|Comparison of detergency characteristics between tri-sodium phosphate (TSP) and LFA-11, a detergent developed for lead-contaminated dust removal.
By: Neil R. Wilson
Data comparing TSP and LFA-11 surface tension, lead solubility, hardness ion tolerance and pH are presented. The significance of these data as it relates to detergency and lead contaminated dust removal for lead abatement activities is discussed. LFA-11 is found to have superior detergency characteristics because it has a lower surface tension, higher lead dissolving ability, better hard water tolerance, and mild pH. It is pointed out that TSP precipitates lead as insoluble lead phosphate.
Detergent LFA-11 was developed to remove lead contaminated dusts such as those found in lead based paint abatement, risk assessment, and in-place management activities. The ingredients of LFA-11 were selected, blended, and optimized for detergency on lead compounds and lead containing particulates. LFA-11 is a phosphate-free, lead-dissolving, biodegradable detergent made from anionic surfactants and organic chelating and sequestering agents.
Tri-sodium phosphate (TSP) is evaluated here because the “Lead Based Paint: Interim Guidelines for Hazard Identification and Abatement in Public and Indian Housing” of September 1990 from the Dept. of Housing and Urban Development specifies the use of “detergents with a high phosphate content (containing at least 5% TSP)” shall be used for washing procedures in conjunction with lead abatement. TSP is a hard surface cleaner that is frequently used by professional painters to clean a surface as part of surface preparation for further painting.
Key mechanisms in detergency are: surface tension lowering; solubilizing soils; emulsifying soils; saponifying fatty soils; inactivating water hardness; and neutralizing acid soils. Because lead contaminated dust (LCD) is not a fatty soil, an oily emulsifiable soil, or an acidic soil, we do not examine emulsifying, saponifying, or neutralizing here. We do review surface tension, solubility, and hardness tolerance to examine compounds for LCD detergency. We also examine pH to determine the mildness of the cleaning solutions evaluated.
Surface tension measurements give an indication of wetting, dispersing, emulsifying and penetrating characteristics of a solution. Within limits, the lower the surface tension, the better the detergent. In practice, good surface tension detergency is achieved in solutions with surface tensions in the range of 30-35 dyne/cm. Surface tension does not vary in a linear fashion with concentration, but is more asymptotic, usually achieving a typical minimum value at well below the concentrations used here. Even higher concentrations, up to 10% or more, TSP would not result in significantly lower surface tension readings.
In Fig. 1, the results of measurements using a calibrated Surface Tensiometer measuring 1% solutions of TSP and LFA-11 are presented.
As can be seen in Fig. 1, LFA-11 achieves the desired surface tension lowering range of 35 dyne/cm. This means that LFA-11 solutions should have good wetting, penetrating, emulsifying, and dispersing characteristics.
Wetting is the ability and speed which liquid displaces air from a solid surface, and it is a desirable detergent property because it shows how fast and how thoroughly a solution is likely to cover a hard surface. Penetrating is the ability of a solution to overcome interfacial tensions and enter into or under small entrances or crevices in a surface or between surfaces and particulate soils. It is a desirable detergent characteristic because it shows how thoroughly a detergent will access soils in difficult to clean porous surfaces such as concrete and how quickly it will penetrate under and lift particulate soils. Emulsifying is the process of dispersing one liquid into another and although a desirable detergent characteristic, it is not so relevant to solid particulate soils such as LCD. Dispersing is the act of increasing the stability of particles in solutions and it is a desirable detergent property because it indicates how well a solution may suspend particles in solution.
As can be seen in Fig. 1, TSP does not have a very low surface tension and would not have very good wetting, penetrating, emulsifying, or dispersing based on surface tension lowering.
Lead solubility measurements indicate the ability of solutions to dissolve solids and form a homogeneous mixture. The opposite of solubilizing would be the formation of a precipitate that causes otherwise dissolved solids to come out of solution as insoluble precipitates. It is desirable to dissolve solids into a homogeneous mixture for easier removal from a surface. LFA-11 solubilizes lead compounds by using chelating and sequestering agents. Chelating agents form metal complex ring compounds with metals or metal compounds. Sequestering agents lift metal or metal compounds into solution by forming a stable soluble metal complex that blocks the reactive sites on metals and inhibits reactions with other compounds such as those that might form insoluble precipitates. In the case of LFA-11, agents were chosen for their activity on lead compounds.
In Fig. 2, the results of lead solubility tests using a slightly water soluble lead salt, lead acetate, are shown. Lead acetate is used to represent a common lead salt in the same oxidation state as many lead pigments found in paints and to show the precipitation effect of TSP and the resulting low solubility characteristics. TSP does not dissolve lead, but instead forms an insoluble lead phosphate precipitate.
