Kinetics of batch microbial degradation of phenols by indigenous Pseudomonas fluorescence

The potential of various organisms to metabolize organic compounds has been observed to be a potentially effective means in disposing of hazardous and toxic wastes. Phenols and their compounds have long been recognized as one of the most recalcitrant and persistent organic chemicals in the environment. The bioremediation potential of an indigenous Pseudomonas fluorescence was studied in batch culture using synthetic phenol in water in the concentration range of (100–500) mg/L as a model limiting substrate. The effect of initial phenol concentration on the degradation process was investigated. Phenol was completely degraded at different cultivation times for the different initial phenol concentrations. Increasing the initial phenol concentration from 100 mg/L to 500 mg/L increased the lag phase from 0 to 66 h and correspondingly prolonged the degradation process from 84 h to 354 h. There was decrease in biodegradation rate as initial phenol concentration increased. Fitting data into Monod kinetic model showed the inhibition effect of phenol The kinetic parameters have been estimated up to initial phenol concentration of 500 mg/ L. The r_smax decreased and K_s increased with higher concentration of phenol. The r_smaxhas been found to be a strong function of initial phenol concentration. The culture followed substrate inhibition kinetics and the specific phenol consumption rates were fitted to Haldane, Yano and Koga, Aiba et al., Teissier and Webb models. Between the five inhibition models, the Haldane model was found to give the best fit. Therefore, the biokinetic constants estimated using these models showed good potential of the Pseudomonas fluorescence and the possibility of using it in bioremediation of phenol waste effluents.     

Microbiological degradation of phenol using two co-aggregating bacterial strains

Phenol biodegradation in an aerobic batch reactor was investigated using mixed two co-aggregating strains (Flavobacterium sp. and Acetobacter sp.). Response surface methodology by the Box–Behnken model was used to evaluate the optimal cell growth and phenol degradation conditions. The optimum temperature, pH value and inoculum size were found to be 33 °C, 6.06 and 13 %, respectively. In the conditions, phenol degradation rate and biomass were predicted to be 96.97 % and 410.78 mg/L within the range examined, respectively. Less toxic acetaldehyde, ethanol and acetic ether were identified as main intermediate products from the degraded samples using GC–MS. Substrate inhibition was calculated from experimental biomass growth and phenol degradation parameters using the Haldane equation. Kinetic parameters derived from nonlinear regression with correlation factors (R ²) were 0.9682 for phenol degradation and 0.9594 for biomass growth, respectively. The phenol concentration to avoid substrate inhibition was 278.17 mg/L.     

Effects of freshwater Synechococcus sp. cyanobacteria pH buffering on CaCO3 precipitation: Implications for CO2 sequestration

In the present study, a mixed-flow steady-state bio-reactor was designed to biomineralize CO₂ as a consequence of photosynthesis from active Synechococcus sp. Dissolved CO₂, generated by constant air bubbling of inorganic and cyanobacteria stock solutions, was the only source of inorganic carbon. The release of hydroxide ion by cyanobacteria from photosynthesis maintained highly alkaline pH conditions. In the presence of Ca²⁺ and carbonate species, this led to calcite supersaturation under steady state conditions. Ca²⁺ remained constant throughout the experiments showing the presence of steady state conditions. Similarly, the Synechococcus sp. biomass concentration remained stable within uncertainty. A gradual pH decrease was observed for the highest Ca²⁺ condition coinciding with the formation of CaCO₃. The high degree of supersaturation, under steady-state conditions, contributed to the stabilization of calcite and maintained a constant driving force for the mineral nucleation and growth. For the highest Ca²⁺ condition a fast crystal growth rate was consistent with rapid calcite precipitation as suggested further by affinity calculations. Although saturation state based kinetic precipitation models cannot accurately reflect the controls on crystal growth kinetics or reliably predict growth mechanisms, the relatively reaction orders obtained from modeling of calcite precipitation rates as function of decreasing carbonate concentration suggest that the precipitation occurred via surface-controlled rate determining reactions. These high reaction orders support in addition the hypothesis that crystal growth proceeded through complex surface controlled mechanisms. In conclusion, the steady state supersaturated conditions generated by a constant cyanobacteria biomass and metabolic activity strongly suggest that these microorganisms could be used for the development of efficient CO₂ sequestration methods in a controlled large-scale environment.     

