On the dissolution kinetics of humic acid particles. Effect of monocarboxylic acids

Quantity and quality variations of dissolved organic matter (DOM) in column leaching process from agricultural soil: Hydrochemical effects and DOM fractionation

Leaching dynamics of dissolved organic matter (DOM) from agricultural soil are influenced by the chemistry of irrigation water. This study used multiple spectroscopy methods to investigate the dynamics of DOM in column leaching process by utilizing selected solutions. DOC was leached out by Na solutions more effectively than by Ca, despite the additional contributions of the ligand exchange of SO₄^2- and the hydrolysis of HCO₃^-. The concentration ranking corresponding to solutions is Na₂SO₄ > NaHCO₃ > NaCl > CaCl₂. Furthermore, the aromaticity and molecular weights of DOM leached by Na solutions increased with the leaching, whereas contrary trends were reared by Ca. Thus, specific cation substantially affects DOM concentration and composition. Parallel factor analysis of the three-dimensional fluorescence spectra (EEM-PARAFAC) determined the characteristics of DOM components. (1) The long-wavelength, humic-like components (HLC, i.e., C1, C3, and C5) were largely accumulated at the early water saturating stage but attenuated toward dynamic stability in the salt-solution leaching (SSL) stage. (2) Reactive functional group-related HLC (i.e., C2, and C4) remarkably increased in response to the solutions chemistry. (3) The protein-like component (PLC, i.e., C6) was released with insignificant dynamic differences between the solutions. Despite approximately stable DOC concentration at the late stage of each SSL, HLC exhibited accelerating release in sodium salts but declining trend in calcium salt whereas PLC showed a roughly constant dynamic in the both. Ca-bridging reactions with the detachment of OM-Ca-clay complex prompting HLC release and its formation to stabilize the HLC well explain the contrast dynamics of HLC with Na or Ca leaching. The diffusion process that was defined for the slow, lasting and small concentration of leaching of inactive components could contribute to the steady dynamic of PLC, given its insensitivity to either the salt cation. This study identified distinct release kinetics of DOM compositions from those of DOC by EEM-PARAFAC and correspondingly the potential mechanisms, which offers promising advantages in monitoring DOM transport and fractionation in dynamic leaching process.

Interaction of pesticides with natural and synthetic solids. Evaluation in dynamic and equilibrium conditions

Interactions between pesticides (paraquat, glyphosate, 2,4-D, atrazine, and metsulfuron methyl) and soil organic and inorganic components have been studied in batch experiments by performing adsorption, dissolution, and chemical and photochemical degradation under different conditions. The obtained results confirm that the affinity of a pesticide to the solid surface depends on the nature of both and shows that each reactant strongly affects the mobility of the other one, e.g., anionic pesticides promote the dissolution of the solid humic acid but if this last is retained into the inorganic matrix enhances the adsorption of a cationic pesticide. Adsorption also seems to protect the bonded specie to be chemical degraded, such as shown in two pesticide/clay systems at constant pH. The use of mesoporous silicas could result in a good alternative for pesticide remediation. In fact, the solid shows high adsorption capacity towards paraquat and its modification with TiO₂ nanoparticles increases not only the pesticide adsorption but also seems to catalyze its degradation under UV light to less-toxic metabolites. UV-VIS spectroscopy was relevant and novel in such sense. Electrostatic interactions, hydrogen and coordinative bonds formations, surface complexations and hydrophobic associations play a key role in the fate of mentioned pesticides on soil and ground/surface water environments.

Removal of natural organic matter (NOM) from an aqueous solution by NaCl and surfactant-modified clinoptilolite

Zeolitic tuffs are found in different parts of the world. Iranian zeolite is a low-cost material that can be frequently found in nature. Surfactant-modified zeolite (SMZ) can be used for the adsorption of natural organic matter (NOM) from aqueous solutions. The adsorption study was conducted to evaluate the adsorption capacity of SMZ; furthermore, the effects of contact time, initial pH, and the initial adsorbent dose on the adsorption process were investigated in a batch system. The kinetic studies showed that the adsorption of NOM on SMZ was a gradual process. The optimum initial pH values for the adsorption of NOM on SMZ were in the acidic ranges. The batch kinetic experiments showed that the adsorption followed the pseudo-second-order kinetic model with good correlation coefficients. The equilibrium data were well described by the Langmuir isotherm model. The results show that the natural zeolite being modified with NaCl and hexadecyltrimethylammonium bromide as a cationic surfactant was an appropriate adsorbent for the removal of NOM.