The lead dissolving ability of a 2% solution of LFA-11 was compared to a 4% solution of TSP. These concentrations were chosen to represent concentrations of cleaner as they might be used in cleaning LCD from hard surfaces. Although a 5% TSP solution is recommended in the Lead Based Paint Interim Guidelines, few contractors compensate for the 8 to 12 waters of hydration present in the technical grade TSP crystals they may use which results in actually having lower TSP concentrations than those resulting from weight/weight dilutions of hydrated TSP. Because of the precipitating effect of TSP, it would not matter if even higher concentrations of TSP had been used.
Fig. 2 shows the results of observing a stirred solution at 20 degrees C with successive additions of 0.1 g of lead acetate crystals to 500 g of each solution until a cloudy solution is observed. With TSP a cloudy solution is observed during the first addition of lead acetate indicating less than 0.1 g of solubility. In fact a change in the character of the large crystals of lead acetate was observed as they dispersed and formed small particles of insoluble lead phosphate. With LFA-11 a clear solution was achieved in a stirred solution at room temperature with 0.5 g of lead acetate.
More tests should be done using other lead compounds such as lead oxides, sulfates and carbonates, and in particular with the insoluble lead compounds where even more dramatic comparisons to TSP would be expected.
Hardness Ion Tolerance
Aqueous detergent solutions are typically made up using tap water. In most of the Midwest, parts of New England, Florida, the Southwest, and in many wellwater sources, the water is often very hard as defined as having greater than 180 ppm calcium carbonate equivalent concentration of calcium, magnesium, iron and manganese ions. Very hard water will interfere with detergency because these ions will form insoluble precipitates with a variety of detergent ingredients and soils, thus rendering the detergent less effective. The ability to tolerate hardness ions is a desirable characteristic for a detergent because this means it will perform effectively when used in very hard tap water.The results shown in Fig. 3, were achieved by measuring hardness ion tolerance by titrating solutions with 1% calcium chloride until the first persistent cloudiness indicating a calcium precipitant was seen. Again, solutions representing concentrations used for LCD removal were evaluated. We used a 2% LFA-11 solution and 4% TSP solution.
As shown in Fig. 3, LFA-11 has a much higher hardness ion tolerance than TSP which indicates that in areas where hard water is common, LFA-11 would possess better water softening properties for good detergency and maintain the capacity to solubilize metal compounds much better than TSP would.
The measurement of ph determines the negative log of the hydrogen ion concentration. In water it is a logarithmic scale going from 0 to 14, acid to alkaline. Mild neutral aqueous solutions have a pH of 7. Solutions that are lower than pH 4 or higher than pH 10 are generally considered to be corrosive for manual cleaning where there may be significant risk of skin or eye contact. In general, a mild solution that is effective for the cleaning required should be used when there is risk of skin or eye contact.
According to the Lead Based Paint Interim Guidelines, the basic Federal law governing waste disposal is the Resource Conservation and Recovery Act (RCRA) of 1976. In RCRA, one of the characteristics that defines a hazardous waste is if it is a corrosive waste defined as a pH less than or equal to 2 or greater than or equal to 12.5. While a typical TSP solution is typically below 12.5 in pH, there are technical crystals of TSP and that are made with excess alkali to prevent caking and give more alkalinity. Some of these technical grades of TSP may have a pH over 12.5 resulting in the formation of a hazardous waste as soon as they are mixed together to make a cleaning solution. Technical grade TSP is what is commonly available at hardware stores which is where the Lead Based Paint Interim Guidelines suggest you obtain high TSP cleaners.
Additionally, TSP is listed as a hazardous substance and is subject to the reporting requirements of the Superfund Amendments and Reauthorization Act of 1980 (SARA) Title III, Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA).
The pH of 1% solutions of TSP and LFA-11 in deionized water were measured using a calibrated pH meter at 20 degrees C. The concentration was chosen to give an even comparison of equal strength solutions. LFA-11 is inherently buffered by the ingredients used to make it, and would not exhibit a substantially different pH at 2% concentrations as used. Depending on how much excess alkali had been used in making a technical grade of TSP, the pH would rise more or less with greater concentrations than 1%.
As can be seen in Fig. 4, even a 1% TSP solution had a pH of 12.2 and the pH of a 1% solution of LFA-11 was 5.4. Although the expected value for a 1% TSP solution is 11.9, we obtained a higher value, presumably because the sample we obtained was made with excess alkali, even though the material we obtained did not in any way indicate that it had been made in this way.
The pH of LFA-11 falls well within the normal standards of pH for a mild manual cleaning detergent, is not any where near a hazardous waste corrosive pH, and does not contain any compounds listed in SARA.
LFA-11 is superior to TSP in detergency characteristics that would be important for lead contaminated dust removal. As compared with TSP, LFA-11 has lower surface tension, higher lead solubility, higher hardness ion tolerance and mild pH. Field tests of LFA-11 are being conducted. So far, as expected, results have been outstanding. Further research on and testing of phosphate-free, lead-dissolving detergents should be evaluated for use in lead contaminated dust removal for lead abatement activities.
(1) LFA-11 was introduced in July of 1993 as Ledizolv® by:
HIN-COR Industries, Inc.