Banca de DEFESA: ÉRICA BARBOSA DE SOUSA

Uma banca de DEFESA de DOUTORADO foi cadastrada pelo programa.
STUDENT : ÉRICA BARBOSA DE SOUSA
DATE: 02/03/2023
TIME: 08:30
LOCAL: Ambiente virtual, GoogleMeet
TITLE:

Determination of total organic carbon in soils: optimization of oxidation with KMnO₄ and new analytical proposal

KEY WORDS:

CHN, soils, Yeomans and Bremner, factorial design.

PAGES: 117
BIG AREA: Ciências Exatas e da Terra
AREA: Química
SUBÁREA: Química Analítica
SPECIALTY: Análise de Traços e Química Ambiental
SUMMARY:

Soil organic matter is of fundamental importance for various environmental functions associated with its cycling process. It is estimated by the total organic carbon (TOC) content and plays a key role in nutrient retention, soil aggregation, water dynamics, and acts as a primary energy source for biological activity. In Brazil, the wet oxidation methods Walkley and Black (WB) and Yeomans and Bremner (YB) are the most used to determine TOC. These methods are based on the organic carbon (CO) oxidation with potassium dichromate in a strongly acidic medium and subsequent titration of the remaining dichromate. However, there are several disadvantages associated with these methods, including environmental concerns due to the use of potassium dichromate, the high cost of storing and disposing of hazardous waste, and the requirement for large volumes of concentrated sulfuric acid, which is a controlled purchase reagent. Furthermore, these wet oxidation methods do not entirely oxidize the OC.  Elemental analysis is the reference and most reliable technique for determining the TOC, as it can fully oxidize OC. However, the costs for its implementation and analysis are high, making its use unfeasible in routine analyses. This work proposes the quantification of TOC of soils using KMnO₄ as an alternative oxidant to K₂Cr₂O₇ and the subsequent spectrophotometric determination of the remaining permanganate at λ=525nm. Ultraviolet-visible spectrophotometry is a well-established technique with low operational cost, which allows accurate and reliable results. Initially, the OC oxidation study was conducted using potassium biphthalate as the organic matter standard and a 2³ factorial design in order to optimize the reaction conditions. The factors studied were time (30 and 60 minutes), temperature (60 and 95 ºC), and acidity (0.125 and 0.250 mol L^-1 of H₂SO₄), with temperature being the factor that most influenced OC oxidation. The optimal reaction conditions were determined as 30 minutes, 95 ºC, and 0.125 mol L^-1 H₂SO₄. The oxidability of the investigated organic compounds by KMnO₄ was evaluated, obtaining results comparable to K₂Cr₂O₇. For the spectrophotometric determination of the remaining permanganate, the influence of sample size was investigated for four different soil classes, with 500 mg being identified as the ideal mass. In the analysis of 16 soil samples, the TOC contents obtained by the proposed method correlated strongly with those obtained by the YB method (r = 0.966) and elemental analysis (r = 0.975). This new approach for determining TOC in soils can replace the methods currently used in soil laboratories since it is reliable, simple, cost-effective, and presents less risk to both the analyst and the environment compared to methods that use potassium dichromate.

COMMITTEE MEMBERS:
Presidente - 2626414 - JOSE GERALDO ROCHA JUNIOR
Externa ao Programa - 387304 - CRISTINA MARIA BARRA - UFRRJExterno ao Programa - 1060711 - MARCOS GERVASIO PEREIRA - UFRRJExterna à Instituição - EVA LUCIA CARDOSO SILVEIRA
Externo à Instituição - LUIZ FERNANDO SILVA CALDAS - IFRJ