Basic Study on the Remediation Technology
437
3.5
Examination of the Mechanisms of Heat-Generated Removal Reactions
Based on our test results, we examined the heat-generated mercuric sulfide
removal mechanisms when iron chloride was added. With our testing system,
since no water was added before the heat was applied and the heating
temperature exceeded 200°C, there was hardly any moisture in the artificially
contaminated soils when the reactions occurred. Therefore, it can be considered
that all substances causing the reactions were bonded solids when the reactions
occurred.
Iron chloride, which was used in our tests as a catalyst, is known to denature
quickly when heated in air (Mizushima 1964). Denaturing starts when it is heated
to around 200°C, but when it is heated to 250 °C it changes and finally converts
to iron oxide and hydrogen chloride gas. Therefore, if iron chloride is heated to
250°C or more in air, bivalent iron chloride becomes trivalent iron oxide and the
electrons that are discharged when iron chloride is denatured by heating can be
used to forcefully form a reducing atmosphere between the solids. This is
obvious from the test results dicussed above, from the fact that the percentage of
mercuric sulfide removed depended on the heating temperature, and the fact that
the generation of iron oxide peaked as the heating temperature rose, as shown in
Fig. 7 of the X-ray analysis results. When iron chloride and mercuric sulfide are
simply mixed in the atmosphere at room temperature, nothing happens. It is
thought, however, that when they are mixed and heated, an oxidation-reduction
reaction atmosphere is forcefully generated, and that an oxidation reduction
occurs between it and the mercuric sulfide. We think that if some mercuric
sulfide is decomposed by an oxidation reduction reaction caused by the
denaturation of iron and the characteristics of the silica, an oxidation reduction
atmosphere can be formed in the soil, which leads to an oxidation reduction
reaction.
There is a test report in which mercuric sulfide was dispersed in water. In
the report, when trivalent iron ions also existed in the water, they were reduced
to bivalent mercury ions, and mercuric sulfide was solubilized at the same time.
It is also reported that mercuric sulfide may solubilize when bivalent iron ions
are oxidized to trivalent iron ions, and that there is a strong correlation
between solubilization and the oxidation-reduction potential of metal ions
(Akagi et al. 1975). Further, as we mentioned earlier, the effect of chloride in
transition metal compounds when artificially contaminated soils are heated
cannot be ignored because the denatured products do not contain chloride
components.
A report on mercury behavior in the discharge of waste treatment facilities
explains that when hydrogen chloride is heated in air to a low temperature of
around 300°C, it oxidizes abruptly and chlorine gas is generated (Urabe and
Yokokawa 1990). Given this, it is likely that some of the hydrogen chloride gas,
which is generated when iron chloride is denatured by heat, is oxidized and
chlorine gas is generated. As mercury reacts furiously with chlorine even at room
temperature, it is necessary to take the oxidation-reduction reaction caused by
chlorine into consideration.
437
3.5
Examination of the Mechanisms of Heat-Generated Removal Reactions
Based on our test results, we examined the heat-generated mercuric sulfide
removal mechanisms when iron chloride was added. With our testing system,
since no water was added before the heat was applied and the heating
temperature exceeded 200°C, there was hardly any moisture in the artificially
contaminated soils when the reactions occurred. Therefore, it can be considered
that all substances causing the reactions were bonded solids when the reactions
occurred.
Iron chloride, which was used in our tests as a catalyst, is known to denature
quickly when heated in air (Mizushima 1964). Denaturing starts when it is heated
to around 200°C, but when it is heated to 250 °C it changes and finally converts
to iron oxide and hydrogen chloride gas. Therefore, if iron chloride is heated to
250°C or more in air, bivalent iron chloride becomes trivalent iron oxide and the
electrons that are discharged when iron chloride is denatured by heating can be
used to forcefully form a reducing atmosphere between the solids. This is
obvious from the test results dicussed above, from the fact that the percentage of
mercuric sulfide removed depended on the heating temperature, and the fact that
the generation of iron oxide peaked as the heating temperature rose, as shown in
Fig. 7 of the X-ray analysis results. When iron chloride and mercuric sulfide are
simply mixed in the atmosphere at room temperature, nothing happens. It is
thought, however, that when they are mixed and heated, an oxidation-reduction
reaction atmosphere is forcefully generated, and that an oxidation reduction
occurs between it and the mercuric sulfide. We think that if some mercuric
sulfide is decomposed by an oxidation reduction reaction caused by the
denaturation of iron and the characteristics of the silica, an oxidation reduction
atmosphere can be formed in the soil, which leads to an oxidation reduction
reaction.
There is a test report in which mercuric sulfide was dispersed in water. In
the report, when trivalent iron ions also existed in the water, they were reduced
to bivalent mercury ions, and mercuric sulfide was solubilized at the same time.
It is also reported that mercuric sulfide may solubilize when bivalent iron ions
are oxidized to trivalent iron ions, and that there is a strong correlation
between solubilization and the oxidation-reduction potential of metal ions
(Akagi et al. 1975). Further, as we mentioned earlier, the effect of chloride in
transition metal compounds when artificially contaminated soils are heated
cannot be ignored because the denatured products do not contain chloride
components.
A report on mercury behavior in the discharge of waste treatment facilities
explains that when hydrogen chloride is heated in air to a low temperature of
around 300°C, it oxidizes abruptly and chlorine gas is generated (Urabe and
Yokokawa 1990). Given this, it is likely that some of the hydrogen chloride gas,
which is generated when iron chloride is denatured by heat, is oxidized and
chlorine gas is generated. As mercury reacts furiously with chlorine even at room
temperature, it is necessary to take the oxidation-reduction reaction caused by
chlorine into consideration.