The Salting-out of Liquid Nitroaromatic Compounds at Temperatures between 5 and 41 ºC

D. J. Luning Prak (prak@usna.edu) and D. W. O’SullivanU.S. Naval Academy, Chemistry Department, 572M Holloway Road, Annapolis, MD 21402.

Abstract

This work was done in support of a 3-year project, funded by SERDP (ER 1431), that seeks to quantify the factors (salinity, dissolved organic matter, sunlight conditions) that affect the transformation of nitrogenous compounds in marine systems. Since most of the transformations occur in the aqueous phase, measurements were made of the solubility of nitroaromatic compounds at temperatures ranging from 5 to 41 °C and salinity values ranging from pure water to 100% seawater (33.1 g salt/kg seawater). The solubility of the nitroaromatic compounds in seawater was lower than that in pure water due to the presence of the salts. The average salting-out coefficients for nitrobenzene, 2-nitrotoluene, and 3-nitrotoluene were 0.12, 0.14, and 0.14 L·mol-1, respectively, which are consistent with measurements for other nitroaromatic compounds. The solubility of these compounds increased with temperature, but the salting-out coefficients did not show any significant variation with temperature over the range examined.

Equilibration Experiments

Liquid nitrobenzene (NB), 2-nitrotoluene (2-NT), or 3-nitrotoluene (3-NT) was pipetted into 14-mL glass vials containing 12 mL of aqueous solution. The vials were:

· shaken on a reciprocal shaker or Labquake tube shaker (Fig. 1) for more than 24 hrs

· placed in a temperature-control room, on lab bench, or in an incubator (temperature ± 0.7 ºC)

· allowed to settled before sampling

Figure 1: Labquake tube shaker

Solubility of NB, 3-NT, and 2-NT in Fresh Water Systems

· Increasing temperature increases solubility of all three nitroaromatic compounds as shown in Figure 3.

· Modeled using:

c = a exp (b T)

c – concentration (mg/L) T – temperature in Kelvin a,b fitting parameters “b” values in Table 1 for liquid 2-NT (0.0124 T-1) and 3-NT (0.00885 T-1) are lower are lower than those of solid 4-NT1, 0.0310 ± 0.0021 K-1 Figure 3. Nitroaromatic solubility increases as

temperature increases

Table 1. Temperature Coefficients for NB, 2-NT, and 3-NT

Nitroaromatic Compound Analysis

Agilent 1100 Series High Performance Liquid Chromatograph (Fig. 2)

· C-18 column; Platinum, 100 Ǻ, 3 m,53 mm by 7 mm, Alltech

· variable wavelength detector set to 254 nm

· Samples (10 L for 2-NT, 3-NT, 4 L for NB) injected into an eluent (50 % MeOH in water)

· Isocratic separation with a flow rate of 0.75 mL/min.

Conclusions

►Solubility of nitroaromatic compounds increases with temperature. ►The average salting-out coefficients are 0.12, 0.14, and 0.014 L/mol for NB, 2-NT, and 3-NT, respectively. The values do not vary significantly with temperature.

Support for this work was provided by the SERDP Office grant ER-1431, and Naval Academy Research Council.

Reference: (1) Luning Prak, D. J.; O’Sullivan, D. W., 2007, “Solubility of 4-Nitrotoluene, 2,6-Dinitrotoluene, 2,3-Dinitroluene, and 1,3,5-Trinitrobenzene in Pure Water and

Seawater,” Journal of Chemical Engineering Data, 52 (6), 2446-2450.

Figure 4. The solubility of all three nitroaromatic compounds decreases as salinity increases and as temperature decreases, 100% was 34 ppt seawater.

Introduction

Nitroaromatic compounds can enter and pollute the aquatic environment through the improper handling practices and the corrosion of underwater unexploded ordnance casings. The impact of this pollution source on the aquatic systems depends on the capacity of the aquatic system to transform these pollutants into less harmful species. Since most of these transformations occur in the aqueous phase, aqueous solubility is a key variable in modeling these transformations. The goal of this work was to quantify the influence of temperature and salinity on the solubility of several nitroaromatic compounds.

Figure 2: HPLC

Values in parentheses represent the 95% confidence interval.

0

500

1000

1500

2000

2500

270 280 290 300 310 320

Temperature (K)

[Nit

roa

rom

ati

c]

(mg

/L)

Nitrobenzene

2-Nitrotoluene

3-Nitrotoluene

0.00885(± 0.00136)(20.4, 65.0)

0.0124(± 0.0020)(10.5, 23.5)(± 0.00120)

b( K–1)

a b( K–1)

a b( K–1)

a (mg/L)

3-2-NB

(±36.4

(±15.70.00961

(±116

(81,166)

b( K–1)

a b( K–1)

a b( K–1)

a

-NT-NT

(mg/L) (mg/L)

Solubility of Nitroaromatic Compounds in Seawater Solutions

· Salt lowers the solubility of NB, 2-NT, and 3-NT as shown in Figure 4.

· Modeled using Setschenow equation: log (Sw/S) = Ks′ I Sw - solute solubility in pure water S - solute solubility in the salt solution Ks’ - the salting-out parameter I - ionic strength of solution

Ks′ values for all four nitroaromatic compounds do not vary significantly with temperature (Table 2) and compare well with other nitroaromatic compounds (0.08 to 0.16 L/mol)1

Table 2. Salting-out Coefficients for NB, 2-NT, and 3-NT at Various Temperatures

SE: Standard error

NB 2-NT 3-NTT

(ºC) Ks’

(L·mol-1)SE T

(ºC) Ks’

(L·mol-1) SE T

(ºC) Ks’

(L·mol-1) SE

5.0 0.13 0.001 4.7 0.15 0.004 20.8 0.12 0.005 20.9 0.15 0.010 20.9 0.14 0.00930.6 0.12 0.011 30.7 0.14 0.002 30.8 0.14 0.00335.6 0.12 0.007 35.5 0.14 0.010 35.5 0.13 0.00740.3 0.12 0.002 40.3 0.14 0.005 40.3 0.14 0.013

Effect of salinity on NB solubility

1200

1400

1600

1800

2000

2200

2400

0 20 40 60 80 100% Seawater

[Nitr

oben

zene

] (m

g/L)

5.0 C 20.8 C 30.6 C 35.6 C 40.3 C

Effect of salinity on 3-NT solubility

300

400

500

600

700

0 20 40 60 80 100% Seawater

[3-N

itrot

olue

ne]

(mg/

L)

20.9 C 30.8 C 35.5 C 40.3 C

Effect of salinity on 2-NT solubility

300

400

500

600

700

800

0 20 40 60 80 100% Seawater

[2-N

itrot

olue

ne]

(mg/

L)

4.7 C 20.9 C 30.7 C 35.5 C 40.3 C