Influence of Zinc and Humic Acids on Dye Adsorption from Water by Two Composts.

Searching for alternative low-cost biosorbents for the removal of textile dyes from wastewater is currently an important subject of research. In this work, we have investigated how the presence of other contaminants in textile wastewaters can affect dye adsorption by biosorbents. We tested the adsorption of three dyes of different types: Basic Violet 10 (BV10), Acid Blue 113 (AB113) and Direct Blue 71 (DB71) by two different composts-municipal solid waste compost and pine bark compost-in the presence of Zn (5 mg L-1) or dissolved organic matter (100 mg humic acids L-1) in batch experiments. Dye adsorption capacity for both composts followed the following sequence: BV10 > AB113 > DB71. In general, dye sorption at the equilibrium was adequately described by the Freundlich model, but not always by the Langmuir model, which did not allow for the estimation of maximum retention capacities in all cases. In general, these were around 1 mg g-1 for DB71, 2 mg g-1 for AB113, and 40 mg g-1 for BV10. Municipal solid waste compost had slightly higher affinity than pine bark compost for the anionic dyes AB113 and DB71, whereas for the cationic dye BV10, pine bark compost presented a much higher adsorption capacity (41.7 mg g-1 versus 6.8 mg g-1). The presence of Zn or dissolved organic matter in the solutions at typical wastewater concentrations did not decrease the dye adsorption capacity of the composts. This result is positive both for the real application of composts to real textile wastewaters and for the validity of the results of biosorbent performance obtained with single-dye solutions.

Assessment of the Chronic Toxicity and Interactions between Arsenic and Riverbed Biofilms.

The toxic effect of exposure to arsenic, As(V), at concentrations of 0 to 30 mg L-1, for 49 days, on epipsammic biofilms, was evaluated in a microcosm experiment. The growth and composition of biofilms developed on sediments containing As concentrations of 31 mg kg-1 and 85 mg kg-1 were compared, using photosynthetic parameters and Live/Dead stains as end points. A toxic effect of arsenic could not be demonstrated; however, biofilm growth was higher over the sediment with higher arsenic concentrations, suggesting the development of pollution-induced community induced tolerance (PICT). Nevertheless, PICT was not observed after exposure to high arsenic concentration in the laboratory, as there were no differences in algal growth between the previous 0 and 30 mg L-1 systems exposed to new 30 mg As L-1 dissolution over 29 days. The algal composition was affected by the added arsenic, and brown algae were the most tolerant compared to green algae and cyanophyceae, as their percentage increased from 25 and 33% in the control samples to 57 and 47% in the samples with the highest added As concentration. In turn, the biofilm development influenced arsenic redistribution and speciation. Arsenic concentration in water decreased with time during the incubation experiment, retained by the sediment particles and the biofilm. In the biofilm, extracellular As was significantly higher (up to 11 times) than intracellular arsenic. As(V) was the predominant species in water and in the biofilm, but products of biotic transformation, namely As(III), DMA(V) and MMA(V), were also found in the solution and in the biofilm in some systems, demonstrating reduction and methylation by the organisms. As a conclusion, a toxic effect was not detected for the concentrations evaluated. Biofilms naturally exposed in the river system to high As concentrations acquire pollution-induced tolerance; however, tolerance was not acquired by exposure to 30 mg L-1 for 29 days in the laboratory.

Comparison of the sorption capacity of basic, acid, direct and reactive dyes by compost in batch conditions.