The solubility of rhodochrosite (MnCO3) and siderite (FeCO3) in anaerobic aquatic environments

Natural groundwaters are often reported to be highly supersaturated with the carbonate minerals siderite (FeCO₃) and rhodochrosite (MnCO₃). The kinetics of precipitation and dissolution were determined in the light of new determinations of the solubility products of siderite and rhodochrosite. Laboratory experiments showed that the precipitation kinetics of siderite and rhodochrosite were much slower than that of calcite, and also much slower than their dissolution kinetics. Experiments with supersaturated solutions failed to reach steady state within 474 days in the case of siderite, whereas steady state for rhodochrosite was reached after 140 days. Suspensions of siderite and rhodochrosite crystals reached steady state after 10 and 80 days, respectively. The solubility product of siderite (−log K_S0(FeCO₃)) was 11.03 ± 0.10 for dried crystals and 10.43 ± 0.15 for wet crystals. For rhodochrosite the solubility product (−log K_S0(MnCO₃)) was 11.39 ± 0.14 for dried crystals and 12.51 ± 0.07 for wet crystals. The solubility product determined from supersaturated solutions was −log K_S0(MnCO₃)=11.65 ± 0.14. The observed slow precipitation kinetics of siderite and rhodochrosite might explain the apparent supersaturation that is often reported for anaerobic aquatic environments.     

Dissolution of illite in saline-acidic solutions at 25 °C

The dissolution rate of illite, a common clay mineral in Australian soils, was studied in saline-acidic solutions under far from equilibrium conditions. The clay fraction of Na-saturated Silver Hill illite (K_1.38Na_0.05)(Al_2.87Mg_0.46Fe³⁺_0.39Fe²⁺_0.28Ti_0.07)[Si_7.02Al_0.98]O₂₀(OH)₄ was used for this study. The dissolution rates were measured using flow-through reactors at 25 ± 1 °C, solution pH range of 1.0-4.25 (H₂SO₄) and at two ionic strengths (0.01 and 0.25 M) maintained using NaCl solution. Illite dissolution rates were calculated from the steady state release rates of Al and Si. The dissolution stoichiometry was determined from Al/Si, K/Si, Mg/Si and Fe/Si ratios. The release rates of cations were highly incongruent during the initial stage of experiments, with a preferential release of Al and K over Si in majority of the experiments. An Al/Si ratio >1 was observed at pH 2 and 3 while a ratio close to the stoichiometric composition was observed at pH 1 and 4 at the higher ionic strength. A relatively higher K⁺ release rate was observed at I = 0.25 in 2-4 pH range than at I = 0.01, possibly due to ion exchange reaction between Na⁺ from the solution and K⁺ from interlayer sites of illite. The steady state release rates of K, Fe and Mg were higher than Si over the entire pH range investigated in the study. From the point of view of the dominant structural cations (Si and Al), stoichiometric dissolution of illite occurred at pH 1-4 in the higher ionic strength experiments and at pH ?3 for the lower ionic strength experiments. The experiment at pH 4.25 and at the lower ionic strength exhibited lower R_Al (dissolution rate calculated from steady state Al release) than R_Si (dissolution rate calculated from steady state Si release), possibly due to the adsorption of dissolved Al as the output solutions were undersaturated with respect to gibbsite. The dissolution of illite appears to proceed with the removal of interlayer K followed by the dissolution of octahedral cations (Fe, Mg and Al), the dissolution of Si is the limiting step in the illite dissolution process. A dissolution rate law showing the dependence of illite dissolution rate on proton concentration in the acid-sulfate solutions was derived from the steady state dissolution rates and can be used in predicting the impact of illite dissolution in saline acid-sulfate environments. The fractional reaction orders of 0.32 (I = 0.25) and 0.36 (I = 0.01) obtained in the study for illite dissolution are similar to the values reported for smectite. The dissolution rate of illite is mainly controlled by solution pH and no effect of ionic strength was observed on the dissolution rates.     

