Fig. 4.

Relationship between the total As removal and the pH. Panel (A) illustrates the trend and linear regression line for As removal as a function of the pH at day 9. The linear regression equation was obtained and the relationship between the As removal and pH can be represented by the linear equation %Asremoval = A – B•pH. The constants and the R-squared are: A = 355.3 %As, B = 42.4 %As, R² = 0.9973. Panel (B) shows the total As removal rate (mmolAs/(L•d)) as a function of the pH. The total As removal rate was calculated using the slope for the first 6 d at pH 6.1, and for the first 9 d at pH 6.5. At pH 7.2, two different As removal rates were observed, high rate, from day 0 to 9 (●), and a lower rate, from day 9 until the end of the experiment (◯). The rate of As removal is related to the pH by linear regression equations considering the higher rate at pH 7.2 (continuous line) and the lower rate at pH 7.2 (dashed line). The constants and the R-squared are: higher rate at pH 7.2, A = 0.4286 mmolAs/(L•d), B = 0.0566 mmolAs/(L•d), and R² = 0.9374; lower rate at pH 7.2, A = 0.5324 mmolAs/(L•d), B = 0.0734 mmolAs/(L•d), and R² = 0.9911.