NITRATION WITH NITRATING MIXTURES 163Clearly, nitro compounds suspended in water are easily removed by passage
through settling tanks or filters. Nitro compounds dissolved in water are most
difficult to remove. Various methods are possible, which make use of various physi-
cal and chemical properties of the nitro compounds. For example, physical methods
which have been suggested and applied include adsorption of the nitro compounds
on absorptive agents (e.g. carbon) and extraction with solvents of low volatility
(e.g. phthalic esters). Among chemical methods, the destruction of nitro compounds
by oxidation with hypochlorous acid is possible. Since nitro compounds are often
resistant to oxidation, a more complex process is sometimes more efficient, con-
sisting first of the reduction of the nitro compound, then its oxidation. Chemical
methods are generally expensive and are rarely used. In particular, oxidation by
hypochlorous acid consumes considerable quantities of chlorine - up to 3 g for
1l of solution.
Biological methods of destroying nitro compounds are of considerable interest.
Up to now they have not been practiced on a large scale, but they are promising
because of their economy. The first investigations into the possibility of destroy-
ing nitro compounds by biological methods were carried out in 1941 by Erikson
[10], who found that nitro compounds such as nitrobenzene, picric acid, and tri-
nitroresorcinol can be used as a nutrient by some Actinomyceres. The observation
was later confirmed by Moore [11] and by Rogovskaya [12], who suggested des-
troying nitro compounds by fermentation.
Simpson and Evans [I 31 reported that nitrophenols such as o- and p-nitrophenols,
2,4-dinitrophenol or such a nitro compound as chloramphenicol could be de-
stroyed by some Pseudomonas species to form nitrous acid salts.
According to Petersen [14], the well known herbicide, dinitro-o-cresol, is rapidly
deactivated in soil by Gram-positive micro-organisms belonging to the Coryne-
bacterium family. This served as a starting point for Jensen and Gundersen [15]
in their research work in which they found that p- nitrophenol, 2,4-dinitrophenol
and picric acid are destroyed by the same micro-organisms with the formation
of nitrites. The same authors have investigated the rate of decomposition of dini-tro-o-cresol. Moreover, they have observed the interesting fact that o- and m- nitro-
phenols, 2,5- and 2,6-dinitrophenols and 3,5-dinitrobenzoic acid do not undergo
such a decomposition. It seems that only those nitrophenols that have the nitro
group in the para position are prone to decomposition.
Since nitro compounds have certain specific properties, which influence the
choice of methods for heating wastes, some special methods are described later
in the technological section dealing with methods of the manufacture of individual
nitro compounds. Diminishing water pollution by washes from the manufacture
of TNT is a most complex problem. This will be discussed in detail on p. 390.