Continuous air rate measurement in flotation cells: Some fundamental considerations

A robust and reliable sensor to measure gas (air) superficial velocity (J_g) continuously in flotation systems is introduced. It is based on the sampling of bubbles by buoyancy with the accumulating air allowed to exit through an orifice. At steady state, pressure drop is measured and related to the J_g by prior calibration. The continuous device is readily automated and extended to a multi-unit set-up. The sensor and data collections are described. The governing gas flow equation and models of the dynamic response to air flow rate set point change and fluctuations in froth depth are derived. Model predictions are confirmed against plant data. By taking a moving average, the J_g remains valid in the face of typical plant disturbances.     

Criteria of Acceptance for Constant Rate of Strain Consolidation Test for Tropical Cohesive Soil

In this study, the rapid consolidation equipment (RACE) was developed as an alternative device to the conventional consolidation test using Oedometer, consuming merely a few hours for the whole precedure to determine the consolidation characteristics of cohesive soil. RACE operates based on the constant rate of strain (CRS) consolidation theory, which is a continuous loading method of testing, requiring a good estimation of the loading rate such that it is ideal for the achievement of steady state condition during testing. The steady state condition is achieved when the c _v values from drained and undrained face of CRS converged with the c_v from Oedometer test. A slightly modification has been made on the normal constant rate of strain (CRS) test by proposing a direct back pressure system to the specimen using a tube to saturate the soil sample. This research has produced a set of criteria for determining the suitable rate for the rapid consolidation test based on the ratio of normalized strain rate, β, and proposed a new coefficient in terms of a ratio of β to clay fraction (CF), as a part of new criteria for testing a fine soil. Four types of sample were tested with different rates of strain using the RACE and their results were compared with those conducted using the Oedometer on the same soil type, from which fairly good agreements were evident in many specimens. It was found from the study that the minimum value of normalized strain rate, β, for the CRS test is 0.005 and for the u _a /σ _v ratio is suggested as 0.01. Also, the maximum β/CF for soils with clay friction lower and higher than 50 % are 0.008 and 0.001, respectively. The minimum β/CF value for both conditions is 0.0001.     

Morphological evolution of dissolving feldspar particles with anisotropic surface kinetics and implications for dissolution rate normalization and grain size dependence: A kinetic modeling study

With previous two-dimensional (2D) simulations based on surface-specific feldspar dissolution succeeding in relating the macroscopic feldspar kinetics to the molecular-scale surface reactions of Si and Al atoms ( [Zhang and Lüttge, 2008] and [Zhang and Lüttge, 2009]), we extended our modeling effort to three-dimensional (3D) feldspar particle dissolution simulations. Bearing on the same theoretical basis, the 3D feldspar particle dissolution simulations have verified the anisotropic surface kinetics observed in the 2D surface-specific simulations. The combined effect of saturation state, pH, and temperature on the surface kinetics anisotropy has been subsequently evaluated, found offering diverse options for morphological evolution of dissolving feldspar nanoparticles with varying grain sizes and starting shapes. Among the three primary faces on the simulated feldspar surface, the (1 0 0) face has the biggest dissolution rate across an extensively wide saturation state range and thus acquires a higher percentage of the surface area upon dissolution. The slowest dissolution occurs to either (0 0 1) or (0 1 0) faces depending on the bond energies of Si-(O)-Si (Φ_Si-O-Si/kT) and Al-(O)-Si (Φ_Al-O-Si/kT). When the ratio of Φ_Si-O-Si/kT to Φ_Al-O-Si/kT changes from 6:3 to 7:5, the dissolution rates of three primary faces change from the trend of (1 0 0) > (0 1 0) > (0 0 1) to the trend of (1 0 0) > (0 0 1) > (0 1 0). The rate difference between faces becomes more distinct and accordingly edge rounding becomes more significant. Feldspar nanoparticles also experience an increasing degree of edge rounding from far-from-equilibrium to close-to-equilibrium. Furthermore, we assessed the connection between the continuous morphological modification and the variation in the bulk dissolution rate during the dissolution of a single feldspar particle. Different normalization treatments equivalent to the commonly used mass, cube assumption, sphere assumption, geometric surface area, and reactive surface area normalizations have been used to normalize the bulk dissolution rate. For each of the treatments, time consistence and grain size dependence of the normalized dissolution rate have been evaluated and the results revealed significant dependences on the magnitude of surface kinetic anisotropy under differing environmental conditions. In general, the normalized dissolution rates are strongly dependent on grain size. Time-consistent normalization treatment varies with the investigated condition. The modeling results suggest that the sphere-, cube-, and BET-normalized dissolution rates are appropriate under the far-from-equilibrium conditions at low pH where these normalizations are time-consistent and are slightly dependent on grain size.     

