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The effects of bubble size, solids particle (impurities) and the respective physical mechanisms on gas–liquid columns have received little attention. Therefore, the objective of this work is to study the effect of solid concentration and the ratio of the bubble diameter to the solid diameter on the mass transfer characteristics. Experiments were conducted in a three phase electroflotation column for the systems gas/water/olive stone. Volumetric mass transfer coefficient, kLa, was measured under different solid concentrations values (1–3 g/l) and ratio of the bubble diameter to the solid diameter values. It was found that the kLa rises when the ratio of the bubble diameter to the solid diameter increase. The presence of solid particle has a negative effect on mass transfer coefficients (kLa,kL and a). The liquid side mass transfer coefficient has the same behavior as the volumetric mass transfer coefficient. The capacity of oxygenation CO was also studied. Models relating the mass transfer coefficients kLa, kL and CO were elaborated in each case using linear regression method.

This work aims to experimentally study the bubble size distribution and the oxygen transfer on the electroflotation process. The distribution of bubbles was measured using a high-speed camera. The measurements were conducted in a three-phase electroflotaion column (water- gas-olive stone) equipped with insoluble electrodes, stainless steel as cathode and titanium, covered with ruthenium oxide, as anode. The volumetric mass transfer coefficient kla was determined for some operating parameters such as current density, solid concentrations and sizes. In order to calculate the global coefficient of mass transfer kl, the specific interfacial area, a, was determined. It was chiefly found that bubble size distribution depends on current density and solid concentration, and the wide range of bubble sizes was found to be affected by the phenomenon of break up and coalescence. kla tended to decrease with the increase of solid concentrations. kl exhibited the same behavior as the volumetric mass transfer coefficient. The experimental results were also fitted with the theoretical models, relating ‘kla’, ‘kl’ and ‘a’ with Reynolds number, Schmidt number and operating conditions.

The effects of solids (impurities) on gas–liquid columns and the respective physical mechanisms have received little attention. Therefore, in this study, the effect of the solids concentration on the mass transfer characteristics in a three-phase electroflotation column was experimentally studied for the systems gas/water/olive stone. Volumetric mass transfer coefficient, kLa, was measured under different solid concentrations (1–3 g/l) and current densities values (60–220 A/m2). The presence of olive stone solids was found to affect negatively kLa. Also, an increase in kLa occurs when the current density increases. The capacity of oxygenation CO and the corrective factor alpha were also studied.

Drought is a major factor limiting crop production worldwide. Barley is a well-adapted cereal that is largely grown on dry marginal land where water and salinity are the most prevalent environmental stresses. This study was carried out to investigate the effects of drought stress and subsequent recovery on growth, photosynthetic activity, water relations, osmotic adjustment (OA), and solute accumulation of wild (Hordeum maritimum) and cultivated barley (H. vulgare L.). In a pot experiment, 60 d old seedlings were subjected to drought stress for 0, 7, 14, 21, or 28 d, and then re-watered to recover for up to 21 d. Plants were harvested at the end of each of these drought/recovery treatments. Drought significantly reduced fresh and dry weights at the whole-plant level, photosynthetic activities, and solute and water potentials, while increasing leaf Na+ and K+ concentrations. The adverse effects of drought on growth were more marked in cultivated barley than in wild barley and the reverse was true for photosynthetic activities. During recovery, all wild barley seedlings completely recovered. For cultivated barley seedlings, rehydration had a beneficial effect on growth and photosynthesis, independent of treatment duration, but complete recovery did not occur. The reduction in leaf solute potential at full turgor in drought-stressed barley, relative to the control, suggests active OA which was more significant in wild barley than in cultivated barley. OA was mainly due to the accumulation of inorganic (K+ in cultivated barley and Na+ in wild barley) and organic (soluble sugars and proline) solutes. The results suggest that OA is an important component of the drought-stress adaptation mechanism in wild barley, but is not sufficient to contribute to drought tolerance in cultivated barley. In the latter species, the results show that even short periods (as little as 7 d) of water deficit stress had considerable long-term effects on plant growth.

The effects of salt stress on the salt marsh halophyte Spartina alterniflora have been well documented. However, plant responses to combined salinity and ammonium toxicity and the underlying mechanisms are relatively unknown. The aim of the present investigation was to study the effects of both salinity (0, 200 and 500 mM NaCl) and nitrogen form (NO3 −, NH4 + or NH4NO3) on S. alterniflora. Plants were cultivated in sandy soil under greenhouse conditions for 3 months. At harvest, growth parameters were measured and leaf samples were analysed for oxidative stress parameters (malondialdehyde, MDA; electrolyte leakage, EL; and hydrogen peroxide, H2O2 concentration) and the activity of antioxidant enzymes (glutathione reductase, GR; superoxide dismutase, SOD; catalase, CAT; ascorbate peroxidase, APX and Guaiacol peroxidase, GPX). In the absence of NaCl, plant growth rate was the highest in the medium containing both nitrogen forms, and the lowest in the medium containing only nitrate. Irrespective of the nitrogen form, plant growth was generally higher at 200 mM NaCl than without salinity. Ammonium-fed plants showed better growth than nitrate-fed plants under high salinity. In the absence of salinity, ammonium-fed plants showed higher SOD, APX, GR, CAT, and GPX activities than nitrate-fed ones. The antioxidant enzymes exhibited higher activity in saline-treated plants. The considerable advantage of NH4 + nutrition to S. alterniflora under saline conditions was associated with high antioxidant enzyme activities, together with low MDA content, EL, and H2O2 concentration. These data clearly demonstrate that NH4 + is more favourable for the growth of S. alterniflora under high salinity than NO3 −. It is suggested that NH4 + nutrition improves the plant’s capacity to limit oxidative damage by stimulating the activities of the major antioxidant enzymes.

The chenopod Beta macrocarpa Guss (wild Swiss chard) is known for its salt tolerance, but the mechanisms involved are still debated. In order to elucidate the processes involved, we grew wild Swiss chard exposed to three salinity levels (0, 100 and 200 mm NaCl) for 45 days, and determined several physiological parameters at the end of this time. All plants survived despite reductions in growth, photosynthesis and stomatal conductance in plants exposed to salinity (100 and 200 mm NaCl). As expected, the negative effects of salinity were more pronounced at 200 mm than at 100 mm NaCl: (i) leaf apoplastic water content was maintained or increased despite a significant reduction in leaf water potential, revealing the halophytic character of B. macrocarpa; (ii) osmotic adjustment occurred, which presumably enhanced the driving force for water extraction from soil, and avoided toxic build up of Na(+) and Cl(-) in the mesophyll apoplast of leaves. Osmotic adjustment mainly occurred through accumulation of inorganic ions and to a lesser extent soluble sugars; proline was not implicated in osmotic adjustment. Overall, two important mechanisms of salt tolerance in B. macrocarpa were identified: osmotic and apoplastic water adjustment.