R read.csv loop with iterating variable in quotation marks

This is my code:

library(data.table)
library(stringr)
parameters <- c("conductivity","calcium","chloride","magnesium","phosphate","potassium","salinity","sodium","sulphate")
for (i in parameters){
  i <- read.csv(str_c("./Data/Parameters/",i,".csv"), sep=",", header=FALSE)
  i <- unique(i)
  i <- subset(i, select=c(1,2,4,6))
  i <- setnames(i, c("site","date", str_c("",i,""), "material"))
  i[,3] <- as.numeric(i[,3])
  i <- subset(i, i > 0)
}

Now, there are two things that don't work here. The first is in the setnames function: it doesn't understand that it needs to label one of the columns of the CSV with the variable name. The second is that it doesn't actually call the imported files with 'conductivity', 'calcium', etc but simply calls them all 'i'. How can I fix this?