Adsorption of hexavalent chromium onto activated carbon derived from <Emphasis Type="Italic">Leucaena leucocephala</Emphasis> waste sawdust: kinetics,equilibrium and thermodynamics

The adsorptive removal of Cr(VI) was studied using activated carbon derived from Leucaena leucocephala (ACLL). The physico-chemical properties of ACLL were determined using proximate analysis and N₂ BET surface area analysis. The N₂ BET surface area of ACLL was determined to be 1131 m² g^?1. The point of zero charge (pH_pzc) of 5.42 indicated that ACLL surface was positively charged for pH below the pH_PZC, attracting anions. The effect of experimental operating parameters such as time of contact, ACLL dose, pH, initial concentration and temperature was investigated. The optimum values of parameters such as concentration of 100 mg L^?1, 300 mg of ACLL dose, time of contact of 60 min, pH of 4 indicated the maximum Cr(VI) uptake of 13.85 mg g^?1. The pseudo-second-order kinetic model best fitted with the Cr(VI) adsorption data. Adsorptive removal of Cr(VI) onto ACLL satisfactorily fitted in the order of Redlich–Peterson > Freundlich > Langmuir > Temkin adsorption isotherm model. The thermodynamic parameters showed the adsorption of Cr(VI) onto ACLL was an endothermic and spontaneously occurred process.     

Relating millimeter-scale turbulence to meter-scale subtidal erosion and accretion across the fringe of a coastal mangrove forest

Within a wave-exposed mangrove forest, novel field observations are presented, comparing millimeter-scale turbulent water velocity fluctuations with contemporaneous subtidal bed elevation changes. High-resolution velocity and bed level measurements were collected from the unvegetated mudflat, at the mangrove forest fringe, and within the forest interior over multiple tidal cycles (flood–ebb) during a 2-week period. Measurements demonstrated that the spatial variability in vegetation density is a control on sediment transport at sub-meter scales. Scour around single and dense clusters of pneumatophores was predicted by a standard hydraulic engineering equation for wave-induced scour around regular cylinders, when the cylinder diameter in the equations was replaced with the representative diameter of the dense pneumatophore clusters. Waves were dissipated as they propagated into the forest, but dissipation at infragravity periods (> 30 s) was observed to be less than dissipation at shorter periods (< 30 s), consistent with the predictions of a simple model. Cross-wavelet analysis revealed that infragravity-frequency fluctuations in the bed level were occasionally coherent with velocity, possibly indicating scour upstream of dense pneumatophore patches when infragravity waves reinforced tidal currents. Consequently, infragravity waves were a likely driver of sediment transport within the mangrove forest. Near-bed turbulent kinetic energy, estimated from the turbulent dissipation rate, was also correlated with bed level changes. Specifically, within the mangrove forest and over the unvegetated mudflat, high-energy events were associated with erosion or near-zero bed level change, whereas low-energy events were associated with accretion. In contrast, no single relationship between bed level changes and mean current velocity was applicable across both vegetated and unvegetated regions. These observations support the theory that sediment mobilization scales with turbulent energy, rather than mean velocity, a distinction that becomes important when vegetation controls the development of turbulence.     

Identification of buried sinkholes using refraction tomography at Ft. Campbell Army Airfield,Kentucky

Karst aquifers are highly susceptible to contamination, with numerous points of entry for contaminants through recharge features such as sinkholes, swallow holes and solutionally enlarged fractures. These recharge features may be filled or obscured at the surface, requiring the use of geophysical or remote sensing techniques for their identification. This study uses seismic refraction data collected at the Ft. Campbell Army Airfield (CAAF), Kentucky, USA, to test the hypothesis that refraction tomography is a useful tool for imaging bedrock depressions beneath thick overburden (greater than 20 m of unconsolidated sediment). Southeast of the main taxiway of CAAF seismic velocity tomograms imaged a bedrock low, possibly a closed depression, at a depth of 25 m that had been earlier identified through delay-time analysis of the same refraction data. Tomography suggests the bedrock low is about 250-m wide by 10-m deep at its widest point. High rates of contaminant vapor extraction over the western extension of this feature suggest a high concentration of contaminants above, and within, this filled bedrock low, the base of which may contain solutionally enlarged fractures (i.e. karst conduits) that could funnel these contaminants to the upper or lower bedrock aquifers. This study thus demonstrates the viability of seismic refraction tomography as a tool for identification of filled sinkholes and bedrock depressions in karst areas.     