Research on biosorption of organic dyes is an important subject for the development of clean technologies for the treatment of textile wastewater. In this work, the process of sorption of four textile dyes of different natures, namely Basic Violet 10 (BV10), Acid Red 27 (AR27), Direct Blue 151 (DB151) and Reactive Violet 4 (RV4) onto two composts, pine bark compost and municipal solid waste compost, has been studied. For this, sorption kinetics and equilibrium sorption at different solution pH values (3.0-7.0) and salinity (0-1.0 M KCl) conditions have been assessed in batch experiments. Sorption rates were relatively slow for BV10, reaching equilibrium only after 24 h, and faster for the rest: around 5-6 h for RV4 and AR27 and 2 h for DB151. Kinetics of dye sorption followed a pseudo-first order model, except that of DB151, which was better described by a pseudo-second order model. The sequence of adsorption capacity for both composts was as follows: BV10 > DB151 > RV4 > AR27. In general, dye sorption at the equilibrium was adequately described by the Langmuir model, what allows to estimate maximum retention capacities for each dye by the composts. At the best removal conditions, pine bark compost presented maximum sorption capacities of 204 mg g-1 for BV10, 54 mg g-1 for DB151, 23 mg g-1 for RV4, and 4.1 mg g-1 for AR27, whereas municipal solid waste compost showed maximum sorption of 74 mg g-1 for DB151, 38 mg g-1 for RV4, 36 mg g-1 for BV10, and 1.6 mg g-1 for AR27. Sorption increased at acid pH in all cases, likely because of modification of charges of the dyes and higher electrostatic attraction, whereas increasing salinity also had a positive effect on sorption, attributed to a solute-aggregation mechanism in solution. In conclusion, organic waste-derived products, like composts, can be applied in the removal of colorants from wastewater, although they would be more effective for the removal of basic cationic dyes than other types, due to electrostatic interaction with mostly negatively-charged composts.

Utilization of Composts for Adsorption of Methylene Blue from Aqueous Solutions: Kinetics and Equilibrium Studies.

Utilization of composts as low-cost adsorbents is an important application in the field of environmental remediation, but these materials have not yet been extensively used for dye removal. In this work, we have studied the characteristics of adsorption of methylene blue onto two composts (a municipal solid waste compost and a pine bark compost). Kinetics and equilibrium batch experiments testing the influence of adsorbent particle size, solution pH and ionic strength were performed. Both composts have a high adsorption capacity for methylene blue, similar to other low-cost adsorbents. Kinetics of adsorption followed a pseudo-first-order model, with maximum adsorption reached after a contact time of two hours. Equilibrium adsorption followed a Langmuir model in general. Reduction of particle size only increased adsorption slightly for composted pine bark. Increase in ionic strength had no effect on adsorption by municipal solid waste compost, but increased adsorption by composted pine bark. Modification of pH between 5 and 7 did not influence adsorption in any case. Overall, the results suggest that electrostatic interaction between the cationic dye and the anionic functional groups in the composts is not the only mechanism involved in adsorption. In conclusion, the use of composts for dye removal is a likely application, in particular for those composts presenting limitations for agricultural use.

Heavy Metal Uptake of Lettuce and Ryegrass from Urban Waste Composts.

Interest in using urban waste composts as amendments in urban agriculture is growing nowadays. However, concerns about the potential transference of pollutants present in urban waste to the food chain are very relevant when they are recycled for food or animal feed production. Thus, for the safe use of urban waste composts, it has to be assured that no risk of metal transference to plants from compost exists. In this work, the transference of heavy metals from urban waste composts to plants has been studied in an experiment with lettuce and Italian ryegrass, grown in substrates based on five metal-rich composts and a manure vermicompost (included for comparison). A two-month pot experiment was performed under controlled light and temperature conditions, and plant growth and uptake of Cu, Pb, Cd and Zn were analyzed. For both species, the concentration of metals in plant tissue followed the sequence Zn > Cu >> Pb ≈ Cd, the same as the metal concentrations in four out of the five composts. Plant concentrations of Zn, Cu and Cd increased with their concentrations in compost, whereas this relation was not observed for Pb. The ratio between element concentration in plant and compost were much higher for Zn, Cd and Cu than for Pb, showing the lower bioavailability of Pb with respect to other metals.

Biotic and Abiotic Factors Influencing Arsenic Biogeochemistry and Toxicity in Fluvial Ecosystems: A Review.

