Juscimar da Silva

The role of Al-Goethites on arsenate mobility

The geochemical fates of Fe and As are so closely correlated that methods of As removal from contaminated water are in general based on the high affinity of this metalloid for Fe (hydr)oxides. Dissimilatory Fe reducing bacteria, however, play a fundamental role in catalysing the redox transformations that ultimately control the mobility of As in anoxic environments. The potential of Al-goethites in adsorbing As(V) compared with hematite, goethite, ferrihydrite, and gibbsite, and the stability of As retained by the Fe compounds under anoxic conditions were investigated in this study. The (hydr)oxides were synthesised, and adsorption isotherms and As(V) adsorption maxima at different pH were measured. Arsenic loaded samples were anaerobically incubated in the presence of Shewanella putrefaciens, and periodically sampled to evaluate the contents of soluble As and Fe. The As(V) adsorption maxima decreased in the following order: Fh > AlGt(13) > AlGt(20) > AlGt(23) > Gb > Hm > Gt. In terms of surface area, Gb, Gt, and Hm showed higher As(V) loading capacity than Fh, suggesting available reactive sites not fully occupied by arsenate on Fh. The same hypothesis can be considered for Al-goethites, as they showed even lower arsenate loading capacity per surface area. The presence of structural Al in the goethites enhanced considerably the As uptake capacity and stability under reducing conditions. Therefore, the Al-goethites showed good potential as adsorbents to remove As from water. S. putrefaciens cells were able to utilise both noncrystalline and crystalline Fe (hydr)oxides as electron acceptors, releasing As into solution. Al-goethites showed a decrease in Fe and As mobilisation as structural Al increased.

Effects of Competing Anions and Iron Bioreduction on Arsenic Desorption

Dissimilatory iron-reducing bacteria play a fundamental role in catalysing the redox transformations that ultimately control the mobility of As in anoxic environments, a process also controlled by the presence of competing anions. In this study, we investigated the decoupling of As from loaded Al and Fe (hydr)oxides by competing anions in the presence of iron-reducing bacteria. Hematite, goethite, ferrihydrite, gibbsite and three aluminium-substituted goethites (AlGts) were synthesised and loaded with arsenate, followed by anaerobic incubation with different phosphate or carbonate-containing media in the presence of catalytic iron-reducing bacteria. Soluble Al, As, Fe and P contents were measured in aliquots by inductively coupled plasma optical emission spectrometry following periodical sampling. Shewanella putrefaciens cells were able to utilise both non-crystalline and crystalline Fe (hydr)oxides as electron acceptors, releasing Fe and As into solution. Phosphate and carbonate affected the Fe bioreduction, probably due to the precipitation of metastable mineral phases and also to phosphate-induced stabilisation on the hydroxide surfaces. Phosphate precipitation acted as a sink for As, thus limiting its mobilisation. The highest fraction of desorbed As by phosphate was observed for gibbsite, followed by AlGts. Similarly, gibbsite showed significant amounts of arsenate displaced by carbonate. In spite of its low crystallinity, ferrihydrite was the most efficient compound in retaining arsenate, possibly due to As co-precipitation. This study provides new insight into the management of As-contaminated soils and sediments containing Al-goethites and gibbsite, where the Fe activity may be too low to co-precipitate As-bearing vivianite. Thus, the dynamics of As(V) in flooded soils are significant in agriculture and environmental management.

A soil geochemical background for northeastern Brazil

Very few area-representative soil geochemical data exist for the southern hemisphere. A sub-continental scale (1.7 × 106 km2) geochemical sampling expedition in northeastern Brazil delivered 101 representative composite soil samples (30–50 cm depth) for non-anthropogenically influenced areas (mainly pasture land). Major, minor, and selected trace elements, determined by WD-XRF, are discussed with respect to lithology, soil type, biome type, climate and land use. These element concentrations vary up to two orders of magnitude, except for Si (factor ≅ 2.6). Silicon is strongly enriched compared to global averages, whereas most other components show a considerable deficiency. Significant deviations occur compared to results obtained from southern Brazil and from Australia – examples for the few representative data from the southern hemisphere. Anthropogenic influences appear negligible. All environmental parameters, except for land use, play an active role in shaping the geochemical composition. Lithology appears to be partly decoupled from the soils due to their age. The soil composition reflects soil type, biome type, and weathering influences. Most plant nutrients, despite their absolute depletion, show the highest values in Caatinga soils, and the lowest in Atlantic Forest soils. The new data form a robust and valuable tool to support future land use management.

Effects of different humic substances concentrations on root anatomy and Cd accumulation in seedlings of Avicennia germinans (black mangrove)

Mangrove areas are among most threatened tropical ecosystems worldwide. Among polluting agents Cadmium is often found in high concentrations in mangrove sediments. Humic substances, complex biomolecules formed in soil and sediments during animal and plant residuals decomposition, have a known biostimulant activity and can be adopted to counteract various plant stresses. This study explores, in controlled conditions, the effect of humic substances on Avicennia germinans seedlings, with or without cadmium contamination. Humic compounds significantly changed plant root architecture, and, when coupled with cadmium, root anatomy and Cortex to Vascular Cylinder diameter ratio. These modifications led to lower Cd uptake by humic substances-treated plants. Humic substances amendment could be effective, depending on their concentrations, on improving plant health in mangrove areas, for forest recuperation and/or dredged sediments phytoremediation purposes.

Determinação de minerais no solo e análise de folhas de couve produzida em Brasília

Collard is a food with a high content of vitamins and minerals, fundamental for the human diet. However, information on the influence of extrinsic and intrinsic factors in its composition during cultivation is still scarce. The objective of this work was to quantify the minerals in collard leaves produced in the Federal District (DF), Brazil: calcium, magnesium, potassium, phosphorus, zinc, manganese, sodium and iron, correlating with the present soil contents. Leaf and soil samples were collected in 16 different rural properties in the Federal District, in which questionnaires were also applied on the cultural practices adopted during plant growing. The average foliar contents per 100 g of fresh leaves were: calcium, 222.8 mg; iron, 1.3 mg; magnesium, 37.6 mg; and zinc, 0.6 mg. This composition meets 28% of daily calcium requirements in children aged from 4 to 8 years: 13% iron, 29% magnesium and 13% zinc, based on the consumption of 100 g of fresh collard leaves. Negative correlation was verified for magnesium contents in soil and leaf, with R = -0.69. A significant correlation was not verified for the other analyzed minerals. Phosphorus and potassium contents in soils were considered high while, contradictorily, low concentrations of magnesium were found. The foliar micronutrients contents were considered low and suggests for an inadequate management of the fertilization, factor corroborated by the answers of the cultural practices questionnaires. Management practices that lead to nutrient balance in soil reflect directly on soil-plant interaction. Up-to-date regional recommendations on soil fertilization and correction could assist farmers and technicians, directly reflecting the achievement of more balanced levels of micro and macro nutrients in vegetables.