Developing a Compost Quality Index (CQI) Based on the Electrochemical Quantification of Cd (HA) Reactivity.

The present work demonstrates the use of Cd2+ as a reactivity probe of the fulvic acids (FAs), humic acids (HAs) and dissolved organic matter (DOM) compost extracts. Significant differences were observed between the extracts, with the HA extract showing the highest reactivity. Comparing the different composts, the largest reactivity variation was again observed for HA then FA and finally DOM extracts. The Cd2+ binding extent was used to calculate the quality of composts and compared with a reference of uncomposted organic fertiliser (FLW), leading to the definition of an operational scale of compost quality. The parameter equivalent mass of fertiliser (mEF) was used for this scale sorted the seven composts from 0.353 to 1.09 kg FLW, for compost of sewage sludge (CSS) and vermicompost of domestic waste (CVDW), respectively. The significance of this parameter was verified through a correlation analysis between binding extent and the effect of compost application on lettuce crop growth in a field trial. The results demonstrate the potentiality of FA and HA extracts as markers of compost bioactivity and the use of Cd2+ as a reactivity probe.

Estimation of phosphate extractability in flooded soils: Effect of solid-solution ratio and bicarbonate concentration.

The Olsen method is widely used to determine bioavailable phosphate (P) in upland soils. It is also used in flooded soils, although different estimates of extractable-P are obtained under anoxic and oxic conditions. In this study, variations in extractable-P in three soils under different redox conditions were evaluated as a function of solid to solution ratio (SSR) (1:5-1:200) and bicarbonate concentration (0.1-1 M). The parameterized CD-MUSIC model was used to describe the data, with optimization of reactive surface area (RSA) and reversibly adsorbed-P (R-PO4). The RSA may vary due to the reductive dissolution of iron minerals and/or the formation of new reactive surfaces upon the establishment of reducing conditions. Changes in SSR and bicarbonate concentration significantly affected extractable-P under both oxic and anoxic conditions; more P was extracted under anoxic than under oxic conditions. The difference was 1.5-2 times greater for the highest SSR considered. In the soil samples with higher organic carbon content, the effect of bicarbonate concentration on extractable-P was remarkable. The large differences in extractable-P under oxic and anoxic conditions were probably due to differences in iron (hydr)oxide content. The CD-MUSIC model successfully predicted the effect of SSR on extractable-P under both conditions. R-PO4 data were fitted for oxic conditions and assumed unchanged for anoxic samples, while RSA data were fitted for both conditions. The RSA values were lower in anoxic than in oxic samples. Overall, our data and model calculations indicate that using wet soil samples obtained in-situ for evaluation of Olsen-P in submerged soils lead to a higher estimation of extractable-P than estimated in oxic soils. If soil testing in the presence of target plants confirms the reliability of in-situ sampling for Olsen-P estimation, the P fertilizer dose applied to submerged soils could be reduced, which is very important from environmental and economic perspectives.

Modeling the effects of humic acid and anoxic condition on phosphate adsorption onto goethite.

Low redox potential in flooded soils may affect phosphate bioavailability by reducing iron oxides or formation of new minerals. To investigate phosphate behavior in anoxic conditions, goethite was selected as a soil model and coated by humic acid (HA) and sodium borohydride was used as a reducing agent. Adsorption experiments were conducted in 0.1 M NaNO3 as a function of pH in oxic (Eh = +254 to +448 mV) and suboxic (Eh = -162 to +167 mV) conditions for four phosphate concentrations (0.05-0.8 mM). CD-MUSIC and NOM-CD models in combination with Extended Stern model were used to describe the experimental data. Results show that by increasing pH and carbon content in the organo-mineral composites, the released phosphate to the solution increases in both oxic and suboxic conditions. In suboxic conditions, as a result of sodium borohydride dissociation in water and consequently boron release to the solution, at high loading of boron and low loading of phosphate, boron can compete with phosphate for the surface reactive sites and decrease its adsorption. On the other hand, ferrous iron can attenuate boron effect and promote phosphate adsorption. The results indicated that goethite surface is resistant to the reductive transformation that may occur at relatively low redox potential due to its high crystalline character and thermodynamic stability. HA may, however, promote the formation of amorphous iron phases, which consequently might induce phosphate adsorption in OM-mineral composites. The derived affinity constants in oxic conditions described the experimental data of suboxic conditions reasonably well.