Effect of the dilution process on the attenuation of contaminants in aquifers

This paper discusses the effects of the dilution process on levels of aquifer contamination attenuation, developing analytical equations for application in theoretical and practical cases. The key finding is that, in aquifers, the dilution process causes a reduction of the contaminant concentration. Analytical equations for the assessment of the contaminant attenuation capacity, by means of the dilution process in an aquifer, are put forward. In particular, the attenuation of the contaminant concentration in an aquifer, and thus the dilution, is described by the volumetric flow rate per unit width perpendicular to the flow direction (q _u). Moreover, a classification of q _u was developed, considering six classes of attenuation capacity due to dilution. For the equations validation, nitrate contamination attenuation due to dilution was analysed in two sample areas in Piedmont (North-West Italy). The results confirmed the hypothesis, evidencing high nitrate concentrations in areas with low q _u values, connected to low dilution rate of the contaminant in the groundwater; vice versa, low nitrate concentrations were evidenced in areas with high q _u values, due to a high nitrate dilution rate.     

Biosorption of ammoniacal nitrogen from aqueous solutions with low-cost biomaterials: kinetics and optimization of contact time

The biosorption of ammoniacal nitrogen (N-NH₄ ⁺) from aqueous solutions by dead biomass of brown seaweed Cystoseira indica and Jatropha oil cake (JOC), which is generated in the process of biodiesel recovery from its seeds, was studied under diverse experimental conditions. The N-NH₄ ⁺ biosorption was strictly pH dependent, and maximum uptake capacity of C. indica (15.21 mg/g) and JOC (13.59 mg/g) was observed at initial pH 7 and 3, respectively. For each biosorbent–N-NH₄ ⁺ system, kinetic models were applied to the experimental data to examine the mechanisms of sorption and potential rate-controlling steps. The generalized rate model and pseudo-second-order kinetic models described the biosorption kinetics accurately, and the sorption process was found to be controlled by pore and surface diffusion for these biosorbents. Results of four-stage batch biosorber design analysis revealed that the required time for the 99 % efficiency removal of 40 mg/L N-NH₄ ⁺ from 500 L of aqueous solution were 76 and 96 min for C. indica and JOC, respectively. The Fourier transform infrared spectroscopy analysis before and after biosorption of ammonium onto C. indica and JOC revealed involvement of carboxylic and hydroxyl functional groups.     

Adsorption characteristics of phenol and heavy metals on biochar from <Emphasis Type="Italic">Hizikia fusiformis</Emphasis>

