Factors that affect physicochemical and acid-base properties of compost and vermicompost and its potential use as a soil amendment.

Composting and vermicomposting have attracted attention in relation to both waste management and the potential to produce organic amendments that could improve soil quality. The main differences between compost depend on the feedstock, the production process, and the degree of maturity. In the present study, samples of compost of different origin (food and green waste, livestock waste, algae waste, urban waste or sewage sludge) or subjected to different composting methods (traditional or using earthworms) were collected for analysis. Additionally, samples collected at various stages of the composting process were compared (raw material, 15 and 30 days of composting, and final compost). Different analysis and techniques were used to establish the chemical composition, physicochemical and acid-base properties of compost samples and the organic matter extracts. The correlations obtained (between the abundance of acid groups in different extracts of the compost or between the cation exchange capacity and the C/N atomic ratio) would allow for predicting the compost behaviour based on certain characteristics, and a reduction in the number of parameters determined experimentally, thus facilitating comparisons between different compost. In addition, the potential value of the compost as amendment was tested with a Haplic Cambisol from a mining area. The application of compost increased the pH, the organic matter and nutrient content, and promoted seed germination and root growth.

Unravelling the metal uptake process in mosses: Comparison of aquatic and terrestrial species as air pollution biomonitors.

Transplanted mosses have been widely shown to be excellent tools for biomonitoring air pollution; however, it is not clear how the functional groups present on their surfaces affect the uptake of metal cations. In the present study, we examined differences in trace metal accumulation in two terrestrial and one aquatic moss species, and investigated whether the differences depended on their physico-chemical characteristics. In the laboratory, we determined C, N and H contents in their tissues and obtained the ATR-FTIR spectra (to identify the presence of functional groups). We also conducted surface acid-base titrations and metal adsorption assays with Cd, Cu and Pb. In the field, we exposed transplants of each species near different air-polluting industries, and determined the mosses enrichment of Al, Cd, Co, Cr, Cu, Fe, Ni, Pb and V. Laboratory results demonstrated higher metal uptake capacity in the terrestrial mosses Sphagnum palustre and Pseudoscleropodium purum, compared to that in the aquatic moss Fontinalis antipyretica, which can be attributed to a greater abundance of acidic functional groups (i.e. negatively charged binding sites) on the surface of the terrestrial mosses. The affinity of moss for certain elements depends on the abundance and nature of surface functional groups. Accordingly, the metal concentrations generally reached higher levels in S. palustre transplants compared to the other species, except for the uptake of Hg, which was higher in F. antipyretica. However, the findings also suggest an interaction between the type of environment (terrestrial or aquatic) and the moss characteristics that may influence the abovementioned trend. Thus, irrespective of the physico-chemical characteristics, metal uptake varied depending on the environment of origin of the mosses "i.e. atmospheric or aquatic". In other words, the findings suggest that species that accumulate more metals in terrestrial environments will accumulate lower amounts of metals in aquatic environments and vice versa.

The ionization constants of alpha-alanine in NaCl at 25 degrees. Effect of the ionic strength based on three models.

In the present work we obtained the experimental pKs of the amino acid alpha-alanine in NaCl at 25 degrees and different ionic strengths. The equilibrium constants have been potentiometrically determined with a commercial glass electrode and the results analysed through three models: two directly based on the Pitzer and Scatchard approaches to the ion specific interaction theory and the other based on a simpler modification of the Debye-Hückel equation. The three models fit the data reasonably well and the extrapolated pK values obtained show a good agreement. The goodness of the ridge regression method cannot be probed in this case.

Effect of ionic strength on the formal potential of the glass electrode in various saline media.

We examined the variation with ionic strength (I, adjusted with KCl, KNO(3), KBr, NaCl or NaClO(4)) of the formal potential (E(const)) for glass electrodes exhibiting a Nernstian response (i.e. E(cell)=E(const)-slog[H(+)]). For this purpose, we investigated the different factors included in the formal potential, so we obtained reported values for the liquid junction potential as a function of ionic strength and determined the logarithm of the activity coefficient for the proton in various saline media, using Pitzer equations.

Analysis of copper and calcium-fulvic acid complexation and competition effects.

The binding of Cu2+ and Ca2+ to a fulvic acid (FA), simulating naturally occurring conditions, was studied. Furthermore, copper-FA complexation in the presence and absence of calcium was compared. For this, potentiometric titrations were carried out using a solution of FA at a concentration of 100 mgL(-1), and of ionic strength 0.1M. The Ca(2+)-FA complexation reaction was carried out at pH 6.5, 7.5 and 8.5 and the Cu(2+)-FA complexation reaction at pH 5.5 and 6.5, in both the presence and absence of calcium ion. The calcium ion had a significant effect on copper binding at [Ca2+]> or =2.5x10(-3)M. The experimental binding curves were analyzed using non-electrostatic discrete site and continuous distribution models. Competition by the calcium ion mainly affected the maximum binding capacity of the Cu(2+)-FA complexation, whereas the slight effect observed on the binding constant appears to indicate that calcium only competes for specific copper sites, thus modifying the binding sites distribution function. It was also found that the number of sites occupied by the copper ion represented only 20% of the total concentration of acid groups ionized on FA at the pH values studied.

Adsorption of MCPA on goethite and humic acid-coated goethite.

Anionic pesticides are adsorbed on the mineral oxide fraction of the soil surface but considerably less on the organic fraction, so that the presence of organic matter causes a decrease in the amount of pesticide adsorbed, and may affect the mechanism of adsorption. In the present study we investigated the adsorption of the weak acid pesticide MCPA on the surface of goethite and of humic acid-coated goethite, selected as models of the mineral oxide fraction and organic components present in soil systems. Adsorption of the anionic form of the pesticide on goethite fitted an S-type isotherm and the amount adsorbed increased as the ionic strength decreased and the pH of the medium decreased. Application of the charge distribution multi site complexation model (CD-MUSIC model) enabled interpretation of the results, which suggested the formation of inner and outer sphere complexes between the pesticide and the singly-coordinated surface sites of goethite. Less pesticide was adsorbed on the humic acid-coated goethite than on the bare goethite and the pattern fitted an L-type isotherm, which indicates a change in the mechanism of adsorption. Simplified calculations with the CD-MUSIC model enabled interpretation of the results, which suggested that the pesticide molecules form the same type of surface complexes as in the previous case.

Effect of pH and ionic strength on the binding of paraquat and MCPA by soil fulvic and humic acids.

The effect of pH and ionic strength on the interaction between pesticides (paraquat and MCPA) and humic substances (soil extracted humic acid and fulvic acid) was interpreted with a simple electrostatic model. Potentiometric titrations were carried out, the charge curves for the humic substances were obtained for three values of ionic strength, and the parameters that define proton binding to humic and fulvic acid were calculated by application of the NICA-Donnan model. The binding isotherms were obtained for paraquat-humic acid and paraquat-fulvic acid at three different pH values and two ionic strengths, and the MCPA-fulvic acid binding isotherms for two pH values and two ionic strengths. Binding experiments were carried out by use of a membrane dialysis technique and the concentrations of pesticide were measured by HPLC. The amount of paraquat bound to the humic substances increased with pH, decreased with increasing ionic strength, decreased in the presence of Ca(2+) and was greater for humic acid than for fulvic acid. Much less binding was observed with MCPA than with paraquat, and therefore the isotherms were not well defined. The application of a simple electrostatic model enabled us to conclude that the effect of pH and ionic strength on binding of paraquat to humic substances is due to the effect that these parameters have on the humic substance charge, and the model provided an excellent reproduction of the experimental binding isotherms.