Matrix solid-phase dispersion as a greener alternative to obtain bioactive extracts from Haematococcus pluvialis. Characterization by UHPLC-QToF.

So far, research on the microalga Haematococcus pluvialis has been focused mainly on the exploitation of its high astaxanthin content, leaving aside the use of other bioactive compounds present. This study is focused on obtaining and characterizing extracts enriched in bioactive compounds from this microalga red aplanospores. This is performed by means of Matrix Solid-Phase Dispersion (MSPD) extraction process, in an environmentally friendly way with low energy consumption and GRAS solvents. The effects of extraction parameters, particularly the extraction solvents (ethanol, ethyl lactate and water) are studied, in order to obtain maximum recovery of the main antioxidant compounds of interest (carotenoids, fatty acids and derivatives). Characterization of extracts is carried out by HPLC-DAD (High Performance Liquid Chromatography Diode Array Detector) and UHPLC-QToF (Ultra High-Performance Liquid Chromatography Quadrupole Time-of-Flight). The results show that MSPD produced extracts with higher bioactive compound recoveries than conventional cell disruption extractions. At the same time, a novel untargeted characterization for this species is performed, identifying compounds not previously dated in H. pluvialis, which include 10-phenyldecanoic acid and the -oxo and -hydroxy derivatives of palmitic acid. This approach, first applied to a freshwater microalgae, characterized by rigid and resistant aplanospores, provided a synergistic and sustainable extract, giving a broader focus on the use of this microalga.

Biochar as low-cost sorbent of volatile fuel organic compounds: potential application to water remediation.

Pyrolysis of waste materials to produce biochar is an excellent and suitable alternative supporting a circular bio-based economy. One of the properties attributed to biochar is the capacity for sorbing organic contaminants, which is determined by its composition and physicochemical characteristics. In this study, the capacity of waste-derived biochar to retain volatile fuel organic compounds (benzene, toluene, ethylbenzene and xylene (BTEX) and fuel oxygenates (FO)) from artificially contaminated water was assessed using batch-based sorption experiments. Additionally, the sorption isotherms were established. The results showed significant differences between BTEX and FO sorption on biochar, being the most hydrophobic and non-polar contaminants those showing the highest retention. Furthermore, the sorption process reflected a multilayer behaviour and a relatively high sorption capacity of the biochar materials. Langmuir and Freundlich models were adequate to describe the experimental results and to detect general differences in the sorption behaviour of volatile fuel organic compounds. It was also observed that the feedstock material and biochar pyrolysis conditions had a significant influence in the sorption process. The highest sorption capacity was found in biochars produced at high temperature (> 400 °C) and thus rich in aromatic C, such as eucalyptus and corn cob biochars. Overall, waste-derived biochar offers a viable alternative to be used in the remediation of volatile fuel organic compounds from water due to its high sorption capacity.

Effects of pH and electrolyte concentration on the binding between a humic acid and an oxazine dye.

The binding between an oxazine dye and a humic acid was studied in aqueous solutions in the pH range 4-10 and in the supporting electrolyte (KCl) range 0.001-0.1M. A rather simple spectrophotometric method was developed to construct binding isotherms under conditions were traditional centrifugation or filtration methods fail. The use of this method is possible because humic acid molecules have the ability of changing the spectrum of dye molecules, and this ability is used to quantify the isotherms. All binding isotherms have a Langmuirian shape. The amount of bound dye is strongly dependent on the ionic strength and less dependent on the pH of the solution. The binding is rather strong and mainly driven by non-electrostatic forces. Whereas the Langmuir binding constant is independent of the pH and electrolyte concentration, the number of assessable sites in humic acid for binding oxazine increases by increasing pH and decreasing electrolyte concentration. These results can be directly related to the flexibility of humic acid molecules, which can swell at high pH and low ionic strength, increasing consequently the availability of binding sites. The results also indicate that humic substances may strongly affect the mobility and fate of dyes and related pollutants in the environment.

Copper binding by peat fulvic and humic acids extracted from two horizons of an ombrotrophic peat bog.

