Adsorption envelopes for arsenic and cadmium versus variable ph values on soil, goethite and zerovalent iron

Abstract EN

Due to arsenic (As) and cadmium (Cd) are considered of human carcinogenic elements, hence a knowledge about their reaction kinetics at the solid-liquid interface under variable pH values is extremely important for understanding the adsorption / desorption processes which controlling As and / or Cd soil-water system.

Thus, the objective of this study was to identify adsorption envelopes for as (as arsenate) and Cd versus variable pH values (3-12) on soil (calcareous in nature), synthetic goethite (colloidal system of iron hydroxide) and zerovalent iron (fine powder of iron metal).

Therefore, series of well-controlled laboratory experiments were carried out on the adsorption / desorption processes for as and Cd to achieve the previous objective.

The obtained data show that the greatest as adsorption by soil (59.5 µg as g-1 soil) was recorded at pH 5, and then it was tended to gradually decrease with increasing the pH value more than 5.

The lowest value of as adsorbed by soil (16.5 µg As g-1 soil? 6.4 % of total added As) was occurred at pH 12.

Adsorption of Cd showed an opposite trend to as, where its adsorbed percent tended to increase with increasing the pH values.

The greatest Cd adsorption (98 µg Cd g-1 soil? 40 % of total added Cd) was noticed at pH 7, with a constant trend for its adsorbed values with increasing pH value more than 7.

On goethite, the adsorption envelopes for As as arsenate at the initial solution concentration equivalent to 10 µg As mL-1 indicated an adsorption of nearly 100 % of total As added in a range of 3-4 for the pH value.

With increasing the pH value > 4, as adsorbed tended to decline sharply to be 30 % of the total as added at the pH value of 12.

In this respect, as adsorption on goethite decreased almost linearly from 3119 µg as g-1 goethite at pH 4 to 938 µg as g-1 goethite at pH 11.

Lowering the as adsorption at the high pH values is attributable to an increase repulsion between the more negatively arsenate species and negatively charged surface sites.

Adsorption of Cd showed an opposite trend to as its adsorbed values increased with increasing the pH value.

However, the greatest value of adsorbed Cd (3099 µg Cd g-1 goethite? 49 % of total Cd added) was noticed at pH 7, then a constant trend for Cd values with increasing the pH more than 7.

As for zerovalent iron (ZVI), the adsorption envelopes for as arsenate at the initial solution concentration equivalent to 10 µg As mL-1 indicated an adsorbed As value of nearly 100 % of total As added in a range of 3-4 for the pH value.

With increasing the pH value more than 4, as adsorption tended to decline sharply to be 16 % of the maximum adsorption of as at pH 12, however, lowering the pH value was associated with a large amount of As retained.

The adsorption envelopes exhibited broad adsorption maxima for as at the pH range of 3-4, which was followed by a gradual decline with increasing the pH value more than 4.

Such decrease coincided approximately with the second pka (6.9) of H3AsO4.

As adsorption decreased linearly from almost 24960 µg as g-1ZVI at pH 4 to 4160 µg as g-1 ZVI at pH11.

The highest Cd adsorption was noticed at pH 7 with a constant trend for its values with increasing the pH value more than 7.

The greatest value of adsorbed Cd was 24973µg Cd g-1 ZVI, which corresponding a percent of 99 % of total Cd added.