Adsorção de cátions metálicos por espécies oxídicas do solo – Uma pequena abordagem teórica.

Por Carlos Pacheco
Discutindo com alguns colegas de trabalho sobre as formas de retenção de cátions metálicos em solos tipicamente “tropicais”, enriquecidos em óxidos férricos e de alumínio, percebi uma frequente “confusão” a respeito dos processos que cercam esse fenômeno. Sobretudo, essa confusão se aplicava às questões relacionadas à adsorção específica de cátions metálicos de elementos enquadrados como metais de transição na tabela periódica por óxidos, oxihidróxidos e hidróxidos componentes da fração argila de tais solos. Nunca é demais lembrar que tais elementos apresentam importância relevante em termos ambientais pois, vários deles, podem apresentar toxicidade acentuada aos seres vivos, representados por aqueles denominados de metais pesados. Outros, por sua vez, são micronutrientes importantes para grande parte dos vegetais. Esses fatos, por si só, já mostram a relevância do assunto. Resolvi então escrever algo sobre o assunto, com linguagem à medida do possível simples, mas com conteúdo suficiente para um blog científico.
A dúvida principal gira em torno de como cátions metálicos são adsorvidos em superfícies reconhecidamente positivas em baixos pHs. Aqui faz-se necessário lembrar que óxidos de ferro e alumínio (usarei essa designação genérica para óxidos, hidróxidos e oxihidróxidos) apresentam elevado Ponto de Carga Zero (PCZ). Esse por sua vez é o pH de equilíbrio entre cargas positivas e negativas. Abaixo do mesmo cargas positivas são predominantes e acima dele as negativas é que os são. Como a maior parte dos solos tropicais são extremamente intemperizados, lixiviados, há naturalmente um empobrecimento em bases e o pH tende a ácido. São comuns pHs abaixo de 6, indicando um pH abaixo do PCZ dos óxidos, que gira em torno de 7 a 9.
Fica evidente, então, o predomínio de cargas positivas na superfície desses minerais. O pensamento que logo vem à cabeça é que solos cuja fração argila apresenta-se enriquecida com óxidos apresentariam baixo potencial de retenção de cátions, afinal, positivo com positivo se repelem. Mas a história não é bem assim. Realmente esse fato se aplica àqueles metais alcalinos e alcalinos terrosos, mas não aos metais de transição. Dessa forma, Ca2+, Mg2+, Na+, K+, entre outros, são facilmente perdidos em tais solos, explicando, de certa forma, a baixa fertilidade predominante à medida que o intemperismo avança.
Em contrapartida, tem sido constatado que solos “oxídicos” apresentam grande capacidade de retenção de elementos-traço por mecanismos similares àqueles que os tornam grandes “drenos” de fosfatos. Em outras palavras, a interação não é puramente eletrostática (interação entre cargas), mas depende de diversos outros fatores abaixo citados, finalmente constituindo uma ligação química forte, tendendo à irreversibilidade, por vários autores citada como próxima à uma ligação covalente. É importante lembrar que, simplificadamente, ligações covalentes são caracterizadas por “compartilhamentos” de elétrons, não havendo a dependência tão evidente de cargas como nas ligações iônicas, por exemplo.

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The tiny soil charges that sustain humanity