The objective of the present study is to evaluate the absorption efficacy of H. fusiformis biochar (HFB) for the removal of phenol and heavy metals from single and mixed solute systems of these species under different experimental conditions. The effects of contact time, pH change, initial phenol concentration, and heavy metal concentration on the adsorption capacity of HFB were investigated. The kinetics and equilibrium models of sorption of the components of the single and mixed solute systems on HFB were also studied. The experimental data were fitted to kinetic and equilibrium models. The batch experiments revealed that 360 min of contact time was sufficient to achieve equilibrium for the adsorption of both phenol and heavy metals. The adsorption of phenol and nickel by HFB followed the pseudo-second-order kinetic model, which was quite adequate for describing the adsorption mechanism. The equilibrium data for the adsorption of phenol and heavy metals fit well to the Langmuir model with regression coefficients of R ² > 0.819. The maximum Langmuir adsorption capacities were 10.39, 12.13, 22.25, 2.24, 2.89, and 22.03 mg/g for phenol, Ni²⁺, Zn²⁺, Cu²⁺, Pb²⁺, and Cd²⁺, respectively. Moreover, HFB exhibited optimal sorption under slightly acidic conditions at pH 6. The HFB used in the present study exhibited higher adsorption capacity for the removal of phenol and heavy metals from aqueous solutions compared to documented sorbents. These results demonstrate that HFB is potentially useful for alleviating the harmful effects of phenol and heavy metal in wastewater treatment systems.     

Isolation of hydrocarbonoclastic bacteria from bilge oil contaminated water

Two bacterial strains, i.e. Pseudomonas mendocina and Ochrobactrum sp. were isolated from bilge oil contaminated water of Mormugao harbour, Goa, India and grown in a culture medium with hexadecane as the sole carbon source. Pseudomonas mendocina was used in further studies as it was the dominant strain. This strain effectively degraded tetradecane, hexadecane and octadecane leaving a residual concentration of about 73 %, 54 % and 40 % respectively in 120 h. Sequence analysis of the dominant bands from the denaturing gradient gel electrophoresis profiles revealed the differences between the genera of bilge oil contaminated sea water and its enrichment culture on hexadecane indicating a shift in community structure based on the type of substrate available. Pseudomonas mendocina amplified for the following catabolic genes namely C23O, nid and ndo. Based on the catabolic gene study the potential of the bacterial strain isolated, i.e. Pseudomonas mendocina seems to be interesting as it will be able to degrade polyaromatic hydrocarbons as well. Physicochemical properties of Pseudomonas mendocina indicates production of exopolysaccharides based on the value of its isoelectric point.     

Dynamics of soil organic carbon following land-use change: insights from stable C-isotope analysis in black soil of Northeast China

Intensive soil tillage is a significant factor in soil organic matter decline in cultivated soils. Both cultivation abandonment and foregoing tillage have been encouraged in the past 30 years to reduce greenhouse gas emissions and soil erosion. However, the dynamic processes of soil organic carbon (SOC) in areas of either continuous cultivation or abandonment remain unclear and inconsistent. Our aims were to assess and model the dynamic processes of SOC under continuous tillage and after cultivation abandonment in the black soil of Northeast China. Soil profiles were collected of cultivated or abandoned land with cultivation history of 0–100 years. An isotope mass balance equation was used to calculate the proportion of SOC derived from corn debris (C₄) and from natural vegetation (C₃) to deduce the dynamic process. Approximately 40% of SOC in the natural surface soil (0–10 cm) was eroded in the first 5 years of cultivation, increasing to about 75% within 40 years, before a slow recovery. C₄ above 30 cm soil depth increased by 4.5%–5% or 0.11–0.12 g·kg^?1 on average per year under continuous cultivation, while it decreased by approximately 0.34% annually in the surface soil after cultivation abandonment. The increase in the percentage of C₄ was fitted to a linear equation with given intercepts in the upper 30 cm of soil in cultivated land. A significant relationship between the change of C₄ and time was found only in the surface soil after abandonment of cultivation. These results demonstrate the loss and accumulation of corn-derived SOC in surface black soil of Northeast China under continuous tillage or cultivation abandonment.     