We studied the binding of Cu(II) to humic acids and fulvic acids extracted from two horizons of an ombrotrophic peat bog by metal titration experiments at pH 4.5, 5.0, 5.5, and 6.0 and 0.1 M KNO3 ionic strength. Free metal ion concentrations in solution were measured using an ion selective electrode. The amounts of base required to maintain constant pH conditions were recorded and used to calculate H+/Cu2+ exchange ratios. The amount of Cu(II) bound to the humic fractions was greater than the amount bound to the fulvic fractions and only at the highest concentrations of metal ion the amount of Cu(II) sorbed by both fractions became equal. The proton to metal ion exchange ratios are similar for all humic substances, with values ranging from 1.0 to 2.0, and decreasing with increased pH. The amount of Cu(II) bound is practically independent of the horizon from which the sample was extracted. The results indicate that the humic substances show similar cation binding behaviour, despite the differences in chemical composition. The copper binding data are quantitatively described with the NICA-Donnan model, which allows to characterize only the carboxylic type binding sites. The values of the binding constants are higher for the humic acids than for the fulvic acids.

Influence of feedstock on the copper removal capacity of waste-derived biochars.

Biochar samples were generated by low temperature pyrolysis of different types of waste. The physicochemical characteristics of the different types of biochar affected the copper retention capacity, by determining the main mechanism involved. The capacity of the biochar to retain copper present in solution depended on the size of the inorganic fraction and varied in the following order: rice biochar>chicken manure biochar>olive mill waste biochar>acacia biochar>eucalyptus biochar>corn cob biochar. The distribution of copper between the forms bound to solid biochar, dissolved organic matter and free organic matter in solution also depended on the starting material. However, the effect of pH on the adsorption capacity was independent of the nature of the starting material, and the copper retention of all types of biochar increased with pH.

Effects of pH and ionic strength on the adsorption of phosphate and arsenate at the goethite-water interface.

The surface properties of a well-crystallized synthetic goethite have been studied by acid-base potentiometric titrations, electrophoresis, and phosphate and arsenate adsorption isotherms at different pH and electrolyte concentrations. The PZC and IEP of the studied goethite were 9.3+/-0.1 and 9.3+/-0.2, respectively. Phosphate and arsenate adsorption decrease as the pH increases in either 0.1 or 0.01 M KNO(3) solutions. Phosphate adsorption is more sensitive to changes in pH and ionic strength than that of arsenate. The combined effects of pH and ionic strength result in higher phosphate adsorption in acidic media at most ionic strengths, but result in lower phosphate adsorption in basic media and low ionic strengths. The CD-MUSIC model yields rather good fit of the experimental data. For phosphate it was necessary to postulate the presence of three inner-sphere surface complexes (monodentate nonprotonated, bidentate nonprotonated, and bidentate protonated). In contrast, arsenate could be well described by postulating only the presence of the two bidenate species. A small improvement of the arsenate adsorption data could be achieved by assuming the presence of a monodentate protonated species. Model predictions are in agreement with spectroscopic evidence, which suggest, especially for the case of arsenate, that mainly bidentate inner-sphere complexes are formed at the goethite-water interface.

Modeling oxyanion adsorption on ferralic soil, part 2: chromate, selenate, molybdate, and arsenate adsorption.

High levels of oxyanions are found in the soil environment, often as a result of human activity. At high concentrations, oxyanions can be harmful to both humans and wildlife. Information about the interactions between oxyanions and natural samples is essential for understanding the bioavailability, toxicity, and transport of these compounds in the environment. In the present study, the authors investigated the reactivity of different oxyanions (AsO4 , MoO4 , SeO4 , and CrO4 ) at different pH values in 2 horizons of a ferralic soil. By combining available microscopic data on iron oxides with the macroscopic data obtained, the authors were able to use the charge distribution model to accurately describe the adsorption of these 4 oxyanions and thus to determine the surface speciation. The charge distribution model was previously calibrated and evaluated using phosphate adsorption/desorption data. The adsorption behavior on ferralic soil is controlled mainly by the natural iron oxides present, and it is qualitatively analogous to that exhibited by synthetic iron oxides. The highest adsorption was found for arsenate ions, whereas the lowest was found for selenate, with chromate and molybdate ions showing an intermediate behavior.