Applicability of the Dual‐Domain Model to Nonaggregated Porous Media

More theoretical analysis is needed to investigate why a dual‐domain model often works better than the classical advection‐dispersion (AD) model in reproducing observed breakthrough curves for relatively homogeneous porous media, which do not contain distinct dual domains. Pore‐scale numerical experiments presented here reveal that hydrodynamics create preferential flow paths that occupy a small part of the domain but where most of the flow takes place. This creates a flow‐dependent configuration, where the total domain consists of a mobile and an immobile domain. Mass transfer limitations may result in nonequilibrium, or significant differences in concentration, between the apparent mobile and immobile zones. When the advection timescale is smaller than the diffusion timescale, the dual‐domain mass transfer (DDMT) model better captures the tailing in the breakthrough curve. Moreover, the model parameters (mobile porosity, mean solute velocity, dispersivity, and mass transfer coefficient) demonstrate nonlinear dependency on mean fluid velocity. The studied case also shows that when the Peclet number, Pe, is large enough, the mobile porosity approaches a constant, and the mass transfer coefficient can be approximated as proportional to mean fluid velocity. Based on detailed analysis at the pore scale, this paper provides a physical explanation why these model parameters vary in certain ways with Pe. In addition, to improve prediction in practical applications, we recommend conducting experiments for parameterization of the DDMT model at a velocity close to that of the relevant field sites, or over a range of velocities that may allow a better parameterization.     

Cloud drop spectra at different levels and with respect to cloud thickness and rain

Measurements on drop size were made in cumulus clouds over Pune (inland region) during the summer monsoon seasons. In this paper, the measurements of the cloud drop spectra made in non-raining clouds at different levels and for different thickness have been studied. Also, those on the days with rain and with no rain (the rain being observed within the clouds) have been compared. The average spectra broadened with height. The concentration of drops >50 μm (N_L), liquid water content (LWC), mean volume diameter (MVD) and dispersion increased with height. The concentration of drops <20 μm (N_S) and total concentration (N_T) decreased with height. The spectra were broader, while N_S and N_T are smaller and the other parameters are greater for thicker clouds as compared to those for thinner. The spectra were broader, while N_S and N_T are smaller and the other parameters are greater on the days with rain with respect to those on the days with no rain. The distributions were bimodal at higher levels, for thicker clouds and on the days with rain, while they were unimodal at lower levels, for thinner clouds and on the days with no rain. The variations of the cloud drop spectra, preceding rain, at initial stage of rain and following rain are discussed.     

Comments on Placenticeras mintoi (Vredenburg, 1906) from the Bagh Beds (Late Cretaceous), central India with special reference to Turonian Nodular Limestone Horizon

The Late Cretaceous (Cenomanian to Coniacian) marine sediments of central India prevalently known as ‘Bagh Beds,’ have been deposited in the E-W extending Narmada Basin. The stratigraphy of these Cenomanian — Coniacian sediments has been reviewed and summarized. The Bagh Beds have been found to consist of three formations: Nimar Sandstone, Nodular Limestone and Corallian Limestone in ascending order. Main emphasis has been given to Nodular Limestone Formation (Turonian), which is the most fossiliferous horizon of the Bagh Beds. Nodular Limestone Formation has more or less alternating bands of varying thickness of nodular limestone and marl. It yielded numerous ammonoid specimens, which have been found to belong to a morphologically highly variable ammoniod taxon Placenticeras mintoi Vredenburg.     

Space charge sheath in plasma-neutral gas interaction

A space charge sheath is found to be formed whenever a high-velocity magnetized plasma stream penetrates a gas cloud. The sheath is always located at the head of the plasma stream, and its thickness is very small compared to the length of the plasma stream. Soon after the sheath is formed it quickly slows down to the Alfvén critical velocity. The plasma behind the sheath continues to move at higher velocity until the whole plasma stream is retarded to the critical velocity. In the interaction at gas density 10¹⁹ m^–3 the sheaths are observed to be accompanied by a single loop of current with current density of 10⁵ Å m^–2. Maximum potential in the sheath ranges between 50 and 200 V.Presently available models for the sheath may explain the initiation of the sheath formation. Physical processes like heating of the electrons and ionization of the gas cloud which come into play at a later stage of the interaction are not included in these models. These processes considerably alter the potential structure in the sheath region. A schematic model of the observed sheath is presented here.Experiments reveal a threshold value of the magnetic field for plasma retardation to occur. This seems to correspond to the threshold condition for excitation of the modified two-stream instability which can lead to the electron heating. The observed current are found sufficient to account for the plasma retardation at a gas density of 10¹⁷ m^–3.