This review is focused on the biogeochemistry of arsenic in freshwaters and, especially, on the key role that benthic microalgae and prokaryotic communities from biofilms play together in through speciation, distribution, and cycling. These microorganisms incorporate the dominant iAs (inorganic arsenic) form and may transform it to other arsenic forms through metabolic or detoxifying processes. These transformations have a big impact on the environmental behavior of arsenic because different chemical forms exhibit differences in mobility and toxicity. Moreover, exposure to toxicants may alter the physiology and structure of biofilms, leading to changes in ecosystem function and trophic relations. In this review we also explain how microorganisms (i.e., biofilms) can influence the effects of arsenic exposure on other key constituents of aquatic ecosystems such as fish. At the end, we present two real cases of fluvial systems with different origins of arsenic exposure (natural vs. anthropogenic) that have improved our comprehension of arsenic biogeochemistry and toxicity in freshwaters, the Pampean streams (Argentina) and the Anllóns River (Galicia, Spain). We finish with a briefly discussion of what we consider as future research needs on this topic. This work especially contributes to the general understanding of biofilms influencing arsenic biogeochemistry and highlights the strong impact of nutrient availability on arsenic toxicity for freshwater (micro) organisms.

Potential use of composts and vermicomposts as low-cost adsorbents for dye removal: an overlooked application.

The use of composts and vermicomposts as adsorbents is an important topic of study in the field of environmental remediation. These materials are rich in organic matter and have functional groups that can interact with organic and inorganic compounds. They also contain microorganisms that can promote biodegradation of organic substances. Composts that cannot be used for agronomic purposes (owing to e.g. low nutrient levels or phytotoxicity) may be valuable for soil remediation or pollutant removal. In this review, we discuss papers on this topic, with the objective of drawing attention to the potential use of composts/vermicomposts and to recommend further investigation on this subject. Few published studies have investigated the use of composts/vermicomposts to remove dyes and other coloured compounds. However, preliminary results show that these materials are potentially good adsorbents, at least comparable to other low-cost adsorbents, and that, in general, basic dyes are more efficiently removed than direct, reactive or acid dyes. The results of the works reviewed also show that dye removal takes place by adsorption mechanisms, in most studies following a Langmuir model, and that the kinetics of removal are fast and follow a pseudo-second order model. However, there remain several uncertainties regarding this application. For example, very few dyes have been studied so far, and little is known about the influence of the properties of composts/vermicomposts on the dye removal process. Moreover, the possible use of compost/vermicompost to enhance biodegradation processes has not been explored. All these questions should be addressed in future research.

Mutual interaction between arsenic and biofilm in a mining impacted river.

Gold mining activities in fluvial systems may cause arsenic (As) pollution, as is the case at the Anllóns River (Galicia, NW Spain), where high concentrations of arsenate (AsV) in surface sediments (up to 270 mg kg-1) were found. A 51 day-long biofilm-translocation experiment was performed in this river, moving some biofilm-colonized substrata from upstream (less As-polluted) to downstream the mine area (more As-polluted site), to explore the effect of As on benthic biofilms, as well as their role on As retention and speciation in the water-sediment interface. Eutrophic conditions (range: 0.07-0.38 mg L-1 total phosphorus, TP) were detected in water in both sites, while sediments were not considered P-polluted (below 600 mg kg-1). Dimethylarsenate (DMAV) was found intracellularly and in the river water, suggesting a detoxification process by biofilms. Since most As in sediments and water was AsV, the high amount of arsenite (AsIII) detected extracellularly may also confirm AsV reduction by biofilms. Furthermore, translocated biofilms accumulated more As and showed higher potential toxicity (higher As/P ratio). In concordance, their growth was reduced to half that observed in those non-translocated, became less nutritive (less nitrogen content), and with higher bacterial and dead diatom densities. Besides the high As exposure, other environmental conditions such as the higher riparian cover at the more As-polluted site could contribute to those effects. Our study provides new arguments to understand the contribution of microorganisms to the As biogeochemistry in freshwater environments.

Use of red mud (bauxite residue) for the retention of aqueous inorganic mercury(II).

