Question

Problem 1. Silver Nanoparticle Fate and Toxicity Several recent efforts to understand the potential risks of nanotechnology have focused on silver- containing nanoparticles, which are being used in consumer products to inactivate bacteria. In this problem, you will assess the aquatic toxicity of commercial silver nanoparticles. For simplicity, you can ignore ionic strength in this problem. You are given two water samples A and B. You put in each water sample 1 g/L of silver nanoparticles and allow the systems to reach equilibrium. Assume that silver nanoparticles consist of a core of Agº with a coating of an Ag(I)-containing mineral. Sample PH 7.5 7.2 [Cl] (M) 0.4 0.003 Sample Description Estuarine water River water a. What is the predominant dissolved Ag(I) species after the nanoparticles are equilibrated with Sample A? b. Assume that the silver nanoparticles were coated with AgCl(s). What is the maximum concentration of dissolved Ag(I) you would expect to find in water from Sample A? c. Consider a sample of silver nanoparticles equilibrated in water sample B. Which of the three Ag(I)-containing minerals (i.e., AgOH(s), Ag2CO3(s) or AgCl)) would you expect to find on the surface of the nanoparticles? Assume closed system and C= 1 mM.

Answer 1

1.) The most abundant anions in estuarine water (river+sea water) are chloride and sulphates. Since silver chloride has a better solubility than silver sulphate, hence, the predominent species in the solution will be silver chloride. However, nitrates are even more soluble than chlorides, but they are in trace amount in the sample and hence, even if silver nitrate is formed, it will be in lower concentration as compared to silver chloride.

2.) Silver chloride has a limited solubility of 1.26 x 10-5 M at 25 °C in water and hence, the concentration of dissolved Ag (I) ion cannot exceed this value. However, increasing the temperature may increase its solubility.

3.) Silver carbonate is more soluble than silver chloride and the concentration of Chloride ion is very low in sample B. Silver hydroxide is not stable in water and decomposes to silver oxide. Hence, the silver nanoparticles will be better coated with silver chloride. Also, Ct=1 mM which is less than actual concentration of Cl- ion (0.003 M) in sample B. It means that chloride gets bound to the surface of silver nanoparticles as AgCl.