BackgroundThe insecticide dichloro-diphenyl-trichloroethane (DDT) is widely used in indoor residual spraying (IRS) for malaria control owing to its longer residual efficacy in the field compared to other World Health Organization (WHO) alternatives. Suitable stabilization to render these alternative insecticides longer lasting could provide a less controversial and more acceptable and effective alternative insecticide formulations than DDT.MethodsThis study sought to investigate the reasons behind the often reported longer lasting behaviour of DDT by exposing all the WHO approved insecticides to high temperature, high humidity and ultra-violet light. Interactions between the insecticides and some mineral powders in the presence of an aqueous medium were also tested. Simple insecticidal paints were made using slurries of these mineral powders whilst some insecticides were dispersed into a conventional acrylic paint binder. These formulations were then spray painted on neat and manure coated mud plaques, representative of the material typically used in rural mud houses, at twice the upper limit of the WHO recommended dosage range. DDT was applied directly onto mud plaques at four times the WHO recommended concentration and on manure plaques at twice WHO recommended concentration. All plaques were subjected to accelerated ageing conditions of 40°C and a relative humidity of 90%.ResultsThe pyrethroids insecticides outperformed the carbamates and DDT in the accelerated ageing tests. Thus UV exposure, high temperature oxidation and high humidity per se were ruled out as the main causes of failure of the alternative insecticides. Gas chromatography (GC) spectrograms showed that phosphogypsum stabilised the insecticides the most against alkaline degradation (i.e., hydrolysis). Bioassay testing showed that the period of efficacy of some of these formulations was comparable to that of DDT when sprayed on mud surfaces or cattle manure coated surfaces.ConclusionsBioassay experiments indicated that incorporating insecticides into a conventional paint binder or adsorbing them onto phosphogypsum can provide for extended effective life spans that compare favourably with DDT's performance under accelerated ageing conditions. Best results were obtained with propoxur in standard acrylic emulsion paint. Similarly, insecticides adsorbed on phosphogypsum and sprayed on cattle manure coated surfaces provided superior lifespans compared with DDT sprayed directly on a similar surface.
Organo-bentonites containing quaternary ammonium surfactants were prepared using purified bentonite from the Boane deposit in Mozambique. The intercalation was effected by intimate mixing of the surfactants into a concentrated dispersion of soda ash activated bentonite. TG indicated organic contents that exceeded CEC expectations by up to 21 mass %. XRD results were consistent with: (i) paraffin-type extended chain intercalation at tilt angles of 27 º to 33 º; and (ii) interdigitated monolayer intercalation of the C14 single alkyl chain surfactant and bilayer intercalation of the C16 double alkyl chains and its 1:1 mixture with the single chain surfactant. However, FTIR analysis revealed disordered alkyl chain conformations. White oil dispersions containing 10 mass % organo-bentonite (inorganic basis) featured high viscosities. They showed strong shear thinning behaviour confirming the ability of the montmorillonite particles to exfoliate.
Single and double chain cationic surfactants were intercalated into Koppies bentonite. The state of the chains in the interlayers was probed using XRD, FTIR spectroscopy and thermogravimetric analysis (TG). The degree of intercalation was determined by TG and exceeded the CEC equivalent value by 30-40 %. The degradation onset temperature of about 200 ºC was independent of the number and length of alkyl chain substituents. The XRD d-spacing was consistent with monolayer interdigitated surfactant intercalation for the C12 and C14 single chain surfactants and with bilayer intercalation of the double chain surfactants. FTIR analysis revealed disordered alkyl chain conformations.
South African Koppies bentonite was organo-modified with single tail and double tail alkyl ammonium cationic surfactants with the latter intercalated both below and above the clay CEC.Corresponding poly(ethylene-co-vinylacetate) nanocomposites were prepared by twin-screw melt compounding. Transmission electron microscopy indicated the presence of mixed nano-and micron-sized clay morphologies. X-ray diffraction studies revealed that the crystallinity of the particles improved and that the d-spacing values increased on incorporating the clays in the polymer matrix. It is postulated that, rather than indicating polymer co-intercalation, this is caused by further intercalation of either excess surfactants or surfactant residues that were released by shear delamination of the clays during compounding. Improved mechanical properties were realized especially when using the clay containing the longer double tail surfactant intercalated at levels in excess of the cation exchange capacity of the clay. The nanocomposites showed improved tensile modulus and elongation at break values at the
WHO-approved insecticides were co-intercalated with hexadecyl-trimethylammonium (HDTMA) chloride in bentonite in an attempt to improve their persistence. XRD and FT-IR confirmed successful co-intercalation of the insecticides. Bioassays revealed that the activity of co-intercalated insecticides deteriorated after just one week of accelerated aging at 40 °C and 90 % ℜH. However, when the pH of the bentonite slurry used for intercalation was reduced with acetic acid to pH = 4.5, the persistence life increased to 113 days. The performance ranking with respect to both knockdown and mortality was as follows: Co-intercalated α-cypermethrin > cointercalated propoxur > co-intercalated DDT > DDT.
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