Remediation of heavy metal(loid)s contaminated soils - To mobilize or to immobilize?

Soil pH is often hypothesized to be a major factor regulating organic matter turnover and inorganic nitrogen production in agricultural soils. The aim of this study was to critically test the relationship between soil pH and rates of C and N cycling, and dissolved organic nitrogen (DON), in two long-term field experiments in which pH had been manipulated (Rothamsted silty clay loam, pH 3.5–6.8; Woburn sandy loam, pH 3.4–6.3). While alteration of pH for 37 years significantly affected crop production, it had no significant effect on total soil C and N or indigenous mineral N levels. This implies that at steady state, increased organic matter inputs to the soil are balanced by increased outputs of CO2. This is supported by the positive correlation between both plant productivity and intrinsic microbial respiration with soil pH. In addition, soil microbial biomass C and N, and nitrification were also significantly positively correlated with soil pH. Measurements of respiration following addition of urea and amino acids showed a significant decline in CO2 evolution with increasing soil acidity, whilst glucose mineralization showed no response to pH. In conclusion, it appears that changes in soil pH significantly affect soil microbial activity and the rate of soil C and N cycling. The evidence suggests that this response is partially indirect, being primarily linked to pH induced changes in net primary production and the availability of substrates. In addition, enhanced soil acidity may also act directly on the functioning of the microbial community itself.

PH regulation of carbon and nitrogen dynamics in two agricultural soils

Dissolved organic matter (DOM) is defined as the organic matter fraction in solution that passes through a 0.45 μm filter. Although DOM is ubiquitous in terrestrial and aquatic ecosystems, it represents only a small proportion of the total organic matter in soil. However, DOM, being the most mobile and actively cycling organic matter fraction, influences a spectrum of biogeochemical processes in the aquatic and terrestrial environments. Biological fixation of atmospheric CO 2 during photosynthesis by higher plants is the primary driver of global carbon cycle. A major portion of the carbon in organic matter in the aquatic environment is derived from the transport of carbon produced in the terrestrial environment. However, much of the terrestrially produced DOM is consumed by microbes, photo degraded, or adsorbed in soils and sediments as it passes to the ocean. The majority of DOM in terrestrial and aquatic environments is ultimately returned to atmosphere as CO 2 through microbial respiration, thereby renewing the atmospheric CO 2 reserve for photosynthesis. Dissolved organic matter plays a significant role in influencing the dynamics and interactions of nutrients and contaminants in soils and microbial functions, thereby serving as a sensitive indicator of shifts in ecological processes. This chapter aims to highlight knowledge on the production of DOM in soils under different management regimes, identify its sources and sinks, and integrate its dynamics with various soil processes. Understanding the significance of DOM in soil processes can enhance development of strategies to mitigate DOM-induced environmental impacts. This review encourages greater interactions between terrestrial and aquatic biogeochemists and ecologists, which is essential for unraveling the fundamental biogeochemical processes involved in the synthesis of DOM in terrestrial ecosystem, its subsequent transport to aquatic ecosystem, and its role in environmental sustainability, buffering of nutrients and pollutants (metal(loid)s and organics), and the net effect on the global carbon cycle.

Dissolved Organic Matter: Biogeochemistry, Dynamics, and Environmental Significance in Soils

The threat of heavy metal pollution to public health and wildlife has led to an increased interest in developing systems that can remove or neutralise its toxic effects in soil, sediments and wastewater. Unlike organic contaminants, which can be degraded to harmless chemical species, heavy metals cannot be destroyed. Remediating the pollution they cause can therefore only be envisioned as their immobilisation in a non-bioavailable form, or their re-speciation into less toxic forms. While these approaches do not solve the problem altogether, they do help to protect afflicted sites from noxious effects and isolate the contaminants as a contained and sometimes recyclable residue. This review outlines the most important bacterial phenotypes and properties that are (or could be) instrumental in heavy metal bioremediation, along with what is known of their genetic and biochemical background. A variety of instances are discussed in which valuable properties already present in certain strains can be combined or improved through state-of-the-art genetic engineering. In other cases, knowledge of metal-related reactions catalysed by some bacteria allows optimisation of the desired process by altering the physicochemical conditions of the contaminated area. The combination of genetic engineering of the bacterial catalysts with judicious eco-engineering of the polluted sites will be of paramount importance in future bioremediation strategies.

Exploiting the genetic and biochemical capacities of bacteria for the remediation of heavy metal pollution

“ No other single chemical soil characteristic is more important in determining the chemical environment of higher plants and soil microbes than the pH There are few reactions involving any component of the soil or of its biological inhabitants that are not sensitive to soil pH This sensitivity must be recognized in any soil-management system” “ Lime is truly a foundation for much of modern humid-region agriculture Knowing how pH is controlled, how it influences the supply and availability of essential plant nutrients as well as toxic elements, how it affects higher plants and human beings, and how it can be ameliorated is essential for the conservation and sustainable management of soils throughout the world” (Brady and Weil, 1999) Under areas where rainfall exceeds evapotranspiration, soil acidification is an ongoing natural process, which can either be accelerated by the activity of plants, animals and humans or can be impeded by careful management practices In areas affected by industrial activities, soil acidification is caused by acid drainage from pyrite oxidation and also from acid precipitation In areas that remain unaffected by industrial pollution, soil acidification in managed ecosystems is mainly caused by the release of protons (H + ) during the transformation and cycling of carbon (C), nitrogen (N) and sulfur (S) Just like in managed ecosystems, soil acidification in natural ecosystems caused by acid drainage and acid precipitation can have adverse impacts where soils have low pH buffering capacity Liming is the most common management practice aimed at neutralizing the acid produced, thereby overcoming the adverse impacts of soil acidification This review brings together fundamental aspects of soil acidification and recent developments on the implications of liming in relation to soil processes, particularly nutrient and heavy metal transformation and bioavailability in soils The article first outlines the various soil, plant and microbial processes that generate acid (protons; H + ions) both under natural and managed ecosystems It then discusses the effects of soil acidity on soil chemical and biological properties The effect of liming to overcome the problems associated with soil acidity is examined in relation to the transformation of nutrient ions and heavy metals The practical implications of liming to overcome heavy metal toxicity have been discussed in relation to the adsorption, leaching and phytoavailability of these metal ions Future research should aim to focus on the development of methods to quantify lime-enhanced (im)mobilization of nutrient ions and heavy metals in soils and to explore further the role of liming in remediating contaminated soils