The effectiveness of the oxide-rich residue from bauxite refining (red mud) to remove inorganic Hg(II) from aqueous solutions was assessed. The aspects studied comprised the kinetics of the process (t = 1 min-24 h), the effect of pH (3.5-11.5), the interacting effect between salt concentration (0.01-1 M NaNO3) and pH and the Hg(II) sorption isotherm. Hg leaching from spent red mud was evaluated using the toxicity characteristics leaching procedure (TCLP) method. The sorption of Hg(II) onto red mud was very fast, with most of Hg(II) (97.0-99.7%) being removed from 0.5-50 µM Hg solutions in few minutes. The kinetic process was best described by Ho's pseudo-second order equation, pointing to chemisorption as the rate controlling step. Hg(II) sorption efficiency was very high (% removal between 93.9 and 99.8%) within all the studied pH range (3.5-11.5) and added Hg concentrations (5 and 50 µM), being optimal at pH 5-8 and decreasing slightly at both lowest and highest pH. The effect of background electrolyte concentration suggests specific sorption as the main interaction mechanism between Hg(II) and red mud, but the increasing non-sorbed Hg concentrations at low and high pH for higher electrolyte concentrations also revealed the contribution of an electrostatic component to the process. The sorption isotherm showed the characteristic shape of high affinity sorbents, and it was better described by the Redlich-Peterson and Freundlich equations, which are models that assume sorbent heterogeneity and involvement of more than one mechanism. The estimated Hg(II) sorption capacity from the Langmuir equation (q m ~9 mmol/kg) was comparable to those of some inorganic commercial sorbents but lower than most bio- or specifically designed sorbents. The leachability of retained Hg(II) from spent red mud (0.02, 0.25 and 2.42 mmol Hg/kg sorbed concentration) was low (0.28, 1.15 and 2.23 µmol/kg, respectively) and accounted for 1.2, 0.5 and 0.1% of previously sorbed Hg, indicating that Hg(II) is tightly bound by red mud once sorbed.

Acute toxicity of arsenic to Aliivibrio fischeri (Microtox bioassay) as influenced by potential competitive-protective agents.

In this study, we investigated the effect of some potential alleviative compounds against the acute toxicity of arsenic (As(V), As(III) and DMA(V)) on Aliivibrio fischeri (formerly Vibrio fischeri), a bioluminescent model bacterium, through the Microtox bioassay. The compounds studied differed in their mechanism of action, and they included the following: phosphate and glycerol, as chemical analogues (and potential competitors) of As(V) or As(III), respectively; citrate, a weak natural organic ligand; and the antioxidant ascorbic acid. Special attention was paid to phosphate effects, a widespread pollutant in natural environments. As(V) was found to be more acutely toxic than As(III) to A. fischeri, in accordance with its higher interaction with the bacteria. Both As(V) and As(III) were found to be much more acutely toxic than DMA(V), which was essentially non-acutely toxic even at very high concentrations. Phosphate presence (at equimolar P/As ratios or higher) resulted in the almost total suppression of bioluminescence inhibition, suggesting it exerts an alleviative effect against As(V) acute toxicity on A. fischeri. Interestingly, the uptake and the percentage of extracellular As(V) were not affected by the addition of phosphate, suggesting that such protective effect does not result from the competition for their common transporters. In contrast, the acute toxicity of As(III) was essentially unaffected by phosphate. Glycerol did not decrease the acute toxicity or the uptake of As(III) by A. fischeri, denoting the likely occurrence of an additional mechanism for As(III) uptake in such bacteria. Similarly, citrate and ascorbic acid essentially did not caused alleviation of As(V) or As(III) acute toxicity. As for environmental and operational implications, P could beneficially protect aquatic microorganisms against acute detrimental effects of As(V), whilst its presence could mask the toxicity due to As(V) when assessed using the Microtox bioassay, thus leading to seriously underestimate the actual ecological and health risks.

Fractionation and mobility of metals in bauxite red mud.

