Dissolved organic matter as a confounding factor in the determination of pollution-induced community tolerance (PICT) of bacterial communities to heavy metals using the leucine incorporation method

PICT methodology using the leucine incorporation method (Leu-PICT) is useful for assessing heavy metal contamination in soils. First, bacterial community is exposed to metal in the soil (selection phase), developing tolerance if metal exerts toxicity. Secondly, in detection phase, bacterial suspensions are obtained, and tolerance is quantified by a second exposition of bacterial community to the metal using Leu-PICT methodology. However, during detection phase when Leu-PICT is performed, some characteristics of bacterial suspensions may change metal bioavailability. We assess the influence of dissolved organic matter (DOM) in bacterial suspensions, as humic acids (HA), on Leu-PICT determination to Cr, Cu, Ni, Pb and Zn. Results showed that the presence of HA in bacterial suspensions causes underestimations of bacterial community tolerance to Cr (increasing Cr toxicity), and overestimations of bacterial community tolerance to Cu, Ni, Pb, and Zn (reducing metal toxicity). In addition, the magnitude of these overestimations was different depending on the metal.Xunta de Galicia | Ref. ED401A-2020/084Ministerio de Economía y Competitividad | Ref. CTM2015-73422-JINFinanciado para publicación en acceso aberto: Universidade de Vigo/CISU

Assessment of polluted soil remediation using bacterial community tolerance to heavy metals as an indicator

The assessment of remediation on metal-polluted soils is usually focused on total and/or bioavailable metal content. However, these chemical variables do not provide direct information about reductions in heavy metals pressure on soil microorganisms. We propose the use of bacterial communities to evaluate the efficiency of three remediation techniques: crushed mussel shell (CMS) and pine bark (PB) as soil amendments and EDTA-washing. A soil sample was polluted with different doses of Cu, Ni, and Zn (separately). After 30 days of incubation, the remediation techniques were applied, and bacterial community tolerance to heavy metals determined. If bacterial communities develop tolerance, it is an indicator that the metal is exerting toxicity on them. Soil bacterial communities developed tolerance to Cu, Ni, and Zn in response to metal additions. After remediation, bacterial communities showed decreases in bacterial community tolerance to Cu, Ni, and Zn for all remediation techniques. For Cu and Ni, soil EDTA-washing showed the greatest reduction of bacterial community tolerance to Cu and Ni, respectively, while for Zn the soil amendment with PB was the most effective remediation technique. Thus, bacterial community tolerance to heavy metals successfully detect differences in the effectiveness of the three remediation techniques.Ministerio de Economía y Competitividad | Ref. CTM2015-73422-JINMinisterio de Economía, Industria y Competitividad | Ref. RYC-2016-20411Región de Murcia | Ref. 21525/EE/21Xunta de Galicia | Ref. ED401A-2020/08

Estimation of baseline levels of bacterial community tolerance to Cr, Ni, Pb, and Zn in unpolluted soils, a background for PICT (pollution-induced community tolerance) determination

The PICT method (pollution-induced community tolerance) can be used to assess whether changes in soil microbial response are due to heavy metal toxicity or not. Microbial community tolerance baseline levels can, however, also change due to variations in soil physicochemical properties. Thirty soil samples (0–20 cm), with geochemical baseline concentrations (GBCs) of heavy metals and from five different parent materials (granite, limestone, schist, amphibolite, and serpentine), were used to estimate baseline levels of bacterial community tolerance to Cr, Ni, Pb, and Zn using the leucine incorporation method. General equations (n = 30) were determined by multiple linear regression using general soil properties and parent material as binary variables, explaining 38% of the variance in log IC50 (concentration that inhibits 50% of bacterial growth) values for Zn, with 36% for Pb, 44% for Cr, and 68% for Ni. The use of individual equations for each parent material increased the explained variance for all heavy metals, but the presence of a low number of samples (n = 6) lead to low robustness. Generally, clay content and dissolved organic C (DOC) were the main variables explaining bacterial community tolerance for the tested heavy metals. Our results suggest that these equations may permit applying the PICT method with Zn and Pb when there are no reference soils, while more data are needed before using this concept for Ni and Cr.This study has been funded by the Spanish Ministry of Economy and Competitiveness through the project CTM2015-73422-JIN (FEDER Funds). David Fernández-Calviño holds a Ramón y Cajal contract (RYC-2016–20411) fnanced by the Spanish Ministry of Economy, Industry and Competitiveness. Claudia CampilloCora holds a predoctoral fellowship fnanced by Xunta de Galicia (ED481A-2020/084). Diego Soto-Gómez was supported by a postdoctoral fellowship from the Spanish Ministry of Education “Juan de la Cierva Formación (FJC2019-039176-I).

