P $11\mathrm{.}2$ Assessing the Speciation of Bromomethane and of Atrazine in a QuartzAssessing the Speciation of Bromomethane and Atrazine in Quartz Sand

Consider a confined volume of pure quartz sand exposed to air $(RH=45\%),$ with a porosity $=$

$0\mathrm{.}5,$ a density of $2\mathrm{.}65gc{m}^{-3}$ and a surface area $A=1m^2{g}^{-1}.$

What fraction fia of $($a$)$ the fumigant bromomethane and $($b$)$ the herbicide atrazine will be present

in the pore air at $15C$ and at $35C$ assuming a linear adsorption isotherm?

Hints: Kiqsurf at $15C$ can be estimated with Eq $11-13$; use Eq$.11-4$ for estimation of the

adsorption enthalpy of the two compounds.

Fraction in air is given as:

$f_{ia}=\frac{C_{ia}{V}_a}{C_{ia}{V}_a+C_{\text{i}\text{q}\text{s}\text{u}\text{r}\text{f}}{A}_{\text{q}\text{s}\text{u}\text{r}\text{f}}}=\frac{1}{1+K_{\text{i}\text{q}\text{s}\text{u}\text{r}\text{f}}\frac{A_{\text{q}\text{u}\text{r}\text{f}\text{f}}}{V_a}}$

The Abraham's parameters for the compounds of interest are provided below. $($As discussed in

class, they can be found in the LSER$-$database on the UFZ website" $-$ just google "LSER UFZ"$).$

Sand

Consider a con!ned volume of pure quartz sand exposed to air $($RH $=45\%),$ with

a porosity $,=0\mathrm{.}5($see Box $5\mathrm{.}4),$ a density of $2\mathrm{.}65$ g cm$3$ and a surface area A $=$

$1$ m$2$ g$1.$ What fraction fia of $($a$)$ the fumigant bromomethane and $($b$)$ the herbicide

atrazine will be present in the pore air at $15!$Cand at $35!$Cassuminga linear adsorption

isotherm? Comment on the result.

Hint: Consult Box $11\mathrm{.}1.$ Use Eq$.11\mathrm{.}4$ for estimation of the adsorption enthalpy of the

two compounds.

log Kiqsurfa$(288$K$,$RH $=45\%)($m$)=0\mathrm{.}92$Li $+3\mathrm{.}20$Ai $+5\mathrm{.}42$Bi $8\mathrm{.}47(11-13)$

and

log Kiqsurfa$(288$K$,$RH $=90\%)($m$)=0\mathrm{.}72$Li $+3\mathrm{.}17$Ai $+4\mathrm{.}34$Bi $8\mathrm{.}47(11-14)$

deltasurfaHi$($kJ mol$1)=9\mathrm{.}83$ logKisurfa$($m$)90\mathrm{.}5(11-4)$