Bring the water to a boil then add the sugar and salt to dissolve. Add the ice to chill and pour over the pork loin.
Refrigerate for 12 to 24 hours. Drain and place back in the dish.
Combine the water, lemon juice, lime juice, orange juice, sugar, salt, and onion. Also, include the citrus rinds after juicing. Heat over a high temperature, then boil for 10 minutes. Remove from the heat and remove the citrus rinds and onion pieces.
Add 8 cups of ice and let the mixture cool completely.
Place the loin in the brine and refrigerate for 4 to 6 hours.
Heat a smoker to 250 degrees F (121 degrees C).
Remove the pork loin from the brine, rinse, and pat dry.
Sprinkle the rub all over the loin and let sit for 30 minutes.
Cook the loin low and slow until an internal temperature of 145 degrees F (63 degrees C), 2 to 3 hours. Let rest 10 to 15 minutes before slicing.