Distribuição do mercúrio total em função da dimensão dos agregados num podzol de floresta temperada

This study determined the distribution of total Hg (HgT) among aggregate size fractions in the A, E, Bh and Bs horizons of a representative temperate forest podzol. The aggregate distribution was dominated by the coarse sand size fraction (average of 55%) followed by fine sand (29%), fine silt (10%), coarse silt (4%) and clay (2%). In general, HgT mean values increased as the aggregate size become smaller: clay (170 ng g-1) > fine silt (130 ng g-1) > coarse silt (80 ng g-1) > fine sand (32 ng g-1) > coarse sand (14 ng g-1). Total Hg enrichment in clay-sized aggregates ranged from 2 to 11 times higher than the values shown by the bulk soil ( 50% of HgT in Bh and Bs horizons. The potential ecological risk index (PERIHg) increased as the aggregate size decreased, with the highest values in the illuvial horizons (45-903) and lowest in the E horizon (3-363). Heterogeneous distribution of Hg in the soil aggregate size fractions must be considered for Hg determination for purposes such as critical loads, background values or environmental risk indices. In addition, Hg accumulation in finer aggregates could be of concern due to its potential mobility in forest soils, either transferred by leaching to groundwater and freshwaters or mobilized by runoff in surface horizons.En este trabajo se analiza la distribución de Hg total (HgT) en fracciones de tamaño agregado en los horizontes A, E, Bh y Bs de un podzol forestal representativo. La distribución de agregados fue dominada por la fracción de tamaño arena gruesa (promedio del 55%), seguida arena fina (29%), limo fino (10%), limo grueso (4%) y arcilla (2%). En general, los valores medios de HgT incrementaron a medida que el tamaño de los agregados disminuía: arcilla (170 ng g-1) > limo fino (130 ng g-1) > limo grueso (80 ng g-1) > arena fina (32 ng g-1) > arena gruesa (14 ng g-1). El enriquecimiento de HgT en los agregados de tamaño arcilla varía entre 2 y 11 veces más que los niveles en la fracción tierra fina (< 2 mm). La acumulación de HgT en los agregados de menor tamaño estaba estrechamente asociada al C orgánico total, al C extraído con pirofosfato Na, a los complejos metal (Al, Fe)-humus y a los oxihidróxidos de Fe y Al. De hecho, estos parámetros variaron significativamente (p < 0,05) con el tamaño de agregado y sus valores más elevados se encontraron en las fracciones más finas. Esto sugiere el papel de estos compuestos del suelo en el incremento de la superficie específica por unidad de masa y de cargas negativas en los agregados más pequeños, favoreciendo la retención de Hg. Los valores del factor de acumulación de Hg (HgAF) fueron de hasta 10,8 en los agregados de tamaño arcilla, siendo cercanos a 1 en las fracciones de tamaño arena. Respecto de los factores de enriquecimiento de Hg (HgEF), estos fueron < 4 (categoría “contaminación moderada”) en la mayoría de los horizontes y tamaños de agregado. El índice de masa por tamaño de agregado (GSFHg) reveló que las fracciones más finas tenían una mayor carga de Hg que el correspondiente a sus masas, siendo destacable la contribución del limo fino que constituía más del 50% del HgT en los horizontes Bh y Bs. El índice de riesgo ecológico potencial (PERIHg) aumentó conforme disminuía el tamaño de agregado, con los valores más altos en los horizontes iluviales (45-903) y los más bajos en el horizonte E (3-363). La distribución heterogénea del Hg entre fracciones de tamaño agregado debe ser tenida en cuenta para la determinación de Hg para fines como cargas críticas, valores de fondo geoquímico o índices de riesgos medioambientales. Además, la acumulación de Hg en los agregados más finos podría ser preocupante debido a su potencial movilidad en suelos forestales, tanto mediante su transferencia por lixiviado a aguas freáticas y superficiales como su movilización por escorrentía en los horizontes superficiales.Neste estudo determinou-se a distribuição do Hg total (HgT) pelos agregado de diferentes frações nos horizontes A, E, Bh e Bs de um podzol representativo de floresta temperada. A distribuição dos agregados foi dominada pela fração areia grossa (média de 55%) seguida de areia fina (29%), limo fino (10%), limo grosseiro (4%) e argila (2%). Em geral, os valores médios da HgT aumentaram à medida que a dimensão do agregado diminuiu: argila (170 ng g-1) > limo fino (130 ng g-1) > limo grosseiro (80 ng g-1) > areia fina (32 ng g-1) > areia grosseira (14 ng g-1). O enriquecimento total de Hg em agregados da dimensão argila variou, sendo de 2 a 11 vezes maior do que os valores na fração terra fina (< 2 mm). A acumulação de HgT em agregados de menor dimensão estava intimamente relacionada com o C orgânico total, C extraído com pirofosfato de Na, aos complexos organo-metálicos (Al, Fe) e aos oxihidróxidos de Fe e Al. De facto, esses parâmetros variaram significativamente (p < 0,05) com o tamanho do agregado e os valores mais altos ocorreram nas frações mais finas. Isto sugere o papel destes compostos do solo no aumento da superfície específica e das cargas negativas nos agregados de menor dimensão, favorecendo a retenção do Hg. Os valores do fator de acumulação de Hg (HgAF) atingiram o valor de 10,8 nos agregados da dimensão argila, sendo próximos da unidade nas frações areia. Os fatores de enriquecimento do Hg (HgEF) apresentam valores < 4 (categoria "poluição moderada") na maioria dos horizontes e dimensão de agregados. O índice de massa por dimensão de agregado (GSFHg) revelou que as frações mais finas apresentam maior carga de Hg do que as suas frações de massa, sendo notável a contribuição do limo fino ao qual correspondiam valores > 50% de HgT nos horizontes Bh e Bs. O índice de risco ecológico potencial (PERIHg) aumentou com a diminuição do tamanho do agregado, com os valores mais altos nos horizontes iluviais (45-903) e mais baixos no horizonte E (3-363). A distribuição heterogénea de Hg nos agregados de várias dimensões deve ser considerada para determinação do Hg no solo com objetivos tais como cargas críticas, valores de fundo ou índices de risco ambiental. Além disso, a acumulação de Hg em agregados mais finos pode ser motivo de preocupação devido à sua potencial mobilidade em solos florestais, quer pela sua transferência por lixiviação para as águas subterrâneas e superficiais, quer por mobilização por escoamento superficial desorganizado em horizontes superficiais.This work was supported by the Consellería de Cultura, Educación e Ordenación Universitaria (Xunta de Galicia) with a Reference Competitive Groups grant (ED431C2017/62) to BV1 Research Group. Xunta de Galicia is acknowledged by the pre-doctoral fellowship of A. G. A. (ED481A-2016/220)S

