Molybdenum-phosphate retention and transport in soils

- Mo sorption is significantly reduced by increasing addition of P.
- Single Mo breakthrough curves were asymmetrical and exhibited extensive tailing.
- The presence of P resulted in increased mobility of Mo on soil columns.
- CMRTM with kinetic reversible and irreversible successfully described Mo behavior.

Transport and retention experiments were carried out to quantify the mobility and sorption of molybdenum (Mo) and phosphate (P) in two different soils. Batch experiment was used to determine sorption isotherms over a wide range of concentrations for Mo and P. For both soils, sorption isotherms were nonlinear with higher affinity of P than Mo. Sorption of Mo was significantly reduced as the amount added P in solution increased. This was observed for both soils and is indicative of competitive sorption for available site. Miscible-displacement experiments were carried out using soil column where a pulse of Mo solution was introduced in each soil. Breakthrough curves (BTCs) indicated extensive sorption of Mo where as much as 50% of that applied was retained by a predominant kaolinatic soil. Subsequently, a pulse of mixed solution (of Mo and P) was introduced in each column. Results of BTCs for Mo indicated enhanced mobility of Mo in the presence of P for both soils. A five step sequential extraction procedure provided evidence that majority of applied Mo was strongly and/or irreversibly retained. A competitive transport model (CMRTM) based on the Sheindorf-Rebhun-Sheintuch (SRS) equation was capable of describing BTCs of Mo and to a lesser extent for P. Future research should focus on improvement of models that accounts for chemical mechanisms of competitive sorption.