Adsorption characteristics of Cu and Zn onto various size fractions of aggregates from red paddy soil

Highlights

• The clay-size aggregate showed faster adsorption rate and larger capacity.

• pH effect the adsorption and desorption of various aggregates at different levels.

• The clay-size aggregate may accumulate metals more easily in natural environment.

• Metal speciation in the clay-size aggregates appeared to be more stable.

Abstract

Soil aggregate is the basic structure unit of soils and the ability of various size fractions are different in the aspect of adsorbing and transferring heavy metals in the environment. In this study, bulk soil from red paddy field was partitioned into four aggregate-size fractions and their adsorption characteristics for Cu and Zn were studied. Our results showed that: Pseudo-second order model was more successful to fit the adsorption process in the kinetic experiments and the isothermal experiments data can be described well with the Freundlich model as a whole. Due to higher contents in organic matter, CEC and free iron oxide, the <0.002 mm fraction was found to have the highest initial sorption rate and maximum adsorption capacity. The adsorption amount of metals increased as the increasing of pH and the percentage of adsorbed metal susceptible to desorption into 0.01 M NaNO₃ was greater for Zn than for Cu, while their variation trends were quite opposite. More specific adsorption sites in the <0.002 mm fraction lead to more desorption amount for this particle size of soil at low pH condition. After 60 days of incubation, Cu and Zn were observed to enrich in the clay-size aggregates with fractions more stable than other particles.

Introduction

Heavy metal pollutants in the environment mainly come from anthropogenic industrial and agriculture activities, they can enter soil system in different ways such as atmospheric sedimentation, waste water irrigation and slag leaching etc. [1]. Past researches have shown that potential mobility of metals in soils was mainly determined by adsorption–desorption process [2]. Through adsorption, heavy metals only accumulate in top-soils in the case of relative low concentration, however, they will migrate to deep soil layers and further contaminate groundwater by rainwater leaching or irrigation when pollution level exceed a certain extent. Metals retained in the top-soils will not only be a potential hazard to plants, animals and micro-organisms, but finally pose a great threat to human health through the food chain [3], [4], [5].

As the basic structural unit of soil, aggregate has been long used to investigate the distribution and migration of organic matter and nutrients in soils [6], [7]. Due to different physical–chemical characteristics, the ability of various particle size aggregates in adsorbing foreign materials (metals, N, P, etc.) may be different and it is commonly believe that fine soil particles have a higher ability to carry heavy metals than coarser particles because of lager specific surface area and more organic matter, Fe/Mn/Al oxides contents [8], [9], [10], [11], [12]. Furthermore, fine soil fractions are often preferentially to transport to deep soil, surface/ground water and air [13], [14]. Therefore, it is very necessary to study the adsorption process between heavy metals and soil aggregates of different size fractions to evaluate associated environmental risk. Up to date, although some related studies have done [7], [15], [16], [17], most of the them were carried out with the subjects of urban soil [8], dust [18] and sediment [19], while arable soil has been neglected. In addition, these studies were mostly focused on the distribution of metals in polluted soils, adsorption properties and relative influence factors have not been evaluated.

The major factors controlling adsorption process include metal speciation and strength, soil type, pH, contact time and temperature, etc., among which pH is considered as the most significant environmental factor due to its strong effects on solubility and speciation of metals both in the soil as a whole and particularly in the soil solution [20]. Usually, the adsorption capacity of heavy metal is positive correlated to soil pH, in other words, low pH means metal ions are more likely to exist in soil solution and consequently more easily to migrate and uptake by plants [21]. In south China, overuse of fertilizer has contributed substantially to croplands soil acidification since the 1980s [22], meanwhile, part of the agricultural soil has suffered serious heavy metal pollution. This situation has result in over-standard concentrations of heavy metals in surface water, groundwater and crops [23], so it is make sense to study the effect of pH on the adsorption–desorption of metals with the objects of soil aggregates in this area.

In this paper, we investigated the adsorption characteristics of Cu and Zn onto various red paddy soil aggregates by kinetics, thermodynamics and pH effect experiments. Moreover, an incubation experiment was conducted to simulate heavy metal pollution in soil close to the natural conditions, and metal speciation in different size fractions was analyzed. By analyzing all the data from the experiments above, it is hope to gain an insight into the adsorption mechanism between soil and heavy metals and assess pollution risk of heavy metals in soil more reasonably.