With tremendous increase in development of nanotechnology, there is a developing enthusiasm towards the application of nanoparticles in diverse areas. Carbon nanotubes, fullerenes, quantum dots, dendrimers, iron oxide, silica, gold and silver nanoparticles are frequently used in different applications such as drug delivery, as ceramic materials, semiconductors, electronics, in medicine, cosmetics, etc. Some of these nanoparticles have shown major toxic effects on fauna, flora and human beings like inflammation, cytotoxicity, tissue ulceration and reduction of cell viability. SWCNT and MWCNT can induce oxidative stress and fibrosis in the lungs of rat and mice. SWCNTs can also induce oxidative stress to the nervous system in human beings. Inflammatory injury and respiratory distress can be observed due to TiO 2 nanoparticles with small diameter. Nanoparticles can also pose detrimental effects on plants such as decreased growth rate, genomic and proteomic changes, etc. Toxicity of nanoparticles arises because of their specific characteristics such as greater 'surface area to volume ratio' compared with bulk particles of the same chemistry. The objective of this review is to critically evaluate the current literature on the toxicity of nanoparticles.

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Summary This study demonstrates the application of an alternative numerical-simulation approach to effectively describe the flow field in a two-scale carbonate-matrix-acidizing model. The modified model accurately captures the dissolution regimes that occur during carbonate-matrix acidizing. Sensitivity tests were performed on the model to compare the output with experimental observations and previous two-scale models in the literature. A nonlinear reaction-kinetics model for alternative acidizing fluids is also introduced. In this work, the fluid-field flow is described by the Navier-Stokes momentum approach instead of Darcy's law or the Darcy-Brinkman approach used in previous two-scale models. The present model is implemented by means of a commercial computational-fluid-dynamics (CFD) package to solve the momentum, mass-conservation, and species-transport equations in Darcy scale. The software is combined with functions and routines written in the C programming language to solve the porosity-evolution equation, update the pore-scale parameters at every timestep in the simulation, and couple the Darcy and pore scales. The output from the model simulations is consistent with experimental observations, and the results from the sensitivity tests performed are in agreement with previously developed two-scale models with the Darcy approach. The simulations at very-high injection rates with this model require less computational time than models developed with the Darcy approach. The results from this model show that the optimal injection rate obtained in laboratory coreflood experiments cannot be directly translated for field applications because of the effect of flow geometry and medium dimensions on the wormholing process. The influence of the reaction order on the optimal injection rate and pore volumes (PVs) of acid required to reach breakthrough is also demonstrated by simulations run to test the applicability of the model for acids with nonlinear kinetics in reaction with calcite. The new model is computationally less expensive than previous models with the Darcy-Brinkman approach, and simulations at very-high injection rates with this model require less computational time than Darcy-based models. Furthermore, the possibility of extending the two-scale model for acid/calcite reactions with more-complex chemistry is shown by means of the introduction of nonlinear kinetics in the reaction equation.

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