A novel organo-zeolite adduct for environmental applications: sorption of phenol

A novel organo-zeolite adduct has been synthesized by sorbing humic acids (HA) onto zeolitic tuff and then heating the resulting complex at 330°C for 1.5h. Desorption tests showed that this procedure effectively immobilized HA on the tuff. The crystal structure of the zeolitic tuff and the chemical structure of HA were not altered during the preparation. Phenol sorption analysis demonstrated that the HA-zeolite adduct had good sorbing properties; moreover, the sorbed amount markedly decreased with increased ionic strength. These results point to prospective application of the HA-zeolite adduct as a low-cost and environmentally friendly sorbent for water purification from phenol and possibly other neutral organic pollutants.

Determination of total and available fractions of PAHs by SPME in oily wastewaters: overcoming interference from NAPL and NOM

BACKGROUND, AIM, AND SCOPE

Polycyclic aromatic hydrocarbons (PAHs) are often found in oily wastewaters. Their presence is usually the result of human activities and has a negative effect on the environment. One important step in addressing this problem is to evaluate the effectiveness of PAH removal by biological processes since these are the most cost-effective treatments known today. Many techniques are presently available for PAH determination in wastewaters. Solid phase microextracion (SPME) is known to be one of the most effective techniques for this purpose. When analyzing complex matrices with substances such as natural organic matter (NOM) and non-aqueous phase liquids (NAPL), it is important to differentiate the free dissolved PAH from matrix-bonded PAH. PAHs associated with the bonded fraction are less susceptible to biological treatment. The present study concerns the development of a simple and suitable methodology for the determination of the freely dissolved and the total fraction of PAHs present in oily wastewaters. The methodology was then applied to an oily wastewater from a fuel station retention basin.

MATERIAL AND METHODS

Headspace SPME was used for analyzing PAH since the presence of a complex or dirty matrix in direct contact with the fiber may damage it. Four model PAHs-anthracene, fluorene, phenanthrene, and pyrene-were analyzed by GC-MS. Negligible depletion SPME technique was used to determine the free fraction. Total PAH was determined by enhancing the mass transfer from the bonded phase to the freely dissolved phase by temperature optimization and the use of the method of standard additions. The PAH absorption kinetics were determined in order to define the optimal sampling conditions for this method. The fitting of the experimental data to a mathematical model was accomplished using Berkeley Madonna software. Humic acid and silicon oil were used as model NOM and NAPL, respectively, to study the effect of these compounds on the decrease of SPME response. Then, the method was evaluated with wastewater from a fuel station spill retention basin.

RESULTS

The SPME kinetic parameters-k (1) (uptake rate), k (2) (desorption rate), and K (SPME) (partition coefficient)-were determined from experimental data modeling. The determination of the free fraction required 15-min sampling to ensure that PAH depletion from sample was below 1%. For total PAH, a 30-min extraction at 100 degrees C ensured the maximum signal response in the GC-MS. For the determination of free and total PAHs, extractions were performed before reaching the SPME equilibrium. The wastewater used in this study had no free fraction of the analyzed PAHs. However, the four studied PAHs were found when the method for total PAH was used.

DISCUSSION

The addition of NOM and NAPL dramatically decreased the efficiency of the SPME. This decrease was the result of a greater partition of the PAHs to the NAPL and NOM phases. This fact was also observed in the analysis of the fuel station spill retention basin, where no free PAH was measured. However, using the method of standard addition for the determination of total PAH, it was possible to quantify all four PAHs.

CONCLUSIONS

The method developed in the present study was found to be adequate to differentiate between free and total PAH present in oily wastewater. It was determined that the presence of NOM and NAPL had a negative effect on SPME efficiency.

RECOMMENDATIONS AND PERSPECTIVES

The presence of binding substances had a great influence on SPME kinetics. Therefore, it is of extreme importance to determine their degree of interference when analyzing oily wastewaters or results can otherwise be erroneous. Other factors influencing the total PAH determinations should be considered in further studies.