Geochemical Composition of the Lomé Lagoon Sediments, Togo: Seasonal and Spatial Variations of Major, Trace and Rare Earth Element Concentrations

The concentrations of major, trace (TE), and rare earth (REE) elements and their seasonal and spatial distribution were studied on the fine fraction (<63 µm) of the sediments of the Lomé lagoons (West Lake, East Lake, and Lake Bè). The sediments were collected on a total of nine sampling sites (three per Lake) during two campaigns (dry season and rainy season). The quality of the sediments was assessed on the basis of the enrichment factor (EF) and the labile or non-residual fraction (NRF) in relation to the values recommended for the quality of the sediments (Sediment Quality Guidelines, SQG). The distribution of rare earth elements shows enrichments in light rare earths superior to those of heavy rare earth elements during any season. Positive Ce anomalies are less noticeable and less variable between seasons than Eu anomalies. La/Yb ratios are positively correlated with the percentage of Al and Fe oxides and with the percentage of fine fractions. The main bearing phases of rare earth elements are, therefore, Al and Fe oxides and the finest fractions of the sediments. The concentrations of trace elements vary little, according to the seasons, but show strong variations from one element to another. The degrees of enrichment obtained are moderate for Bi, Cr, Ga, Mo, Pb, Sn, and Zn (1.5 < EF < 5) to significant for As, Cd, and Sb (5 < EF < 20) for all sites of Lake Bè. For the sites of West Lake, the degrees of enrichment obtained are moderate for As, Cd, Cu, Mo, and Pb (1.5 < EF < 5) to a significance for As, Bi, Cd, Pb, Sb, Sn, and Zn (5 < EF < 20). Only the East Lake sites show high degrees of enrichment for elements such as Sb and Sn (20 < EF < 40). Trace elements (TE) such as As, Cd, Cu, and Ni have total concentrations within the range of variation of the SQG concentrations (particularly Probable Effect Level (PEL) and Effect Range Median (ERM)), whereas Cr, Pb, and Zn total concentrations are higher. The ranking of priority sites with respect to the sediment contamination is determined according to ERM and PEL quotients in relation to the probability of toxicity for benthic organisms. For almost all the sites, the priority is lowest to medium-low with regard to As, Cd, and Cu and medium-high (Cr and Ni) to highest (Pb and Zn), particularly for the East and West Lakes. Moreover, the NRF can represent significant percentages of the total TE concentrations: 5% to 15% for As, Bi, Ni, V, Mo, and Sc, 15% to 25% for Co, Cu, and Sr, 25% to 40% for Pb and Zn and, lastly, 47% to 55% for Cd.

Mineralisation of soil organic carbon in two Andisols under oil palm: an incubation study into controlling factors

Understanding the factors controlling stability against mineralisation of soil organic matter is important for predicting changes in carbon stocks under changed environment or management. Soil carbon dynamics in oil palm plantations are little studied and have some characteristics that are unusual compared with other agricultural soils, such as high management-induced spatial variability and warm moist conditions. The aim of this work was to determine the factors controlling the mineralisability of the intermediate-stability carbon fraction of volcanic ash surface soils (0–5 and 15–20 cm depth) from oil palm plantations in Papua New Guinea. Soils with carbon contents of 2.2–35.2%, from areas with low and high organic matter inputs, were incubated for up to 812 days and soil respiration was measured periodically. Mean carbon turnover rates were 0.18–1.58, 0.07–0.23 and 0.03–0.07 a–1 on Days 54, 379 and 812 respectively. Turnover rate was initially (Day 54) correlated with pre-incubation total carbon content (r = 0.88), the ratio of permanganate-oxidisable carbon to total carbon (r = 0.62) and the ratio of oxalate-extractable Al and Fe to total carbon (r = –0.51 and –0.54 respectively), but the correlations decreased with time, being insignificant on Day 812. In the soils that had changed from C4 grassland 25 years previously, turnover rate was negatively correlated with δ13C, which increased with depth, but δ13C did not change significantly over the course of the incubation. Temperature sensitivity of mineralisation varied little, despite large differences in soil properties and changes in mineralisation rate. This suggested that turnover rates were affected to similar extents by biochemical recalcitrance and physical protection, as these two factors influence temperature sensitivity in opposing directions. Physico-chemical protection of organic matter appeared largely related to interaction with poorly crystalline Al and Fe oxides.

