waste originated from lead ore beneficiation and smelting was launched for decades in the Ribeira de Iguape river or deposited directly on the soil surface near this river, without any protection. This created sources of soil and water contamination by potentially toxic metals. The use of local soil for construction of sealant barriers aiming to waterproof and retain the contaminants represents an alternative to control this type of contamination. It is important to study the region soils, their capacity of retention of potentially toxic metals, and low cost alternatives to improve their physical and chemical properties. The addition of organic matter to the soil is an alternative to increase cation retention. Therefore, the purpose of this research was to verify if a residual soil collected in the Eldorado Paulista municipality, in the Ribeira Valley, presents suitable characteristics to be used as a sealant barrier for mining waste disposal; and to assess the improvement of its adsorption capacity of lead and cadmium, when mixed with peat (an organic material with high metal retention capacity, and low cost), in proportions of 10 and 20% (w/w). Characterization tests were carried out to determine the physical, chemical and mineralogical properties of the soil. For the evaluation of the adsorption capacity of lead and cadmium by the soil and soil-peat mixtures, batch equilibrium tests were performed. The studied soil is a clayey residual soil. Its clayey fraction is predominantly composed of kaolinite. It presents some favorable characteristics to be used as a sealant barrier for retention of metallic cations, such as: predominance of fines (54.5% clay), suitable plasticity index (25.1%), oxidizing medium (Eh = +333 mV), predominance of negative charges on the surface of the colloidal particles (ΔpH = -0.9 and PZSE = 3.6) and low hydraulic conductivity when compacted, which reached the order of 10-9 m/s. However, some soil properties are less suitable for metal retention, such as low cation exchange capacity (41.4 mmolc/dm3), low organic matter content (9 g/kg) and low pH (4.6). The addition of peat to the soil in proportions of 10 and 20% (w/w) increased the organic matter content by 4.3 times and 6.4 times, respectively. CTC also increased. It ranged from 41.4 mmolc/dm3 (natural soil) to 107.7 mmolc/dm3 (mixture with 10% peat) and to 143.1 mmolc/dm3 (mixture with 20% of peat). No changes in pH were observed. It was also noticed the improvement of the adsorption capacity of lead and cadmium. The lead removal efficiency of the natural residual soil was from 54% to 100% at initial concentrations from 22 to 232 mg/L of this metal. The mixture with higher peat content (20%) removed 95.3% to 100% of the lead at initial concentrations from 31 to 267 mg/L. The distribution coefficient Kd increased from 4.57 cm3/g (for the natural soil) to 58.57 cm3/g (for the mixture with 20% of peat). The Kf increased from 75.75 to 508.86 cm3/g and the KL increased from 0.0276 to 0.4118 cm3/g, due to the addition of peat in 20%. With regard to cadmium, the natural soil removed 30.5 to 72.1% of this metal for initial concentrations of 17 to 170 mg/L. The mixture with 20% of peat increased the Cd removal efficiency to 54.9 to 88.9% at initial concentrations from 20 to 150 mg/L. Kd was 1.56 cm3/g for the natural soil and increased to 6.42 cm3/g for the mixture with the highest content of peat. Kf increased from 30.06 to 63.88 cm3/g, and the KL increased from 0.0301 to 0.0483 cm3/g. It was concluded that the addition of peat to the soil in proportions of 10 and 20% caused improvement of properties related with metalic cation retention and favored Pb and Cd adsorption. (AU)

• FAPESP's process: 16/17894-2 - Geological and geotechnical characterization of mixtures of residual soil from Eldorado Paulista (SP) with reactive organic materials
• Grantee: Jéssica Pelinsom Marques
• Support Opportunities: Scholarships in Brazil - Master