Cu and As(V) Adsorption and Desorption on/from Different Soils and Bio-Adsorbents

This research is concerned with the adsorption and desorption of Cu and As(V) on/from different soils and by-products. Both contaminants may reach soils by the spreading of manure/slurries, wastewater, sewage sludge, or pesticides, and also due to pollution caused by mining and industrial activities. Different crop soils were sampled in A Limia (AL) and Sarria (S) (Galicia, NW Spain). Three low-cost by-products were selected to evaluate their bio-adsorbent potential: pine bark, oak ash, and mussel shell. The adsorption/desorption studies were carried out by means of batch-type experiments, adding increasing and individual concentrations of Cu and As(V). The fit of the adsorption data to the Langmuir, Freundlich, and Temkin models was assessed, with good results in some cases, but with high estimation errors in others. Cu retention was higher in soils with high organic matter and/or pH, reaching almost 100%, while the desorption was less than 15%. The As(V) adsorption percentage clearly decreased for higher As doses, especially in S soils, from 60–100% to 10–40%. The As(V) desorption was closely related to soil acidity, being higher for soils with higher pH values (S soils), in which up to 66% of the As(V) previously adsorbed can be desorbed. The three by-products showed high Cu adsorption, especially oak ash, which adsorbed all the Cu added in a rather irreversible manner. Oak ash also adsorbed a high amount of As(V) (>80%) in a rather non-reversible way, while mussel shell adsorbed between 7 and 33% of the added As(V), and pine bark adsorbed less than 12%, with both by-products reaching 35% desorption. Based on the adsorption and desorption data, oak ash performed as an excellent adsorbent for both Cu and As(V), a fact favored by its high pH and the presence of non-crystalline minerals and different oxides and carbonates. Overall, the results of this research can be relevant when designing strategies to prevent Cu and As(V) pollution affecting soils, waterbodies, and plants, and therefore have repercussions on public health and the environment.Spanish Ministry of Economy and CompetitivenessS

