Deactivation of phosphonium biocides

Solutions of organophosphorus biocides which also contain dissolved oxygen are deactivated by adding a catalytic amount of activated carbon.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention concerns a process for the deactivation of certain biocides 
which are used for the treatment of water. 
2. Background Information 
In our patent GB 2145708 we have described the use of certain hydroxy alkyl 
phosphines and phosphonium salts as biocidal agents against bacteria, 
algae, fungi and other lower organisms. These compounds may be used in 
cooling and other systems which contain water for prolonged periods, and 
may prevent the build-up of living organisms which would otherwise reduce 
the efficiency of such systems. As outlined in EP 0293152, these compounds 
can also be applied for the purposes outlined above to ecologically 
sensitive systems containing higher life-forms such as fish, without 
causing them any harm. An advantage of the compounds is that they achieve 
a rapid kill of the target organism whilst having a relatively low 
toxicity to higher organisms such as fish. 
However, in systems in which treated water is discharged as effluent it 
would be advantageous to deactivate these biocides prior to discharge. It 
is known that certain hydroxy alkyl phosphines and phosphonium salts react 
with dissolved oxygen and are thereby deactivated in acidic water. This 
property is not directly applicable to water systems, such as cooling 
systems, which are often operated at a higher pH e.g. 7 to 10. In this pH 
range the oxidative decomposition of the biocides essentially does not 
proceed. It is possible to acidify the effluent before discharge, and 
allow the oxidation to occur but subsequent adjustment to a neutral pH 
would be required before discharge. This process would be cumbersome, 
require continuous tight control and would be costly in terms of acid and 
alkali consumption. In addition the dissolved solids level in the 
discharged water would be increased. 
SUMMARY OF THE INVENTION 
The present invention provides a method of deactivating a solution 
comprising an organophosphorus biocide of formula 
EQU (R.sup.1 R.sup.2 R.sup.3 R.sup.4 P.sup.+).sub.y X 
or 
EQU R.sup.1 R.sup.2 R.sup.3 P 
wherein R.sup.1, R.sup.2, R.sup.3 and R.sup.4 each independently represent 
a C.sub.1-24 hydroxyalkyl, alkyl or alkenyl group, provided that at least 
one hydroxyalkyl group is present, and X is an anion of valency y, and 
dissolved oxygen, which comprises contacting with a catalytic amount of 
activated carbon. The activated carbon is preferably in the powdered or 
granular form, typically &gt;80% of less than 200 US mesh (0.075 mm) in size. 
The term "activated carbon" usually covers carbons having a specific 
surface area of from 300-2500 m.sup.2 /g, typically 500-1500 m.sup.2 /g. 
DETAILED DESCRIPTION OF THE INVENTION 
The biocides to which this invention is applied preferably contain no more 
than one alkyl or alkenyl group (i.e. preferably at least 2 or 3 hydroxy 
alkyl groups). 
The hydroxyalkyl groups are preferably 1-hydroxyalkyl and are most 
preferably hydroxymethyl groups. 
If any of R.sup.1, R.sup.2, R.sup.3 and R.sup.4 in the above formula are 
not hydroxyalkyl they are preferably C.sub.1-3 alkyl or alkenyl (e.g. 
methyl, ethyl or allyl). Particularly preferred are tetrakis 
(hydroxymethyl) phosphonium, methyltris(hydroxymethyl) phosphonium, ethyl 
tris(hydroxymethyl) phosphonium and allyl tris (hydroxymethyl) phosphonium 
salts. Compounds of this type have been described in EP 0139404 and EP 
066544. 
In addition to deactivating the biocide this process provides a mechanism 
for the reduction of the oxygen content of aqueous systems, and can also 
be used to provide an anaerobic solution, which has considerable 
application in that it reduces the corrosive action of the said solution. 
It also provides a mechanism for the reduction of the odour of such 
systems. 
The anion X may be a monovalent anion such as chloride or bromide, or an 
organic carboxylate, e.g. an alkane carboxylate, preferably of 2-5 carbon 
atoms such as acetate, bisulphite, bisulphate, an organic sulphonate, such 
as methanesulphonate or a benzene, toluene or xylene sulphonate, or 
dihydrogen phosphate; a divalent anion such as sulphate, sulphite, 
monohydrogen phosphate, a phosphonate or an organic dicarboxylate; a 
trivalent anion such as phosphate, or an organic carboxylate with 3 
carboxyl groups, such as citrate; or a polyvalent anion such as 
polyacrylate or polymaleate. 
Typical catalytic or deodorising amounts of activated carbon are those 
which provide a weight ratio of carbon: THP in the range 200:1 to 1:500, 
more preferably 20:1 to 1:50. 
The catalyst may additionally contain co-catalysts, promoters or additives 
designed to assist in this process or others. Additives, such as scale or 
corrosion inhibitors, flocculants, dispersants, antifoams, oxygen 
scavengers, biocides, demulsifiers, reverse demulsifiers or lubricants may 
also be added to the solution which contains the phosphorus biocides. In 
either case the added material may be solid, liquid or gaseous in nature 
and may be added periodically or continuously. 
The process may be carried out in batch or continuous mode preferably with 
a contact time of 2 minutes to 30 hours, especially 40 to 200 minutes. 
Batch mode involves simple addition of the catalyst to the solution and 
its separation, for example by filtration, after deactivation has taken 
place. Operation in continuous mode may, for example, involve passing the 
solution through a bed containing the catalyst. 
The catalyst or catalyst mixture may require reactivation or replacement. 
Any reactivation procedure, which can be chemical or physical in nature 
(for example steaming), may be conducted in-situ or after removal of the 
spent catalyst from its operational environment, and can also involve the 
replacement of the additives etc. This reactivation can be periodic or 
continuous, and may alternatively or additionally involve treatment of the 
phase containing the aforementioned phosphorus compounds. 
Reactivation may also involve additional physical or mechanical treatment 
of the catalyst and/or the phase containing the aforementioned phosphorus 
compound, and this may be continuously or periodically applied, using a 
rotating degradation chamber for example. 
Solutions of the aforementioned biocides have a characteristic 
phosphine-like odour. Addition of the catalyst to such solutions reduces 
this odour even in the absence of dissolved oxygen or another suitable 
oxidant. Accordingly, this method also provides a mechanism for the 
deodorization of the aforementioned biocidal solutions, thus improving the 
applicability of the biocides and further facilitating discharge of 
treated water to the environment. 
This invention provides a catalytic process for the destruction of 
organophosphorus biocides when in solution, thus improving the 
environmental characteristics of discharges containing them. It also has 
the advantage that it reduces the odour of such solutions, and can be 
adopted for use in aerobic and anaerobic systems. It may have application 
in industrial processes in general, but especially in the paper industry, 
the beneficiation of ores, the paint and ink industry, the processing of 
wood, the electronics industry, mining, quarrying, cooling systems, oil 
production, extraction, processing and exploration, metal finishing 
processes and any other process which involves water.