Comparison of Nanoscale Zero-Valent Iron, Compost, and Phosphate for Pb Immobilization in an Acidic Soil

Lead is one of the most potentially toxic metals present in soils. In situ Pb immobilization techniques reducing its bioavailability to soil organisms are of increasing interest. The present work compares the effectiveness of nanoscale zero-valent iron (nZVI), compost, and phosphate for Pb immobilization in an acidic, artificially polluted soil after different contact times (15 and 45 days). The availability and mobility of Pb were evaluated by the Tessier extraction procedure and the toxicity characteristics leaching procedure (TCLP). The impact on soil properties and soil phytotoxicity was also evaluated. The phosphate was the most effective treatment in all sampling times, reaching Pb reductions in more available fractions of 72%, followed by compost (40%) and nZVI (32%). Comparing the two sampling times, a significant reduction of available Pb in phosphate-treated soils was observed at a longer contact time. Soil properties changed depending on the treatment. In general, the application of compost improved the soil fertility, soils treated with nZVI showed an increase of pH and available sodium and iron concentration, and the treatment with phosphate increased available phosphorus concentration in soil, but was less than that obtained by the compost treatment. Regarding the soil phytotoxicity, Vicia sativa showed moderate phytotoxicity to untreated Pb-polluted soils, and the different treatments decreased it. In conclusion, at the experimental conditions, the use of phosphate resulted as more effective than compost and nZVI for reducing Pb availability in an acidic soil. Longer-term assays are necessary to evaluate the stability of the process.