Kinetics of Mn(II) oxidation by Leptothrix discophora SS1

The kinetics of Mn(II) oxidation by the bacterium Leptothrix discophora SS1 was investigated in this research. Cells were grown in a minimal mineral salts medium in which chemical speciation was well defined. Mn(II) oxidation was observed in a bioreactor under controlled conditions with pH, O₂, and temperature regulation. Mn(II) oxidation experiments were performed at cell concentrations between 24 mg/L and 35 mg/L, over a pH range from 6 to 8.5, between temperatures of 10°C and 40°C, over a dissolved oxygen range of 0 to 8.05 mg/L, and with L. discophora SS1 cells that were grown in the presence of Cu concentrations ranging from zero to 0.1 μM. Mn(II) oxidation rates were determined when the cultures grew to stationary phase and were found to be directly proportional to O₂ and cell concentrations over the ranges investigated. The optimum pH for Mn(II) oxidation was approximately 7.5, and the optimum temperature was 30°C. A Cu level as low as 0.02 μM was found to inhibit the growth rate and yield of L. discophora SS1 observed in shake flasks, while Cu levels between 0.02 and 0.1 μM stimulated the Mn(II) oxidation rate observed in bioreactors. An overall rate law for Mn(II) oxidation by L. discophora as a function of pH, temperature, dissolved oxygen concentration (D.O.), and Cu concentration is proposed. At circumneutral pH, the rate of biologically mediated Mn(II) oxidation is likely to exceed homogeneous abiotic Mn(II) oxidation at relatively low (≈μg/L) concentrations of Mn oxidizing bacteria.     

Anaerobic digester sludge as nutrient source for culturing of microalgae for economic biodiesel production

After decades of ‘living dangerously’, the human kind has paused to think of Mother Nature. Alternate energy sources are being developed as a part of this realization. The use of indigenous sources of nutrients would considerably bring down the cost of production. A mixed consortium of Chlorella vulgaris, Arthrospira platensis, and tap water-originated Scenedesmus dimorphus was cultured using natural seawater, anaerobic digester sludge, and its growth was compared with synthetic commercial media like Zarrouk’s media, NaNO₃ media, and NH₄Cl media. A spectrophotometric method was standardized for regular biomass estimation. The dry biomass of a 15-day pure, batch culture of Arthrospira was found to yield around 600 mg/L in anaerobic digester sludge, whereas the pure batch culture of chlorella’s growth was hindered mainly due to the presence of bacteria and grazers. Regular microscopic observation and biomass monitoring revealed a drastic reduction in grazing activity, with the use of autoclaved AD sludge, resulting in a strong and stable microalgal mixed consortia. The consortia growth in AD sludge was found to be better than with the synthetic media with no cost of nutrient. The mixed consortia yielded a biomass up to 600 mg/L and lipid of 21.18%. The lipid generated from AD sludge had around 95% unsaturates and contained around 5% omega-3 fatty acids. The use of anaerobic digester sludge in a non-sterile condition reduces the total cost of the biodiesel production process as a whole and introduces a decentralized system for waste water treatment as well.     

The isotopic composition of waters from the El Tatio geothermal field,Northern Chile

On the basis of isotopic and chemical analyses of 45 spring, well and meteoric water samples from the El Tatio geothermal field in Northern Chile, four main processes giving rise to the formation of a wide range of thermal discharges can be distinguished. (1) Deep dilution of a predominant, primary high chloride (5500 mg/l, 260°) supply water derived from precipitation some 15 km east of El Tatio with local groundwater produces a secondary chloride water. (4750 mg/l, 190°) feeding springs over a limited area. (2) Single step steam separation from these two waters leads to isotopic shifts and increases in chloride contents to 8000 and 6000 mg/l respectively. (3) Absorption of this separated steam and carbon dioxide into local ground water and mixing with chloride waters at shallow levels produces a series of intermediate temperature (160°), low chloride, high bicarbonate waters. (4) Absorption of steam containing H₂S into surface waters leads to the formation of zero chloride, high sulfate waters; the isotopic enrichment observed is governed by a kinetic, steady state evaporation process.     

