Penyisihan Pb dalam Air Limbah dengan Teknik Pertukaran Ion (Studi Kasus Air Limbah Pabrik Aki PT. GS Battery, Inc., Sunter-Jakarta Utara) = Removal of Pb in Wastewater by Ion Exchange Techniques (A Case Study on Wastewater of Battery Factories PT. GS Battery, Inc., Sunter, North Jakarta)