Heavy metals are elements that have a density ranging between 3.5 g/cm3 to above 7 g/cm3. These heavy metals have a long half-life and they are non-biodegradable. Membrane filtration, chemical precipitation, adsorption, electro-coagulation, chelation, inverse osmosis and ion exchange are some of the methods to remove heavy metals from raw water. However, the preferred methodology for the removal of heavy metals is adsorbtion owing to its high efficiency of removal of heavy metals.
Through an ASTM 18 stainless steel commercial ground coﬀee seeds was sieved. This was repeatedly leached with excess 0.01 M NaOH solution for a duration of 10-30 mins at a temperature of around 60◦C. Until the yellow colour of the solution disappeared, the NaOH was continuously decanted and was added again. In a 0.01M HNO3 the coffee ground seeds were suspended and then it was filtered and washed repeatedly with deionized water until a pH of 6.0 was achieved. If these are washed with just deionised water, the adsorbent continued to release soluble coffee and hence this complicated the process of adsorbtion. Finally it was dried at 100◦C for 2 h in an oven and cooled to room temperature and with an aid of a parafilm was sealed.
Known concentration of heavy metals such as Cu2+, Zn2+, Cd2+ and Pb2+ were added in a batch process to the suspension of spent ground coffee seeds which was dissolved in water. At room temperature this was equilibrated overnight with occasional stirring. The pH was adjusted to around 5 by adding NaOH or HCl as it was concluded by previous experiments, that this was the optimum pH at which the adsorbtion rate of the heavy metals by the spent coffee adsorbent was maximum. At pH below 4 the adsorbtion rate was very less and when the pH exceeded 10, the rate of adsorbtion once again started reducing. Buffers were not used to control the pH as they would compete as ligands for the metal cations. After equilibration, the suspensions were ﬁltered and the ﬁltrate obtained was analyzed for metal ions by the process of atomic absorption spectrometry.
The amount of heavy metal adsorbed was calculated by the diﬀerence obtained. The amount of metal ion adsorbed was fitted into the Langmuir isotherm.
1/X =1/bXmce +1/Xm
where X is the amount of metal ion adsorbed per g of adsorbent,
Xmis the maximum amount of metal ion that can be adsorbed per g of adsorbent,
Ceis the final equilibrium concentration of the metal ion in solution (M)
b is effectively the equilibrium constant for adsorption.
From the above equation 1/X was plotted against Ceand the values for b and Xm were calculated. It was found that at a concentration of 10mg/L and lesser of the coffee ground seeds the adsorbtion rate of the heavy metals was constant. As the concentration increased, the rate of adsorbtion increased. At around 100 mg/L maximum adsorbtive capacity of the coffee ground seeds exceeded the maximum. Coffee ground seeds in this way have proved to remove heavy metals such as lead copper etc.