Modeling oxyanion adsorption on ferralic soil, part 1: parameter validation with phosphate ion.

Surface complexation models have proved to be valuable tools for predicting processes that occur at the solid-solution interface. Use of such models has become more widespread and nowadays more complex systems are studied, in an attempt to explain processes such as the competition between different species for mineral surfaces and the effect of the presence of organic matter. The aim of the present study was to analyze the mobility of phosphate in ferralic soils. The charge distribution model parameters for phosphate-goethite adsorption were used to predict phosphate mobility on samples from 2 horizons of a ferralic soil containing large amounts of iron oxides. The soil reactivity was attributed to the iron oxides, and some specific parameters were determined by means of phosphate adsorption-desorption experiments and included in the model. Adsorption of phosphate in the upper horizon, which contained more organic carbon and phosphate than the deeper one, was modeled by using the information obtained for the soil and the charge distribution model parameters derived for phosphate-goethite interaction with no need of further optimization. In contrast, some extra fitting parameters were required to improve the modeling of the phosphate adsorption in the deeper horizon.

Effect of organic matter and pH on the adsorption of metalaxyl and penconazole by soils.

Soil organic matter (SOM) is considered to be the primary adsorbent of non-ionic pesticides, and it is therefore thought to determine the concentration of such pesticides in the soil solution and how they are transported throughout the medium. It is generally assumed that the sorption capacity of different soils is the same per unit mass of SOM; however, the reactivity also depends on the SOM composition and the pH of the medium. We carried out experiments to study the effects of pH and ionic strength on the adsorption of the non-ionic fungicides metalaxyl and penconazole on four soils containing different amounts of organic carbon. The adsorption isotherms fitted a Freundlich equation. For pH>5, partitioning of the fungicides between the solid phase and the soil solution did not vary with the pH, while at lower pH, the fraction adsorbed on the solid phase increased as the pH decreased. The response was related to the effect of pH on the ionization of the carboxylic groups of the SOM and therefore to the hydrophilic nature of the SOM. Analysis of the charge effect on the partitioning of both fungicides revealed a common response in all four soils. Adsorption appears to be related to the magnitude of the charge developed at the SOM due to ionization of the carboxylic acid groups.

Comparison of arsenate, chromate and molybdate binding on schwertmannite: surface adsorption vs anion-exchange.

The presence of iron oxides may play an important role in controlling the mobility and availability of contaminants in soils and waters affected by acid mine drainage. The present study describes the uptake of arsenate, chromate and molybdate from solution by synthetic schwertmannite. Batch experiments were performed at different pH values in order to obtain the adsorption isotherms for the three oxyanions. In addition to the formation of surface complexes between the oxyanions and the iron surface reactive groups, it is also expected that anion exchange will occur between sulphate anions from the schwertmannite structure and the oxyanions present in the solid/solution interface. Comparison of the experimental adsorption results for the different oxyanions showed large differences, not only the amount adsorbed, which was much higher for arsenate, but also in the sulphate exchange with the anions in solution. In case of chromate, the main mechanism of adsorption process is the exchange reaction with the sulphate groups present in the schwertmannite. The observed results suggest a different adsorption mechanisms for each of the three oxyanions, with important implications for the mobility of these anions in acid mine drainage systems.

Adsorption of paraquat on soil organic matter: effect of exchangeable cations and dissolved organic carbon.

Herbicides that interact with soil organic matter do so with both the solid and the dissolved fractions, so that the distribution of herbicide between the soil solution and solid phases is determined by competitive effects. In the present study, adsorption experiments were carried out with the cationic herbicide paraquat and untreated and acid-washed samples of a peat soil, at different values of pH and ionic strength. Less herbicide was adsorbed onto the untreated peat than onto the acid-washed peat; the difference was due to the presence of exchangeable cations, as demonstrated in experiments carried out by adding Ca(2+) to suspensions of acid-washed peat. The results were interpreted by an electrostatic model and the fitting parameters indicated that the adsorption constants were the same for both samples of peat, although the number of binding sites available was different. Simultaneous resolution of the adsorption equilibrium of paraquat for the soil organic matter (SOM) and of the binding equilibrium between paraquat and dissolved organic matter (DOM) enabled the distribution of paraquat between the solid and solution phases to be determined. The increased solubility of the SOM with increasing pH led to a decrease in the fraction of paraquat retained on the peat surface above pH 5.5, which favors the mobility of the herbicide in the soil.