Everybody knows, or rather should know, that soils sustain vegetation by providing not only physical support, but mainly by providing nutrients to plants. To build organic matter, besides carbon (which is provided by CO2 from the atmosphere), plants need inorganic nutrients, such as nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), iron (Fe) and some other elements taken up in smaller quantities. These elements originate in soil by the same process that forms soils: weathering.
Soils form from the chemical and physical weathering (decomposition) of rocks. When rocks weather physically, they simply break into smaller particles, without any mineralogical changes, but when rocks are chemically weathered, principally by CO2-rich water, newly formed (secondary) minerals appear by the chemical transformation of rock minerals or by the precipitation of dissolved elements. Some of these new minerals, generally belonging to the phyllosilicate mineral family, are very small and present peculiar features, such as exposed electrical charges.
Clays are the main bearers of charges in soil, although soil organic matter also exposes charges. The mineral nutrients in soils also have charges, for example, nitrogen is generally taken up by plants as NO3- (nitrate) or as NH4+ (ammonium). If these compounds are not held (adsorbed) by soil charges, they can be easily taken up by plants or washed away by infiltrating water from rainfall or irrigation. If all minerals could be washed (leached) by water, plants would “starve” and we would have no vegetation. That’s probably the most crucial importance of soil charges: they hold mineral nutrients and when soil solution is depleted of nutrients, they free (exchange) them, so that plants always have “food”. However, the supply of natural mineral nutrients is limited, and when too much is used or lost, man generally has to fertilize soils. Soil charges are also used to hold pollutants, like heavy metals and excess nitrate, and this is an important reason to maintain soils healthy.

Carbon sequestration by soils

Global warming is mainly caused by the increase of CO2 (carbon dioxide) concentration in the atmosphere, although other greenhouse gases also contribute to climate changes, like methane (CH4), which is even a more potent greenhouse gas than CO2. Both these gases originate from the burning of carbonaceous compounds, which may be fossil or modern in origin. This burning may be biological, in which case it is the product of respiration, or man made, as in the burning of oil or coal.
Most biological burning of carbon compounds comes from the decomposition of organic matter by microorganisms, such as bacteria and fungi, which act mostly upon oganic matter originated from plants. Some of this organic matter is highly resistant to microbial decomposition and ends up, after yet unclear biochemical changes, as soil organic matter, also known as humus or humic substances.
To prevent global climate changes, a number of measures have been suggested to slow or even stop CO2 increase in the atmosphere, from planting (and conserving) forests to fertilizing the ocean with iron to cause algal blooms. These measures would increase the carbon sequestered as plant biomass, since photosynthesis is basically the uptake of CO2 from the air to build plant tissues utilizing energy from the sun. What most people don’t know is that there is much more carbon stored as soil organic matter than as standing vegetation biomass.
There is possibly more carbon stored in some soils of the Amazon Rain Forest than in the forest itself! Current estimates for carbon stored in terrestrial vegetaion are 550 billion metric tons, while soils store at least 1200 billion metric tons, more than twice as much. What’s more, soil carbon storage is probably underestimated, because the carbon in deep layers of some tropical soils is generally not considered in soil carbon stocks estimates.
Carbon dioxide emitters from developed countries can balance their emissions by buying carbon credits from developing countries. Generally, these carbon credits are in the form of planting forests or paying to conserve them. Obviously, this is a good arrangement for both. But those who sell carbon credits may be losing money, as the planting of forests generally increase the carbon content in soils, which is not being considered. Soil organic matter is more stable than the organic matter in vegetation, so it’s more efficiently sequestered.
What some people do not understand about some tropical soils called Oxisols is that they are deep. These soils are the result of centuries of intense weathering, especially chemical weathering, under high temperatures and rainfall. Organisms also play an important role in the formation of these soils, notably plants exuding organic acids and other substances. The activity of mesofauna in these soils is impressive, and termites, earthworms and ants are very active in mixing soil material throughout the soil profile, making the Oxisols rather homogeneous vertically.
A large area of tropical rain forests, including Amazonia, is on Oxisols. Until recently, soil carbon stocks estimation only considered soil depth down to around one meter. It happens, though, that Oxisols may be much deeper than that and they can and do store organic carbon in deep subsurface horizons, which are more or less horizontal layers in the soil profile. In fact, subsurface carbon stocks in Oxisols may be twice the surficial stocks, as I found in my doctorate research.
Recent research has demonstrated that deep soil carbon is very stable, which is good when one considers carbon sequestration. Besides, this carbon is spacially separated from the agents that could decompose or mineralize it. These soils also have good natural physical characteristics, which can be maintained even after land use changes if they are properly managed, so greatly decreasing erosion risks. All this, in our view, make tropical Oxisols promising sinks for carbon sequestration and medium to long term storage in helping decrease atmospheric CO2 excess concentrations.

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