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Treatment of arsenic contaminated drinking water from the sources around gold mining areas of Geita and Mara, Tanzania:

Removal efficiency of locally available materials, bauxite, gypsum and magnesite.

Time: Tue 2023-12-19 13.00

Location: Sahara, Teknikringen 10B, Stockholm

Video link: https://kth-se.zoom.us/j/65995145722

Language: English

Subject area: Land and Water Resources Engineering

Doctoral student: Regina Filemon Irunde , Vatten- och miljöteknik, Chemistry department, University of Dar es Salaam - Tanzania

Opponent: Dr. Sudip Chakraborty, University of Calabria, Vicenza, Italy

Supervisor: Professor Prosun Bhattacharya, Vatten- och miljöteknik; Professor Em. Felix Mtalo, Department of Water Resources Engineering, College of Engineering and Technology, University of Dar es Salaam, Dar es Salaam, Tanzania

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QC 20231127

Abstract

In recent years, high arsenic (As) levels of about 300 µg/L have been reported around the gold mining areas of Geita and Mara regions within the Lake Victoria basin (LVB) in Tanzania. During a sampling campaign at Geita and Mara, the groundwater and surface water samples revealed the presence of high As concentrations as well as Fe and sulfide. Several wells are abandoned because of odor due to high content of sulfide, as well as red color, due to high iron content. About 53% of the analyzed As water samples exceeded the WHO guideline for drinking water. The release of As is primarily attributed to the weathering of sulfide minerals like arsenopyrite related to gold mining activities. In some parts of the LVB, an increasing number of cancer cases are being reported, and clinical investigations are now on the national agenda to identify the possible causes. Water treatment using bauxite, gypsum, and magnesite shows promising results, especially bauxite and magnesite, which could lower As concentrations to below 0.1 µg/L. Both bauxite and magnesite worked efficiently on As removal even at higher concentrations above 5 mg/L, while gypsum is preferable for treatment of low As concentrations. Furthermore, magnesite has a unique chemical character of influencing other materials to have high efficiency of As removal; however, it raises the pH of the water up to 10. Addition of 5 g/L magnesite to water containing 5 mg/L As, could lower the As concentration to below 10 µg/L within 30 min. The As removal increased with dosage and contact time up to 98 % in 4 hours, which is in agreement with Visual MINTEQ simulation. The performance of calcined magnesite, and gypsum fitted well with Freundlich adsorption isotherm, which indicates the presence of chemical reaction as controlling factor for As removal, while bauxite fitted Langmuir isotherm indicates monolayer surface coverage. The kinetic reactions were observed to follow pseudo-second-order. The statistic obeys linear regression with R2 ranging between 0.7 and 0.9. The artificial neural network revealed pH as a most influencing parameter for As removal from water. The mini-scale column revealed that a flow rate of 0.5 – 1 mL/min for 30 min gave an adsorption capacity ranging between 0.07 and 0.14 µg/g, which follows Thomas linear model with rate constant of kTH of 29.48 to 211.25 mL/min µg. The release of elements from spent magnesite, gypsum, and bauxite, such as magnesium (Mg), aluminum (Al), iron (Fe), and calcium (Ca) were found to be below WHO standards after water treatment. However, the desorption process of As from spent magnesite and gypsum was a challenge, which means there was formation of strong bond between Mg-O-As and Ca-O-As. This study is based on 5 papers that provide significant insights to the scientific community, policymakers, and the community living around As contaminated areas to learn about the occurrences of As and simple remediation techniques evaluated in this study.

urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-339968

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