Speciation of adsorbed arsenic(V) on red mud using a sequential extraction procedure

The distribution of sorbed arsenic(V) among different geochemical fractions for arsenic(V)-loaded red mud, an oxide-rich residue from bauxite refining that has been proposed as an adsorbent for arsenic, was studied as a function of sorbed arsenic(V) concentration using a sequential extraction procedure. The release of previously sorbed arsenic(V) was also studied as a function of pH and arsenic(V) concentration. Most sorbed arsenic(V) (0.39–7.86 mmol kg–1) was associated with amorphous and crystalline Al and Fe oxides (24.1–43.8% and 24.7–59.0% of total sorbed arsenic, respectively). Exchangeable arsenic was the smallest fraction (0.4–5.2% of total sorbed arsenic). The distribution of sorbed arsenic(V) was related to the arsenic surface coverage. For arsenic surface coverages >∼30% the percentage of arsenic(V) associated with the amorphous Al oxide fraction increased and that associated with the crystalline oxide fraction decreased. The arsenic(V) exchangeable fraction increased from 1.4 to 756 μmol kg–1 as surface coverage increased from 388 to 7855 μmol kg–1. The release of sorbed arsenic(V) from red mud was greater at alkaline pH values (maximum release of ∼33% of previously sorbed arsenic at pH = 12), but for high arsenic(V) initial concentration (0.2 mM arsenic) considerable amounts of arsenic (6.5% of previously sorbed arsenic) were released at pH 4, in accordance with the dissolution of amorphous Al oxides in the red mud. The results obtained suggest a greater mobility of sorbed arsenic(V) as its surface concentration approaches saturation.

Ti3AlC2 — A Soft Ceramic Exhibiting Low Friction Coefficient

The friction behavior of a high-purity bulk titanium aluminum carbide (Ti3AlC2) material dryly sliding against low carbon steel was investigated. Tests were performed using a block-on-disk type high-speed friction tester under sliding speed of 20 m/s and 60 m/s, several normal pressures from 0.1 to 0.8 MPa. The results showed that the friction coefficient is as low as about 0.18 for sliding speed of 20 m/s and only 0.1 for 60 m/s, and that almost not changes with the normal pressure. The reason could be related with the presence of a surface layer on the friction surface. The layer was analyzed to consist of Ti, Al and Fe oxides, which played a lubricate part inducing the friction coefficient decrease on the friction surface.

PHOSPHORUS REMOVAL FROM DOMESTIC WASTEWATER IN HORIZONTAL SUBSURFACE FLOW CONSTRUCTED WETLAND AFTER 8 YEARS OF OPERATION – A CASE STUDY

Horizontal subsurface flow constructed wetlands can effectively treat high levels of biochemical oxygen demand (BOD) and suspended solids. They are also effective as phosphorus trap but usually for a short time. This phenomenon was observed in the presented case study, an example of subsurface flow reed bed filled with “improved” site soil where it was assumed that the permeability of bed would increase as a result of reed penetration. Fine grained site soil was initially effective trap for phosphorus from wastewater. However, during operation clogging of bed media proceeded and phosphorus sorption capacity used up. In general, the longevity of subsurface flow wetlands as phosphorus sinks depends on the hydraulic load, phosphorus load and the type of the media used in bed construction. To be effective as phosphorus sorbent, substrate should contain high levels of Ca, Al and Fe oxides and possess suitable sorption capacity, quick time of reaction and suitable permeability.