Kinetics of biotite dissolution and Fe behavior under low O2 conditions and their implications for Precambrian weathering

Biotite dissolution experiments were carried out to better understand the dissolution kinetics and Fe behavior under low O₂ conditions, and to give an insight into the Precambrian weathering. Mineral dissolution with a continuous flow-through reactor was employed at 25 °C for up to 65 days varying partial pressure of atmospheric oxygen (PO₂), pH (6.86 and 3.01) and Fe content in mineral (1.06 and 0.11 mol of Fe per O₁₀(OH,F)₂ for biotite and phlogopite, respectively) independently for the examination of their effects on biotite dissolution. Low PO₂ conditions were achieved in a newly developed glove box (PO₂ ? 6 × 10⁻⁴ atm; referred to as anoxic conditions), which was compared to the present, ambient air conditions (0.2 atm of PO₂; oxic conditions). The biotite dissolution rate was slightly faster under anoxic conditions at pH 6.86 while it was not affected by PO₂ at pH 3.01. There was no direct effect of Fe content on dissolution rate at pH 6.86 while there was a small difference in dissolution rate between biotite and phlogopite at pH 3.01. The 1.5 order-of-magnitude faster release rate of Fe under anoxic conditions for biotite dissolution at pH 6.86 resulted from the difference in ratio of Fe³⁺ precipitates remaining in the reactor to Fe dissolved (about 60% and 100% under anoxic and oxic conditions, respectively), which is caused mainly by the difference in PO₂. The results infer that the Fe²⁺ and Fe³⁺ contents in the Paleoproterozoic paleosols, fossil weathering profiles, are reflected by atmospheric oxygen levels at the time of weathering.     

Effects of magnesium ions on near-equilibrium calcite dissolution: Step kinetics and morphology

Dissolution kinetics at the aqueous solution-calcite interface at 50 °C were investigated using in situ atomic force microscopy (AFM) to reveal the influence of magnesium concentration and solution saturation state on calcite dissolution kinetics and surface morphology. Under near-equilibrium conditions, dissolved Mg²⁺ displayed negligible inhibitory effects on calcite dissolution even at concentrations of . Upon the introduction of , the solution saturation state with respect to calcite, , acted as a “switch” for magnesium inhibition whereby no significant changes in step kinetics were observed at Ω_calcite<0.2, whereas a sudden inhibition from Mg²⁺ was activated at Ω_calcite?0.2. The presence of the Ω-switch in dissolution kinetics indicates the presence of critical undersaturation in accordance with thermodynamic principles. The etch pits formed in solutions with exhibited a unique distorted rhombic profile, different from those formed in Mg-free solutions and in de-ionized water. Such unique etch pit morphology may be associated with the anisotropy in net detachment rates of counter-propagating kink sites upon the addition of Mg²⁺.     

Alginate biopolymer production by Azotobacter chroococcum from whey degradation

The potential of three Azotobacter chroococcum strains for whey degradation and alginate production were investigated. After dilution, samples were spread plated on isolation agar and Manitol agar and incubated at 30 °C for 24 h. Microorganisms were screened for their ability to whey degradation and alginate production based on colony morphology, negative and capsule staining, ability to decrease the apparent turbidity of the fermentation broths in batch and semi continuous culture by spectrophotometer assay at 400 nanometer and tensiometer assay. Of the three strains tested for whey degradation, only Azotobacter chroococcum 1723 produced significant apparent growth on whey broth and could decrease about 70 % of turbidity in fermentation broth during 6 days in batch culture. Colonies of this strain was characteristically yellow, large, moucoid and slimy on whey agar than Manitol agar after 24 h at 30 °C. Transmission electron microscopy assay and Carbazole reagent were used to recognize the alginate biopolymer. After optimizing environmental factors such as pH, salt concentration and temperature, this strain was able to produce exopolysaccharide greater than 5 mg/mL. Optimum results were obtained when using whey broth as a fermentation medium without extra salt, temperature at 35 °C and pH 7. Increasing inorganic and organic nitrogen sources (yeast extract and NH₄NO₃) reduced whey degradation at least 30%. Transmission electron microscopy assay showed a net-structured polysaccharide capsule around the cells. Semi-continuous culture results demonstrated that, alginate production as well as whey degradation was decreased (1 mg/mL and 30 %).