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Toxicity to microorganisms

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The toxicity of DTPA to microorganisms was assessed in several studies, including OECD 209 activated sludge respiration inhibition tests, and several biodegradation screening tests. In the activated sludge respiration inhibition study the 3-hour EC50 was greater than 1280 mg/L for DTPA-FeHNa, corresponding to 1313 DTPAFe(NH4)2. A NOEC was established at 400 mg/L DTPA acid, corresponding to 488 mg/L DTPA-Fe(NH4)2.

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In an OECD 209 test, a 3 -hour EC50 value could not be determined for DTPA-FeHNa, since only minor effects were observed at 1280 mg/L, the highest concentration tested. This was confirmed by DTPA acid in another OECD 209 test. In an OECD 302B inherent biodegradation test, no inhibition of activated sludge inoculum was observed at 100 mg/L DTPA acid (123 mg/L DTPA-Fe(NH4)2). In batch biodegradation studies using microbial inocula from pulp mill activated sludge, municipal activated sludge, and soil, no microbial inhibition was reported for DTPA acid concentrations of 400 mg/L (488 mg/L DTPA-Fe(NH4)2). Using pulp mill activated sludge pre-exposed to DTPA, no inhibition was reported in reaction mixtures containing 725 mg/L DTPA acid (886 mg/L DTPA-Fe(NH4)2). These results are consistent with the structurally similar compound EDTA, where EDTA did not significantly inhibit the respiration of activated sludge in an OECD 209 test up to a nominal concentration of 500 mg/L (EC20, EC50, EC80 >500 mg/L).