Soil acidification and liming interactions with nutrientand heavy metal transformationand bioavailability

The chemical behaviour of metals is primarily governed by their retention and release reactions of solute with the soil matrix. Liming increased the soil pH, resulting in increased adsorption of Zn...

In situ immobilization of metals in contaminated or naturally metal-rich soils.

In situ immobilization of heavy metals in contaminated soils is a technique to improve soil quality Synthetic zeolites are potentially useful additives to bind heavy metals This study selected the most effective zeolite in cadmium and zinc binding out of six synthetic zeolites (mordenite-type, faujasite-type, zeolite X, zeolite P, and two zeolites A) and one natural zeolite (clinoptilolite) Zeolite A appeared to have the highest binding capacity between pH 5 and 65 and was stable above pH 55 The second objective of this study was to investigate the effects of zeolite addition on the dissolved organic matter (DOM) concentration Since zeolites increase soil pH and bind Ca, their application might lead to dispersion of organic matter In a batch experiment, the DOM concentration increased by a factor of 5 when the pH increased from 6 to 8 as a result of zeolite A addition A strong increase in DOM was also found in the leachate of soil columns, particularly in the beginning of the experiment This resulted in higher metal leaching caused by metal-DOM complexes In contrast, the free ionic concentration of Cd and Zn strongly decreased after the addition of zeolites, which might explain the reduction in metal uptake observed in plant growth experiments Pretreatment of zeolites with acid (to prevent a pH increase) or Ca (to coagulate organic matter) suppressed the dispersion of organic matter, but also decreased the metal binding capacity of the zeolites due to competition of protons or Ca

Metal immobilization in soils using synthetic zeolites.

Organic matter is an important component of soil with regard to the binding of contaminants. Hence, the partitioning of organic matter influences the partitioning of soil contaminants. The partitioning of organic matter is, among other factors, influenced by the ionic composition and ionic strength of the soil solution. This study focuses on the behavior of organic matter after a change in the ionic composition of the soil solution, particularly in Ca concentration and pH. Different amounts of Ca(NO3)2 and NaOH were added to soil suspensions. The dissolved organic carbon (DOC) concentration increased with increasing pH (addition of NaOH), whereas an increase in Ca (addition of Ca(NO3)2) had the opposite effect. A stronger increase in DOC was observed if a single dose of NaOH was added, compared to a gradual addition of the same amount of NaOH. Cation binding by organic matter in the supernatant was calculated using the NICA-Donnan model. The log DOC concentration appeared to be correlated to the Donnan po...

Solid-solution partitioning of organic matter in soils as influenced by an increase in pH or Ca concentration.

The free metal concentration in the soil solution is often considered a key parameter for metal uptake by and toxicity to soft-bodied soil organisms. The equilibrium partitioning theory, which assumes a relationship between the contaminant concentration in pore water and the contaminant concentration in the body tissue, can be used to describe uptake by earthworms. This theory has proved useful for organic chemicals, but its applicability is less clear for metals. In this study, the Cd concentration in soil pore water (pw) was varied by increasing the soil pH by the addition of lime (Ca(OH)2) and by adding manganese oxide (MnO2), which has a high metal binding capacity. Both lime (0.135% w/w) and MnO2 (1% w/w) decreased [Cd2+]pw by a factor of 25, while CdWorm was reduced only by a factor of 1.3 in lime-treated soils and 2.5 in MnO2-treated soils. Cadmium uptake was weakly related to the free metal concentration (R2adj = 0.66). Adding pH as an explanatory variable increased R2adj to 0.89, indicating that Cd uptake from pore water is pH dependent, which might be attributed to competition of protons and Cd at the surface of the earthworm body. However, previous earthworm experiments in reconstituted groundwater showed a conspicuously smaller pH dependency of Cd uptake. The differences in metal uptake between earthworms in lime- and MnO2-treated soils are therefore more likely to reflect the predominance of pH-independent intestinal uptake of Cd. Equilibrating the soil with a solution of 0.01 M CaCl2 and 0.1 M triethanolamine (buffered at pH 7.2), simulating the conditions prevailing in the worm intestine, yielded free Cd concentrations that were closely (R2adj = 0.83) and linearly related to the Cd concentration in the earthworm tissue.

Cadmium uptake by earthworms as related to the availability in the soil and the intestine

Free metal ions in aqueous and terrestrial systems strongly influence bioavailability and toxicity. Most analytical techniques determine the total metal concentration, including the metal ions boun...

L. Oste

Papers

In situ immobilization of metals in contaminated or naturally metal-rich soils.

Metal immobilization in soils using synthetic zeolites.

Solid-solution partitioning of organic matter in soils as influenced by an increase in pH or Ca concentration.

Cadmium uptake by earthworms as related to the availability in the soil and the intestine

Measuring and Modeling Zinc and Cadmium Binding by Humic Acid