What is the effect of concentration on the rate of diffusion? (the rate of concentration of a substance not the concentration gradient)
I am a little uncertain about what you mean by the part of your question in parentheses, specifically 'the rate of concentration of a substance'. This seems to imply that you are measuring the rate at which a substance is becoming concentrated. This is still just diffusion because during diffusion a substance is moving from areas of high concentration to areas of low concentration, so by measuring the rate at which a substance is becoming concentrated in the area of low concentration, you are measuring the rate of diffusion.
Back to your question of how concentration affects the rate of diffusion. The higher the concentration of a substance in relation to its surroundings, the higher the rate of diffusion. For example, diffusion is constantly occurring between your cells and your blood stream, exchanging oxygen and carbon dioxide. During times of intense activity, the cells produce higher amounts of carbon dioxide so the concentration of carbon dioxide inside the cell is higher than normal. Diffusion of carbon dioxide from the cell to the blood stream will occur at a higher rate than when the concentration of carbon dioxide is at more normal levels.
I hope this helps!