Oak leaves drop as atmospheric Hg deposition pahtway to a forest soil

Se estudia la contribución de la caída de hojas en la transferencia de Hg atmosférico hacia el suelo de un bosque dominado por Quercus robur en el NO de España durante 2015, 2016 y 2017. Las muestras de material senescente (litterfall) se recolectaron mensualmente, del cual se separaron las hojas. El flujo anual de caída de hojas varió entre 313 y 450 g m-2, con máximos en enero y diciembre. El flujo anual de Hg en 2015 fue el más alto (21.2 μg m-2) y estuvo notablemente influenciado por el flujo de biomasa foliar. En el suelo, la concentración de Hg en los horizontes Oi (promedio 49 n g-1) fue menor que en los horizontes Oe+Oa (promedio 72 ng g-1), consistente con la diferente humificación de la materia orgánica del suelo. En los horizontes Oe+Oa, el Hg almacenado equivaldría a su deposición a través de las hojas durante 5 años.Centro de Investigaciones Geológica

Oak leaves drop as atmospheric Hg deposition pahtway to a forest soil

Se estudia la contribución de la caída de hojas en la transferencia de Hg atmosférico hacia el suelo de un bosque dominado por Quercus robur en el NO de España durante 2015, 2016 y 2017. Las muestras de material senescente (litterfall) se recolectaron mensualmente, del cual se separaron las hojas. El flujo anual de caída de hojas varió entre 313 y 450 g m-2, con máximos en enero y diciembre. El flujo anual de Hg en 2015 fue el más alto (21.2 μg m-2) y estuvo notablemente influenciado por el flujo de biomasa foliar. En el suelo, la concentración de Hg en los horizontes Oi (promedio 49 n g-1) fue menor que en los horizontes Oe+Oa (promedio 72 ng g-1), consistente con la diferente humificación de la materia orgánica del suelo. En los horizontes Oe+Oa, el Hg almacenado equivaldría a su deposición a través de las hojas durante 5 años.Centro de Investigaciones Geológica

Oak leaves drop as atmospheric Hg deposition pahtway to a forest soil

Se estudia la contribución de la caída de hojas en la transferencia de Hg atmosférico hacia el suelo de un bosque dominado por Quercus robur en el NO de España durante 2015, 2016 y 2017. Las muestras de material senescente (litterfall) se recolectaron mensualmente, del cual se separaron las hojas. El flujo anual de caída de hojas varió entre 313 y 450 g m-2, con máximos en enero y diciembre. El flujo anual de Hg en 2015 fue el más alto (21.2 μg m-2) y estuvo notablemente influenciado por el flujo de biomasa foliar. En el suelo, la concentración de Hg en los horizontes Oi (promedio 49 n g-1) fue menor que en los horizontes Oe+Oa (promedio 72 ng g-1), consistente con la diferente humificación de la materia orgánica del suelo. En los horizontes Oe+Oa, el Hg almacenado equivaldría a su deposición a través de las hojas durante 5 años.Centro de Investigaciones Geológica

Specific Adsorption of Heavy Metals in Soils: Individual and Competitive Experiments

The partitioning between the soil solid phase and the soil solution determines the mobility of pollutants like heavy metals. If nonspecific sorption takes place, the reactions are easily reversible and heavy metals are released to soil solution increasing the probability of leaching through soil profile. Mobility and leaching are also favoured if other metals are in the system and competition for specific adsorption sites takes place. In this study, desorption equilibrium experiments were conducted after adsorption ones. The specific adsorption was evaluated through the amounts of the still adsorbed Cu, Pb, Cr, Ni and Zn after desorption experiments in ten different soils. In addition, competition adsorption and desorption binary experiments were conducted for evaluating the metal competition in three of the soils. Pb and Cu are the metals adsorbed and retained in higher amounts in all the studied soils. In slightly neutral soils, Cr is retained in lesser amounts while in acidic soils Zn is the metal less retained. Results showed that despite the high and variable amounts of organic matter in the soils, soil pH is the most important variable in neutral soils. In acidic soils, soil properties different than pH play important roles and specific sorption of Pb is related to the cationic exchange capacity of the soils while that of Zn to the clay content. Instead, the release of Cu during desorption experiments is probably due to the more soluble organic fraction of the soils. The individual retention of Cu, Zn, Ni and Pb is higher than when they are in competition, except if Cr is present. In this case, the amount of those four metals and that of Cr increased. Therefore, the presence of Cr together with cationic heavy metals favoured the adsorption of those metals in multi-metal polluted areas. Specific adsorption is also important during competition as soil affinities increase during competition experiments