Assessing Adsorption Behavior of Molybdenum in Soils: An Emerging Metal Contaminant

Abstract

Molybdenum (Mo) is an essential micronutrient for most living organisms but also an emerging contaminant in the environment. A narrow range exists between critical requirements for plants (0.5 mg kg-1) and threshold toxicity of molybdenum (10 mg kg-1) to ruminants, considered as “molybdenosis”. Adsorption is one of the most important chemical processes in soils that affect the mobility of metals/contaminants in soil. To evaluate the sorption behaviour of molybdenum in soils, twenty bulk surface soil samples (0–15 cm) with diverse physical and chemical properties were collected from different parts of India. A laboratory experiment was conducted to study the sorption behaviour of Mo in soil under two temperature (20 and 30○ C) conditions. Five graded Mo concentrations (1, 2, 5, 10, 20, and 50 mg L-1) were prepared using (NH4 ) 6 Mo7 O24 in a 0.005 M Ca(NO3 ) 2 background solution. Maximum adsorption of Mo was observed in the soils where pH ranges from 4-5, while in alkaline soils (pH > 8) negative adsorption phenomena were found. Freundlich isotherm fitted better than Langmuir isotherm in a wide range of soil’s pH. A highly significant negative correlation was observed between soil pH and adsorption parameters, while a significant positive correlation was found between SOC and adsorption parameters. The thermodynamic parameters i.e., free energy (∆G○ ), enthalpy (∆H○ ), and entropy (∆S○ ) were determined using sorption data in two different temperature conditions. It was observed that molybdenum sorption in soil is a spontaneous endothermic reaction. This study highlighted the role sorption mechanism in the evaluation of mobility and availability of molybdenum in different soil chemical environments.