Red mud (RM) is a strongly alkaline residue generated in enormous amounts worldwide from bauxite refining using the Bayer chemical process. RM is composed mainly of Fe, Ti and Al oxides and hydroxides, but it also contains an array of trace metals and metalloids at different concentrations. The purpose of this paper is to assess the potential mobility of metals in RM, with special emphasis on pH effect. The 'operational' distribution and leachability of metals within/from RM was studied by applying a sequential extraction procedure (SEP) and several leaching tests (rapid titration, equilibration acidification, batch leaching with acetic acid and also the toxicity characteristics leaching procedure (TCLP) and the DIN 38414-S4 procedures, used as reference methods) carried out at different pH, solid/liquid ratio, extraction period and type of acid (HCl or acetic acid). Chemical analysis showed that, in addition to the major metals Fe, Al and Ti, RM contains several trace metals, some of them (Cr, Cu and Ni) in concentrations exceeding the regulatory limits. SEP showed that a majority of the metals in the RM (between the 32.2±8.5 for Cd and 95.3±0.4% for Ni) were found in the residual fraction, suggesting that they are not readily mobile under normal environmental conditions. Leaching tests performed at different pH showed that a significant fraction of the metals is mobilised from RM only under very strong acid conditions (pH<2), whereas Al is released in considerable amounts at pH<5.3. Among the trace metals, Cr requires special attention because of its relative high concentration in RM and the higher concentrations of this metal mobilised at low pH. The leaching tests using acetic acid showed that the standard TCLP largely underestimates the release of trace metals from RM, and therefore it is not advisable to evaluate the actual potential leaching of trace metals from this residue.

Evolution of organic matter during the mesophilic composting of lignocellulosic winery wastes.

Winery wastes were composted in the laboratory during five months in order to study the composting process of lignocellulosic wastes. In a first experiment, spent grape marc was composted alone, and in a second one, hydrolyzed grape marc, which is the residue generated after the acid hydrolysis of spent grape marc for biotechnological purposes, was composted together with vinification lees. During the composting of spent grape marc, total organic matter did not change, and as total N increased only slightly (from 1.7% to 1.9%), the reduction in the C/N ratio was very low (from 31 to 28). The mixture of hydrolyzed grape marc and lees showed bigger changes, reaching a C/N ratio around 20 from the third month on. Water-soluble organic matter followed the usual trend during composting, showing a progressive decrease in both experiments. Although the mixture of hydrolyzed grape marc and lees presented the highest initial water-soluble carbon concentrations, the final values for both experiments were similar (8.1 g kg(-1) for the spent grape marc, and 9.1 g kg(-1) for the mixture). The analysis of the humification parameters did not allow an adequate description of the composting process, maybe as a consequence of the inherent problems existing with alkaline extractions. The total humic substances, which usually increase during composting as a consequence of the humification process, followed no trend, and they were even reduced with respect to the initial values. Notwithstanding, the fractionation of organic matter into cellulose, hemicellulose and lignin enabled a better monitoring of the waste decomposition. Cellulose and hemicellulose were degraded mainly during the first three months of composting, and the progressive reduction of the cellulose/lignin ratio proved that the main evolution of these wastes took place during the first three months of composting.

Plant tests for determining the suitability of grape marc composts as components of plant growth media.

Five grape marc composts prepared by different procedures (composting and vermicompost at several scales) were tested as potential components of plant growth media. The five composts had high organic matter content (>90%), low electric conductivity (<1 dS m(-1)) and a pH between 7 and 8. Different chemical and biochemical analyses performed indicated the higher stability of those composts submitted to a longer composting process or to a vermicomposting process (lower water soluble organic matter, respiration and dehydrogenase activity). In order to determine the suitability of the composts as substrate components, plant growth tests were performed by blending the composts with peat or commercial substrate at two compost rates (25% and 50%). The mixtures were sown with barley (Hordeum vulgare L.) and grown under controlled conditions in an incubation chamber. No prejudicial effects derived from the use of composted grape marc were observed, whatever the procedure of composting used. The results showed that four out of the five composts would be suitable for use in plant growth substrates elaboration, as they did not reduce productivity with respect to the control substrates.

