**2.15. Ecotoxicological profile**

of T and O sheets. 1:1 (or T-O) clay minerals consist of a single T sheet linked to a single O sheet, and 2:1 (or T-O-T) clay minerals consist of a single O sheet sandwiched between two T

between two tetrahedral silica sheets. Two types of charges originate on the smectite clay particle: (1) permanent negative charges on the surface due to isomorphous cation substitu-

charges on the edges due to broken Si-O, Al-OH, and Mg-OH groups. At pH < Zero Point of Charge (ZPC), these edge charges become positive with anion exchange capacity, while at pH > ZPC, they become negative with a cation exchange capacity. Adapted with permission

*Clay structure and reactivity*. The reactivity of clays is largely a function of their swelling capacity. Kaolinite (of the 1:1 clay family) and talc and pyrophyllite (of the 2:1 clay family) possess no structural charges and consequently are non-swelling and of low adsorption capacity. The high-layer charge on vermiculite and illite restricts their swelling and gelling tendency although their surface area and CEC are relatively high. Smectites are characterized by their relatively low-layer charge which allow their particles to undergo complete dissociation in water and give them interesting rheological/gel-forming properties and surface reactivity. Halloysite is formed of hydrated 1:1 layers which role up into nanotubes (alumina sheet on the inside and silica sheet on the outside surface) and sepiolites (and palygorskite) are characterized by their inverted 2:1 ribbon structures. Such arrangements confer large SSA, porosity,

We focused on the investigation of the potential toxic effects caused by nanoclay particles currently used in tons in several industrial applications. Bentonite, montmorillonite, and

**Figure 12.** The structure of smectites. Clays are formed of layered tetrahedral (T) and octahedral (O) sheets.

or Al3+

for Mg2+ in Laponite) balanced by

in the interlayer gallery. (2) Positive (amphoteric)

) is sandwiched

In the case of smectites, an octahedral sheet of metal oxides (usually Mg2+

tion in the tetrahedral and/or octahedral sheets (e.g., Li<sup>+</sup>

180 Current Topics in the Utilization of Clay in Industrial and Medical Applications

and sorptive capacity. Adapted with permission [117].

exchangeable cations such as Na or Ca2+

[116]. Copyright 2014, John Wiley and Sons.

sheets (**Figure 12**, **Table 3**).

**2.14. Morphology**

No ecotoxicological studies of nanoclay have been identified as of November 30, 2010. Since clay is a naturally occurring material for which environmental organisms have adapted throughout evolution, the inherent toxicity is expected to be low; however, issues related to small particle sizes may occur [78].