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

Adsorption of cationic pesticides in soils is generally attributed to mineral clays and organic matter components. However, iron oxides may also contribute to such adsorption or affect it by associating with other components. Using goethite and humic acid as models for iron oxides and organic matter respectively, we studied the adsorption of the cationic pesticide paraquat on goethite and humic acid-coated goethite. At pH 4.0 the adsorption on goethite was not significant, and at pH 10.0, although the surface of the oxide was negatively charged, much less pesticide was adsorbed than on mineral clays. At this pH the adsorption of paraquat decreased as the ionic strength increased, and application of the charge distribution multisite complexation model (CD-MUSIC model) enabled interpretation of the results. At pH 4, the adsorption of paraquat on the humic acid-coated goethite was similar to the adsorption on mineral clays, but was considerably less than the adsorption on humic acid in solution. The lower adsorption on solid organic matter is attributed to a decrease in the number of "active" binding sites on the humic acid as a result of the binding to iron oxide.

Analysis of phosphate adsorption onto ferrihydrite using the CD-MUSIC model.

Ferrihydrite nanoparticles may dominate the ion binding properties of the natural oxide fraction present in soil and aquatic systems. A correct description of the adsorption properties of ferrihydrite nanoparticles may be useful for gaining a better insight into the adsorption processes in natural systems and at the same time will be essential for developing surface complexation models able to describe these processes. In the present study, phosphate speciation in ferrihydrite has been analyzed combining the available spectroscopic data and molecular information with modeling calculations. For this purpose, a new data set that analyzes the effect of pH and ionic strength on the phosphate adsorption onto ferrihydrite has been used. Description of the phosphate adsorption process onto ferrihydrite nanoparticles, for the entire pH and ionic strength range, has been made taking into account the presence of protonated and nonprotonated bidentate surface complexes. The presence of monodentate complexes, protonated and nonprotonated, was also analyzed, but no significant improvement in the description of the results was observed. The surface complexation constants obtained with the CD-MUSIC modeling calculations are comparable to the values found in the literature for phosphate surface complexes in goethite particles.

Study of the acid-base properties of fulvic acid-like substances extracted from senescent leaves of eucalyptus and oak.

The acid-base properties of two fulvic acids (FA) extracted from senescent leaves of eucalyptus and oak were characterized by carrying out potentiometric titrations at two FA concentrations and four ionic strengths (0.005 M <[KNO(3)] <1.0 M). Experimental data were analyzed by means of the master curve approach, which includes an electrostatic spherical double layer model, and the Langmuir-Freundlich isotherm was used to fit the data. The contribution of the electrostatic effect to the proton binding reaction was lower than that observed for soil fulvic acids. The chemical heterogeneity of both samples was described by two acid sites with p Ks of about 4 and 7.5, the most abundant being the carboxylic site of p K = 4.

Analysis of the effect of pH on Cu2+-fulvic acid complexation using a simple electostatic model.

A simple electrostatic model was used to study the effect of pH on the binding of Cu2+ to fulvic acid in solutions containing similar amounts of dissolved organic carbon (DOC) as natural media, such as aquatic environments and soil solutions. Complexation behavior was affected by increased pH because of changes in the electrostatic interaction resulting from an increase in the negative charge on the fulvic acid molecule. Solutions of soil-extracted fulvic acid (FA), at concentrations of 25 and 35 mg L(-1), ionic strength 0.005 M, and pH 5.0, 5.5, 6.0, 6.5, 7.0, and 7.5, conditions that simulate those of natural freshwaters, were titrated with copper ion using differential pulse anodic stripping voltammetry. Assuming the formation of 1:1 complexes, the conditional binding parameters (stability constant and binding capacity) were calculated for each pH value. Use of a 1:1 electrostatic model allowed estimation of the contribution of the electrostatic effect to the ion binding reaction, at each pH value, as well as calculation of a binding constant that was not dependent on pH and which thus represented the contribution of the chemical heterogeneity. Furthermore, it was found that only a small proportion of the FA acid functional binding sites are involved in the formation of copper complexes.