Effect of ageing on the nature and interlayering of mixed hydroxy A1-Fe-montmorillonite complexes

Mixed hydroxy Al-Fe-montmorillonite complexes (5 mmol of Al+Fe per g of clay) were prepared at pH 5.0 by mixing different amounts of Al(NO3)3 and Fe(NO3)3 to give initial Fe/Al molar ratios (R) of 0, 0.1, 0.25, 0.5, 1.0, 4.0, 10.0 or ∞. The effect of ageing on the interlayering, mineralogy, chemical composition, CEC and external surface area of the complexes was studied. The degree of interlayering of freshly prepared and aged complexes (up to 120 days at 50°C) was always greater in the samples with R ranging from 0.5 to 4. Gibbsite and hematite (with traces of goethite) formed in the samples with R ⩽ 1 and R∞, respectively. No crystalline Fe oxide was detected by XRD analyses in the complexes with R ranging from 0.1 to 10, whereas no crystalline Al and Fe oxides were formed in the complexes at R = 4–10. However, some evidence suggests the possible formation of a microcrystalline Fe oxide/oxyhydroxide undetectable by XRD analyses in the complexes richer in Fe. The CEC and the surface area of the freshly prepared mixed hydroxy Al-Fe-montmorillonite complexes increased with increasing R. Large quantities of Al+Fe were solubilized by oxalate in the complexes with R ⩾ 0.5, even after ageing for 120 days at 50°C, indicating that interlayering of Al-Fe species at certain Fe/Al molar ratios was particularly stable.

Zinc buffer capacity of vertisols

The soil Zn buffer capacity is an important factor that regulates the concentration of plant-available Zn in soil solution. It is measured variously as Zn buffer power, Zn sorption or Zn desorption capacity. This study was conducted to determine Zn buffer power, and Zn sorption capacity and Zn desorption capacity in Vertisols as influenced by soil properties. The Zn buffer power, defined as the slope of the line relating soil solution Zn concentration to DTPA-extractable Zn, varied from 217 to 790. Soil pH was found to be the major soil parameter responsible for the variation in Zn buffer power. The sorption of Zn by these Vertisols was satisfactorily described by the Freundlich equation. The calculated values of the Freundlich parameters were closely related to the soil pH and amorphous Al and Fe content. Desorption of Zn by a series of successive extractions with DTPA was described by the Mitscherlich equation. The calculated values of desorption capacity were negatively correlated with soil pH and positively correlated with the contents of Al and Fe oxides. Work published elsewhere showed that the parameters of both the Zn buffer power and Zn desorption capacity accounted for as much as 62% of the variation in relative yield of wheat from Zn application to Vertisols.

Oxygen as an exchangeable ligand in soil

An attempt was made to clarify the adsorption of oxygen ligands onto the Al- and Fe-oxides in the soil. In the ligand exchange the central ions Al3+ and Fe3+ of the oxides are Lewis acids or electron acceptors and the ligands are Lewis bases or electron donors. The binding of ligand oxygen by the central ion is primarily dependent on the nucleophilic strength of the ligand oxygen. The proton is a hard Lewis acid as are Al3+ and Fe3+. The pKa-value of the acid corresponding to the anion shows the ability of ligand oxygen to bind a proton and also the ability to bind Al3+ and Fe3+. The greater the nucleophilic strength of the ligand, the more covalent the nature of the bond between ligand oxygen and the central ion. However, the covalent character of this bond does not make the adsorption of a ligand by oxides an exothermic reaction. The entropy changes in adsorption determine the exchange equilibrium of the anions. The nucleophilic strength of ligand oxygen also determines the rate of exchange. If an anion has a high nucleophilic strength, it is rapidly adsorbed by oxides but the desorption is slow. The theory explains the relationship between the adsorption of anions to oxides and the pH-value in the equilibrum solution. The theory also illuminates the factors, which determine the leaching of an anion from the soil and the uptake rate of an anion from the soil by plants.