Oxidation of a Dimethoxyhydroquinone by Ferrihydrite and Goethite Nanoparticles : Iron Reduction versus Surface Catalysis

Hydroquinones are important mediators of electron transfer reactions in soils with a capability to reduce Fe(III) minerals and molecular oxygen, and thereby generating Fenton chemistry reagents. This study focused on 2,6-dimethoxy hydroquinone (2,6-DMHQ), an analogue to a common fungal metabolite, and its reaction with ferrihydrite and goethite under variable pH and oxygen concentrations. Combined wet-chemical and spectroscopic analyses showed that both minerals effectively oxidized 2,6-DMHQ in the presence of oxygen. Under anaerobic conditions the first-order oxidation rate constants decreased by one to several orders of magnitude depending on pH and mineral. Comparison between aerobic and anaerobic results showed that ferrihydrite promoted 2,6-DMHQ oxidation both via reductive dissolution and heterogeneous catalysis while goethite mainly caused catalytic oxidation. These results were in agreement with changes in the reduction potential (EH) of the Fe(III) oxide/Fe(II)aq redox couple as a function of dissolved Fe(II) where EH of goethite was lower than ferrihydrite at any given Fe(II) concentration, which makes ferrihydrite more prone to reductive dissolution by the 2,6-DMBQ/2,6-DMHQ redox couple. This study showed that reactions between hydroquinones and iron oxides could produce favorable conditions for formation of reactive oxygen species, which are required for nonenzymatic Fenton-based decomposition of soil organic matter

Determination of dust deposition and associated nutrients in natural forest and plantation - a case study from the moist semi-deciduous forest zone in Ghana.

Every year in December to February the dust laden Harmattan blows from Sahara towards southwest to the countries along the east west going coast of the Gulf of Guinea. Only few investigations have quantified external dust deposition in forests and plantations in that region and hereby the nutrient input by dust. This paper investigates three different methods for sampling and quantifying dust deposition in natural forests and plantations in the moist semi-deciduous forest zone in Ghana. The three methods tested were the following: bowls with water, plastic mats with straws and funnels for determining canopy drip. The study shows that the canopy drip method overestimates the external dust deposition due to a significant enrichment of organic matter during the throughfall. Therefore, it is necessary to combine the canopy drip method with sampling from outside the forest to determine the external organic matter content in the dust deposited to a forest. The study showed that the dust deposition in the forest and in the rubber and cocoa plantations was 1.5 to 2.0 times bigger than on the plastic mats located outside the forested area and further, that the external nutrient input by dust to the natural forests is very low in the Kade area, only about 0.2 kg ha-1 total P and the readily available Ca, Mg and K are respectively 0.4 kg ha-1, 0.2 kg ha-1 and 0.7 kg ha-1

Deposition of nutrients from Harmattan dust in Ghana, West Africa

In order to measure dust's nutrient input on farmland in different agro-ecological zones, Harmattan dust was sampled by mats with plastic straw in Ghana between 2002-2006. The inputs of total nutrients by Harmattan dust in Ghana per Harmattan period were about 1-2 kg Ca ha-1, 0.5-2 kg K ha-1, 0.5-1.5 kg Mg ha-1 and less than 0.5 kg P ha-1. Compared with the annual input of nutrients by precipitation, the dust accounted for 10% or less of Ca, Mg and K but approximately 20%-40% of P. The input of nutrients by dust was only valid for areas with vegetation, because in areas with none or sparse vegetation, loss of soil due to wind erosion and hereby loss of nutrients might be significant. In farmland areas with bare and vegetated fields there seemed to be an internal redistribution of the nutrients and not a net gain of nutrients from outside the area (long-range transported dust). The input of P by dust might be of some importance in the traditional shifting cultivation systems, while the inputs of other three nutrients of Ca, Mg and K were so low that they must be considered insignificant. In the intensive agriculture systems with huge inputs of manures and fertilizers the nutrient input by dust is insignificant and could be neglected

