Arsenic (V) and Phosphate Sorption to Swedish Clay Soils – Freundlich Sorption Modelling (Civil Project)

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This study is an attempt to analyze and interpret the behavior of the two elements arsenic and phosphorus when released into the environment. Both of them may occur naturally in the environment but also may be added to the environment for certain purposes e.g. as pesticides and fertilizers respectively or through anthropogenic sources.

When in excess, arsenic can be toxic to plants and organisms in the soil and some of it when leaches to groundwater or transported to surface water bodies through runoffs may pose a threat to aquatic organisms. Likewise, phosphorus when in excess result into eutrophication of surface water bodies and groundwater as well which has been a major problem in the Baltic Sea.

In order to be able to predict their mobility a study on their chemical and physical characteristics under different conditions is important. The soil composition is an important aspect of nutrient management because some of the minerals present i.e. hydr(oxides) of aluminium and iron tend to hold and store both arsenic and phosphorus in the soil, while plant uptake and harvest may remove them (especially phosphorus) from the soil.

This study was focused on Swedish agricultural soils and the samples for investigation were collected from two locations, one is Broknäs from which samples were collected from different horizons i.e. A 0-30 cm, C 60-90 cm and C 47-67 cm samples from an area known as Bogesundslandet, NE of Stockholm (59°24’N, 18°18’E) and E21:2 was collected from the county of Östergötland (58°27’N,14°57’E), southern Sweden not far from Lake Vättern, from where the A horizon was collected. Batch experiments were performed to check pH and concentration dependence of the sorption/desorption of As and P.

Two varieties of the Freundlich equation (Basic and Competitive) were used to model the results obtained. It was observed that the dependence of arsenate and phosphate sorption/desorption on pH show a similar but not identical trend for both anions. At low pH, the dependence of dissolved P and As did not agree, for unknown reasons.

Possibly, the low pH value may mobilize otherwise un-reactive P that at higher pH are blocked by some aluminium/iron precipitate. The Freundlich modeling results showed that there is direct competitive adsorption between As and P ions, at least in the A horizon. However use of the competitive Freundlich equation did not result in meaningful results in the C horizon, which may indicate different As and P sorption mechanisms. However, further studies on this are recommended.
Source: KTH
Author: Mwamila, Luhuvilo

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