An exploration of arsenic-phosphate
chemistry relevant to remediation of
contaminated soils at substations
Man-Tzu Huang
Energy Systems Engineering Institute
Problem Statement
Discussion
Results
- Phosphate has similar chemical properties and
Arsenate
(As
(V))
is
the
predominated
form
of
Arsenic level has been detected from the
arsenic under oxidizing condition. Arsenic can be similar dissociation constants with arsenic.
substations through out the United State and
traced from the agricultural, industrial use, the
Canada. The use of herbicide which contains
- The dissociation constants for orthophosphoric
arsenic trioxide (As2O3) is the primary contaminated natural deposits on the minerals. The
acid, H3PO4 (pk1=2.12,pk2=7.20,pk3=12.40),
biogeochemistry
of
arsenic
in
soil
is
very
source. In order to eliminate the on-site arsenic in
complicated. It undergoes adsorption-desorption, are very similar to those of arsenic acid,
the soil, the common remedial action for power
reduction-oxidation, dissolution-precipitation,
H3AsO4 (pk1=2.2,pk2=6.97,pk3=11.53
company is to excavate, remove arseniccontaminated soil from the site, and backfill the site acid-base reactions, and biomethylation. The
- When adding phosphate to arsenic-soil,
biogeochemistry
of
arsenic
schematic
was
with fill material. However, arsenic concentration
phosphate is more competitive to the anion
has been increase slowly after the remediation. The showed in figure 2. Also according to the
sorption site and it will replace with arsenic via
remobilization of the existing arsenic in the original isothermal thermodynamics, arsenate is
adsorbed more strongly to soil than arsenite.
soil was likely activated by phosphate from the fill
ligand exchange causing the arsenic desorption
The following showed the isothermal equations;
material. The purpose of this project is to
from soil which can be observed from figure 5
understand the chemistry between phosphate and
below.
arsenic-soil and the interaction of arsenic and soil.
Langmuir adsorption isotherm equation:
n= KcM/(1+Kc)
Extent of Problem
Arsenite (As (III)) and arsenate (As (V)) are the two
dominated forms of arsenic. Both chemical forms
are very toxic and arsenate is carcinogenic to
human body. When arsenic is fixed in soil, it is safe
to the environment. However, arsenic can easily be
transported to groundwater by arsenic desorption.
Long term exposure to arsenic drinking water may
cause cancer of skin, lung, and kidney.
Freundlich equation:
n= (Kc)B
Where n is the amount of As adsorbed (umol/kg),
c is the equilibrium As concentration (uM), K and
B are adjustable parameters and M is the
adjustable adsorption maximum.
Figure 5: Water-extractable As in unlimed Fe and P amended Thurmont (A)
and Burch (B) soils during incubation a6 26 C.
- The proposed arsenic and phosphate chemical
equation can be express as following.
Figure 2, Experimental As (III) and As (V) adsorption isotherms
for Fallbrook soils Lines are Langmuir isotherms fitted to
experimental data using the ISOTHERM computer program
Figure 1, Schematic diagram of arsenic biogeochemistry in soils
Methodology of Analysis
The project was approached by literature
review on the following topic.
- Chemistry of arsenic/soil and arsenic/phosphate
- Arsenic transport (desorption/adsorption process,
- Arsenic thermodynamics,
When arsenic in the soil, arsenic is bound with
metals, mostly Al and Fe, via ligand exchange
and form the inner-sphere complexes as
mononuclear monodentate, mononuclear
bidentate and binuclear bidentate which are
showed in figure 3.
M-OH+M-OAsO3H2+ 2H2PO4- -----
2(M-OPO3H2)(s) + H2AsO4-+OH,where M is metal such as Al and Fe.
- The dominated process for arsenic movement
in soils is non-equilibrium which was
determined from the breakthrough curve (see
figure 4). It undergoes two ligand exchange
processed with OH2 and OH and the equations
were showed below.
M-OH2++H2AsO4 -  M-OAsO3H2+H2O
M-OH+ H2AsO4 - - M-OAsO3H2+OH-
, where M is the metal atom at oxide
surface
Conclusions
After all the chemistry of arsenic, phosphate
and soil, it may be feasible to find another
chemical to bind with phosphate and fix arsenic
in the soil. Power company should take this
issue seriously and come up with efficient
remediation to avoid this problem.
Figure 6, arsenic poisoning
on human body
Figure 3, Binding mechanism of arsenic in soils.
Figure 4, Breakthrough curves of AsO4 at four different pore water
velocities ( 0.2,1,10, and 90 cm hr-1).