Effect of different wastewater composition on kinetics, capacities, and mechanisms of phosphorus sorption by carbonated bauxite residue

International audienceThis study aims at evaluating the effect of different wastewater composition on kinetics, capacities, and mechanisms of P sorption by carbonated bauxite residues (CBR). A series of batch experiments was performed to investigate P sorption behaviors from solutions prepared with different aqueous matrices (deionized water, tap water, and real wastewater) and different initial P concentrations (from 10 to 200 mg P/L). Also, a series of sequential P extractions was performed to investigate P fractionation of CBR before and after its use in P sorption experiments, and hence to elucidate the main P removal mechanisms. The results indicate that initial P concentration is the most influential parameter controlling kinetics, capacities, and mechanisms of P removal in batch experiments. Kinetic constant of P sorption increases exponentially with decreasing initial P concentration below 100 mg P/L, thus indicating a faster achievement of P sorption equilibrium. Equilibrium P sorption capacities increase linearly from about 0.2 to about 3.9 mg P/g CBR with increasing initial P concentration from 10 to 200 mg P/L, thus indicating that P saturation of CBR was not reached. Ca phosphate precipitation is the main P removal mechanism at higher initial P concentrations (> 10 mg P/L), whereas phosphate adsorption on CBR surface becomes more relevant over the total amount of P removed at lower initial P concentrations. Overall, the findings of this study allow to evaluate kinetic constants, sorption capacities, and removal mechanisms under different operating scenarios, thus providing crucial information for the design and operation of P treatment units

Direct uptake of organically derived carbon by grass roots and allocation in leaves and phytoliths: 13C labeling evidence

International audienceIn the rhizosphere, the uptake of low-molecular-weight carbon (C) and nitrogen (N) by plant roots has been well documented. While organic N uptake relative to total uptake is important, organic C uptake is supposed to be low relative to the plant's C budget. Recently, radiocarbon analyses demonstrated that a fraction of C from the soil was oc-cluded in amorphous silica micrometric particles that precipitate in plant cells (phytoliths). Here, we investigated whether and to what extent organically derived C absorbed by grass roots can feed the C occluded in phytoliths. For this purpose we added 13 C-and 15 N-labeled amino acids (AAs) to the silicon-rich hydroponic solution of the grass Festuca arun-dinacea. The experiment was designed to prevent C leakage from the labeled nutritive solution to the chamber atmosphere. After 14 days of growth, the 13 C and 15 N enrich-ments (13 C excess and 15 N excess) in the roots, stems and leaves as well as phytoliths were measured relative to a control experiment in which no labeled AAs were added. Additionally , the 13 C excess was measured at the molecular level, in AAs extracted from roots and stems and leaves. The net uptake of labeled AA-derived 13 C reached 4.5 % of the total AA 13 C supply. The amount of AA-derived 13 C fixed in the plant was minor but not nil (0.28 and 0.10 % of total C in roots and stems/leaves, respectively). Phenylalanine and methionine that were supplied in high amounts to the nu-tritive solution were more 13 C-enriched than other AAs in the plant. This strongly suggested that part of AA-derived 13 C was absorbed and translocated into the plant in its original AA form. In phytoliths, AA-derived 13 C was detected. Its concentration was on the same order of magnitude as in bulk stems and leaves (0.15 % of the phytolith C). This finding strengthens the body of evidences showing that part of organic compounds occluded in phytoliths can be fed by C entering the plant through the roots. Although this experiment was done in nutrient solution and its relevance for soil C uptake assessment is therefore limited, we discuss plausible forms of AA-derived 13 C absorbed and translocated in the plant and eventually fixed in phytoliths, and implications of our results for our understanding of the C cycle at the soil– plant–atmosphere interfac

Hydrochemical constraints between the karst Tabular Middle Atlas Causses and the Saïs basin (Morocco): implications of groundwater circulation

The karst Tabular Middle Atlas Causses reservoir is the main drinking-water supply of Fez-Meknes region (Saïs Basin) in Morocco. Recent analyses showed a decline in associated groundwater chemical quality and increased turbidity. To understand this hydrosystem, four surveys were undertaken during fall and spring, 2009–2011. Hydrogeochemical studies coupled with isotopic analyses (δ18O, δD and 222Rn) showed that the aquifers between the causses (mountains) and the Saïs Basin are of Liassic origin and at the southern extremities are of Triassic origin. Five recharge zones of different altitudes have been defined, including two main mixing zones in the south. Deuterium excess results suggest local recharge, while a plot of δ18O versus δD characterizes a confined aquifer in the eastern sector. 222Rn results reveal areas of rapid exchanges with an upwelling time of less than 2 weeks. A schematic conceptual model is presented to explain the groundwater circulation system and the behavior of this karst system