Ex situ treatment of hydrocarbon-contaminated soil using biosurfactants from Lactobacillus pentosus.

The utilization of biosurfactants for the bioremediation of contaminated soil is not yet well established, because of the high production cost of biosurfactants. Consequently, it is interesting to look for new biosurfactants that can be produced at a large scale, and it can be employed for the bioremediation of contaminated sites. In this work, biosurfactants from Lactobacillus pentosus growing in hemicellulosic sugars solutions, with a similar composition of sugars found in trimming vine shoot hydrolysates, were employed in the bioremediation of soil contaminated with octane. It was observed that the presence of biosurfactant from L. pentosus accelerated the biodegradation of octane in soil. After 15 days of treatment, biosurfactants from L. pentosus reduced the concentration of octane in the soil to 58.6 and 62.8%, for soil charged with 700 and 70,000 mg/kg of hydrocarbon, respectively, whereas after 30 days of treatment, 76% of octane in soil was biodegraded in both cases. In the absence of biosurfactant and after 15 days of incubation, only 1.2 and 24% of octane was biodegraded in soil charged with 700 and 70,000 mg/kg of octane, respectively. Thus, the use of biosurfactants from L. pentosus, as part of a well-designed bioremediation process, can provide mechanisms to mobilize the target contaminants from the soil surface to make them more available to the microbial population.

Carbon and nitrogen mineralization in a vineyard soil amended with grape marc vermicompost.

Vineyard soils in many areas suffer from low organic matter contents, which can be the cause of negative effects such as increasing the risk of erosion, so the use of organic amendments must be considered a good agricultural practice. Even more, if grape marc is recycled as a soil amendment in the vineyards, benefits from a good waste management strategy are also obtained. In the present study, a grape marc from the wine region of Valdeorras (north-west Spain) was used for the production of vermicompost, and this added to a vineyard soil of the same area in a laboratory study. Mixtures of soil and grape marc vermicompost (2 and 4%, dry weight) were incubated for ten weeks at 25°C and the mineralization of C and N studied. The respiration data were fitted to a first-order kinetic model. The rates of grape marc vermicompost which should be added to the vineyard soil in order to maintain the initial levels of organic matter were estimated from the laboratory data, and found to be 1.7 t ha(-1) year(-1) of bulk vermicompost (if the present mean temperature is considered) and 2.1 t ha(-1) year(-1) of bulk vermicompost (if a 2°C increment in temperature is considered), amounts which could be obtained recycling the grape marc produced in the exploitation.

Distribution and availability of trace elements in municipal solid waste composts.

Trace element contamination is one of the main problems linked to the quality of compost, especially when it is produced from urban wastes, which can lead to high levels of some potentially toxic elements such as Cu, Pb or Zn. In this work, the distribution and bioavailability of five elements (Cu, Zn, Pb, Cr and Ni) were studied in five Spanish composts obtained from different feedstocks (municipal solid waste, garden trimmings, sewage sludge and mixed manure). The five composts showed high total concentrations of these elements, which in some cases limited their commercialization due to legal imperatives. First, a physical fractionation of the composts was performed, and the five elements were determined in each size fraction. Their availability was assessed by several methods of extraction (water, CaCl(2)-DTPA, the PBET extract, the TCLP extract, and sodium pyrophosphate), and their chemical distribution was assessed using the BCR sequential extraction procedure. The results showed that the finer fractions were enriched with the elements studied, and that Cu, Pb and Zn were the most potentially problematic ones, due to both their high total concentrations and availability. The partition into the BCR fractions was different for each element, but the differences between composts were scarce. Pb was evenly distributed among the four fractions defined in the BCR (soluble, oxidizable, reducible and residual); Cu was mainly found in the oxidizable fraction, linked to organic matter, and Zn was mainly associated to the reducible fraction (iron oxides), while Ni and Cr were mainly present almost exclusively in the residual fraction. It was not possible to establish a univocal relation between trace elements availability and their BCR fractionation. Given the differences existing for the availability and distribution of these elements, which not always were related to their total concentrations, we think that legal limits should consider availability, in order to achieve a more realistic assessment of the risks linked to compost use.

