Arsenic is a toxic element that can occurs in the environment as a result
of either natural processes or anthropogenic activities. The accumulation of
arsenic in form of arsenate As(V) in soils and sediments threatens the health of
plants, wildlife and human. Goethite (α-FeOOH), zerovalent iron (fine powder
of iron metal) and clay minerals play an important role in controlling the
concentration of soluble arsenic in pure water due to it is formed inner sphere
surface complexes. Extraction of arsenic using phosphate, sulphate, molybdate
and DTPA had been suggested as a procedure to assess its amounts. Arsenate
was equilibrated with soils at 10-100 µg As ml-1 surface coverage and extracted
by 0.005M DTPA. On the other hand, it was equilibrated with goethite and
zerovalent iron at 10 µg As ml-1 and pH values of 5 and 9 as well as it was
extracted by phosphate, sulphate and molybdate solutions at a ratio of I : 100 ≈
As : each solution and pH values ranging from 3 to 12.
Regarding soils, desorbed As (µg g-1) from both the studied loamy and
clay loam soils by DTPA gradually increased with increasing initial As (V)
concentration. A slight increase in the desorbed As from the loamy soil was
observed with increasing its initial concentration from 30 to 40, 50 to 60 and 60
to 100 µg As ml-1. Whereas, a relatively high increase in the desorbed As(V)
from the clay loam soil was noticed with increasing the initial concentration of
As(V) from 20 to 30,70 to 80 and 90 to 100 µg As ml-1.
Concerning goethite, the values of As(V) desorbed by using phosphate
(at pH 5) depend on the pH values, where the relative increase percentages of
As(V) desorbed from goethite reached 31.6, 33.0 and 70.6% at pH values of 3,
up to 7 and 12, respectively. The corresponding relative increase percentages of
As(V) desorbed in case of phosphate (concentration of 100 times as arsenate
and at pH 9) were greatly affected and reached 43.7, 46.0 and 75.5%,
respectively. In case of sulphate, the relative increase in As(V) desorbed was
negligible either at pH 3 or with increasing its value up to 7, where the relative
increase percentage was 0.34% at both pH values, and then greatly increased to
59.8%) at pH 12. As for molybdate (at initial concentration ratio of 100 : 1 of
MoO4 : As), it has a greatest effect on As(V), where the relative increase
percentages of As(V) desorbed reached 32.0% at pH 3, and then sharply
decreased to 0.10% with increasing pH up to 7. Above pH 7, the relative
increase percentages of As(V) desorbed increased again to 58.9% at pH 12.
With respect to zerovalent iron, phosphate (at concentration ratio of 100
solution : 1 As and pH 5) had a moderate effect on As(V) desorbed at pH 3,
where the relative increase percentage was 10.1%, and it tended to decrease
with increasing pH value up to 7 (7.08%). Above pH 7, the relative increase
percentage of As (V) increased again to 48.1% at pH 12. In case of phosphate
(at initial adsorption pH of 9), the relative increase percentages were greatly
affected, i.e., 23.50, 22.09 and 55.70% at pH values of 3, 7 and 12,
respectively. Sulphate (at initial ratio of 100 : 1 As and pH 5) exhibited a very
less quantity of As(V) desorbed at pH values of 3 and up to 9 as its relative
increases were 0.28 and 0.08%, respectively. Above pH 9.0, the relative
increase of As(V) greatly increased to 45.5% at pH 12. While, sulphate at
initial pH 9 showed a very less relative increase percentages in As(V) desorbed
at pH values of 3 up to 8 reached 0.93 and 1.79%, respectively. Above pH 8, the relative increase of As(V) greatly increased to 65.8% at pH 12. As(V)
desorbed as a result of molybdate (at initial adsorption pH 5) addition had no
change with increasing pH value up to 7 (0.00), and then a pronounced increase
was occurred (39.9%) at pH 12. In case of molybdate (at a ratio 100 MoO4 : 1
As and pH 9), a greatest relative increase was achieved for As(V) desorbed at
pH 3 (51.93%), and then it sharply decreased to 1.30% with increasing pH up < br />to pH 7. Above pH7, As(v) desorbed tended to increase again (55.80%) at pH
12.