Photocatalytic fixation of NOx in soils.

Nitrogen oxides (NOx = NO + NO2) are important atmospheric pollutants that are directly harmful to human health. Recently in urban and industrial areas, synthetic materials have been developed and deployed to photocatalytically oxidize NOx to nitrate (NO3-) in order to improve air quality. We show that the natural presence of small amounts (≤5%) of titanium oxides, such as anatase and rutile, can also drive NOx oxidation to nitrate in soils under UV-visible irradiation. The NO uptake coefficients ranged between 0.1 × 10-6 for sandy soils to 6.4 × 10-5 in the case of tropical clay soils; the latter comparable in efficiency to current industrial man-made catalysts. This photocatalytic N-fixation mechanism offers a new strategy for NOx mitigation from the atmosphere by transforming it into nitrate, and simultaneously provides an energy efficient source of essential fertilizer to agriculture.

Photochemical emission and fixation of NOX gases in soils.

Gaseous nitrogen oxides (NOx), which result from the combustion of fossil fuels, volcanic eruptions, forest fires, and biological reactions in soils, not only affect air quality and the atmospheric concentration of ozone, but also contribute to global warming and acid rain. Soil NOx emissions have been largely ascribed to soil microbiological processes; but there is no proof of abiotic catalytic activity affecting soil NO emissions. We provide evidence of gas exchange in soils involving emissions of NOx by photochemical reactions, and their counterpart fixation through photocatalytic reactions under UV-visible irradiation. The catalytic activity promoting NOx capture as nitrate varied widely amongst different soil types, from low in quartzitic sandy soils to high in iron oxide and TiO2 rich soils. Clay soils with significant amounts of smectite also exhibited high rates of NOx sequestration and fixed amounts of N comparable to that of NO (nitric oxide) losses through biotic reactions. In these soils, a flux of 100 µg NNO m-2 h-1, as usually found in most ecosystems, could be reduced by these photochemical reactions by more than 60%. This mechanism of N fixation provides new insight into the nitrogen cycle and may inspire alternative strategies to reduce NO emissions from soils.

Pot evaluation of synthetic nanosiderite for the prevention of iron chlorosis.

BACKGROUND: Iron chlorosis is a problem that affects crops grown on calcareous soils. In this work, we assessed the effectiveness of nanosized siderite (FeCO3) to prevent iron chlorosis, the underlying hypothesis being that the oxidation products of siderite in soil are poorly crystalline, and hence plant-available, iron oxides. RESULTS: Nanosized siderite was prepared by mixing FeSO4 and K2CO3 solutions, either pure or doped with phosphate (siderite SID and SIDP, respectively). The average specific surface area was ∼140 m² g⁻¹ for SID and ∼220 m² g⁻¹ for SIDP. Experimental oxidation in a calcite suspension yielded goethite for SID and a mixture of lepidocrocite and goethite for SIDP. Two pot experiments in which a SID or SIDP suspension was applied to a calcareous soil at a rate of ∼2 g Fe kg⁻¹ showed nanosiderite to prevent iron chlorosis in chickpea. In a pot experiment with five successive crops, one initial application of ∼0.7 g Fe kg⁻¹ soil in the form of SID or SIDP was as effective as FeEDDHA in preventing Fe chlorosis. The residual effect of nanosiderite when applied to the first crop alone clearly exceeded that of FeEDDHA. CONCLUSION: Nanosiderite suspensions applied at rates of ∼0.7 g Fe kg⁻¹ soil were highly effective in preventing iron chlorosis and have a great residual effect.