Sediment and nutrient deposition in Lake Volta in Ghana due to Harmattan dust

Harmattan is a dust-laden north-easterly wind that blows from the Sahara towards the Gulf of Guinea in the period November to March. It is the dominant wind in the north of Ghana while at the coast in the south it only occurs sporadically and here westerly or south-westerly winds dominate. Some of the dust is trapped in the vegetation, in lakes and other inland waters, and a little on the bare land. In this study, we determine the amount of sediment and nutrients that are deposited by this wind in big water bodies, exemplified by the dust deposition in the Harmattan period in the man-made Lake Volta. These depositions are compared with similar inputs by the rivers. On average, approximately 146,000. t of mineral matter and 42,000. t of organic matter are deposited per year in the Lake during the Harmattan periods. This is the equivalent of approximately 1% of the suspended sediment input by the rivers. The total amounts of Ca, P and Mg deposited during the Harmattan period are3000. t, 350. t and 810. t, respectively. About 40% of the Ca deposited is readily or plant available, for P and Mg it is about 50% and 20%, respectively. If the amount of readily available nutrients coming from the Harmattan dust is held against the influx of readily available (dissolved) Ca, Mg and P from the rivers to Lake Volta, it appears that the Harmattan dust accounts for only 0.7‰ Mg, 4.8‰ Ca, and 2.1% P. It must therefore be concluded that the contribution of nutrients to Lake Volta by the Harmattan dust is very limited

A Review of Phosphorus Removal Structures: How to Assess and Compare Their Performance

Controlling dissolved phosphorus (P) losses to surface waters is challenging as most conservation practices are only effective at preventing particulate P losses. As a result, P removal structures were developed to filter dissolved P from drainage water before reaching a water body. While many P removal structures with different P sorption materials (PSMs) have been constructed over the past two decades, there remains a need to evaluate their performances and compare on a normalized basis. The purpose of this review was to compile performance data of pilot and field-scale P removal structures and present techniques for normalization and comparison. Over 40 studies were normalized by expressing cumulative P removal as a function of cumulative P loading to the contained PSM. Results were further analyzed as a function of retention time (RT), inflow P concentration, and type of PSM. Structures treating wastewater were generally more efficient than non-point drainage water due to higher RT and inflow P concentrations. For Ca-rich PSMs, including slag, increased RT allowed for greater P removal. Among structures with low RT and inflow P concentrations common to non-point drainage, Fe-based materials had an overall higher cumulative removal efficiency compared to non-slag and slag materials.201

Generation of hydroxyl radicals from reactions between a dimethoxyhydroquinone and iron oxide nanoparticles

The hydroxyl radical (·OH) is a powerful oxidant that is produced in a wide range of environments via the Fenton reaction (Fe2+ + H2O2 → Fe3+ + ·OH + OH-). The reactants are formed from the reduction of Fe3+ and O2, which may be promoted by organic reductants, such as hydroquinones. The aim of this study was to investigate the extent of ·OH formation in reactions between 2,6-dimethoxyhydroquinone (2,6-DMHQ) and iron oxide nanoparticles. We further compared the reactivities of ferrihydrite and goethite and investigated the effects of the O2 concentration and pH on the generation of ·OH. The main finding was that the reactions between 2,6-DMHQ and iron oxide nanoparticles generated substantial amounts of ·OH under certain conditions via parallel reductive dissolution and catalytic oxidation reactions. The presence of O2 was essential for the catalytic oxidation of 2,6-DMHQ and the generation of H2O2. Moreover, the higher reduction potential of ferrihydrite relative to that of goethite made the former species more susceptible to reductive dissolution, which favored the production of ·OH. The results highlighted the effects of surface charge and ligand competition on the 2,6-DMHQ oxidation processes and showed that the co-adsorption of anions can promote the generation of ·OH