Trace elements in compost regulation: the case of Spain.

We use the example of the Spanish Decree on Fertilizers (2005) to discuss the need of new horizontal regulations for assessing the quality of all the materials reaching the soil as amendments of fertilizers, in particular in what concerns potentially toxic trace elements in compost. Here it is proposed that the new regulations take into account the following: establishing maximum legal limits for the total loads for each element when compost is added to agricultural soils; establishing the maximum total concentration and bioavailability of each element in the final compost; and that they take into account the soil characteristics for establishing those limits.

Reduction of water repellence of hydrophobic plant substrates using biosurfactant produced from hydrolyzed grape marc.

This work demonstrates that the biosurfactant produced by Lactobacillus pentosus from grape marc hydrolysates can be successfully employed in reducing the water repellence of hydrophobic substrates, rather than chemical surfactants, as it can be produced from low-cost residual materials and it is less toxic than chemical surfactants. The method employed to measure the water repellence of the 11 plant substrates, consisting of pine bark, peat, and composts from various origins (biodegradable fraction of municipal solid waste, green waste, sewage sludge, manure, pine bark, and grape marc), was the molarity of ethanol droplet method (MED). Peat, pine bark, and the composts obtained from grape marc and pine bark were severely hydrophobic, having contact angles over 104 degrees , whereas the composts from municipal solid waste were less hydrophobic, with contact angles under 101 degrees . When hydrophobic substrates were treated with the biosurfactant from L. pentosus, the water repellence of the plant substrates was reduced in all but two cases (the least hydrophobic composts), achieving in most of the cases results better than those obtained using chemical surfactants.

Magnetic susceptibility as an indicator of heavy metal contamination in compost.

One of the main restrictions to the agronomic use of compost is the excess of heavy metals, which are often present due to inadequate separation of biodegradable fractions from non-degradable or inert materials. Magnetic susceptibility (MS) measurements are a simple technique that has been reported as a useful tool for assessing anthropogenic pollution, especially heavy metal pollution on soil and sediment samples. The close relationship of MS with heavy metal contamination has been proved by combined analyses of chemical and magnetic data. In this study, the MS and total heavy metal concentrations of eight composts from different origins were determined; all composts were passed under a magnet to remove the magnetic material, and total heavy metals were determined again. In our work, high correlations were found between magnetic susceptibility and total Cd, Zn, Pb, Cr and Ni, thus confirming the applicability of MS measurement as a proxy for heavy metal contamination in compost quality assessments. The application of a magnet over the composts reduced the MS as well as the heavy metal content, the reduction of Fe and MS being the most significantly correlated. Thus, the inclusion of an additional magnetic separation step in the post-process compost finishing could be envisaged.

Properties of slate mining wastes incubated with grape marc compost under laboratory conditions.

The effect of the addition of spent grape marc compost (GMC) and vermicompost (GMV) as amendments to slate mining wastes was evaluated in a laboratory incubation experiment. Mixtures of slate processing fines (SPF), with three doses of each amendment (4%, 8% and 16% compost, dry weight), plus a control were incubated at 25 degrees C in the laboratory for 90 days. The changes in the chemical and biological properties of the mixtures (pH, total C, total N, inorganic N, available nutrients, microbial biomass carbon and dehydrogenase activity) were investigated during the incubation period, and once it was finished, the phytotoxicity of the mixtures was determined by the germination of Lolium multiflorum Lam. seeds. The addition of the amendments significantly increased the nutrient concentrations of the SPF and enhanced biological activity by increasing microbial biomass and enzymatic activity. Results improved with higher doses; within the composts, GMV showed a better performance than GMC. These results prove the suitability of grape marc-derived amendments for the biochemical amelioration of mining wastes, and highlight the benefits of organic amendment in restoration projects.