Abstract
Within the UK, approximately 10 million tonnes of sludge dry solids are currently produced annually. Heavy metal accumulation in the soil-plant-animal system is a major concern in sewage sludge application on agricultural land. Remediation technologies that adopt 'in-situ' immobilisation of these heavy metals are best suited to sewage sludge and sludge-amended soils. Heavy metals found within different environmental media are believed to be immobilised by natural zeolites through ion exchange - thus providing one 'in-situ' remediation technology. This research has tested the effectiveness of using the natural zeolite 'clinoptilolite' in the remediation of heavy metal (Cu, Cd, Ni, Pb and Zn) contaminated sludge-amended soils. Moreover, through greenhouse trials the efficiency of clinoptilolite to immobilise heavy metals, and the impact of the exchangeable zeolitic cations (Ca, K, Mg and Na) on soil and plant health, were assessed. Clinoptilolite was found to contain predominantly Na and K exchangeable cations, with 10-15% impurities (quartz, cristobalite, etc) and low levels of heavy metals (< 25 mg/kg). Batch equilibrium studies showed that clinoptilolite could effectively remove Cd, Cu, Ni, Pb and Zn from synthetic aqueous metal solutions under controlled pH and contact time experiments. For pH 5, 5.5, 6 rapid heavy metal removal occurred within the first 10 minutes of contact with a 10 mg/l mixed metal solution. Clinoptilolite selectivity for different contact times was: Pb > Cu > Cd > Zn > Ni. These experiments also demonstrated Na and K release into the mixed metal solution as a function of contact time. Increasing the concentration of the mixed metal solution resulted in a change in metal selectivity in every case, highlighting the competition of heavy metals for zeolite exchange sites. Using sludge-amended soil (RFC) containing heavy metals at levels exceeding UK guidelines, geochemical fractionation studies showed a significant association of Cu, Zn and Ni (43-67%) with the residual fractions, with Pb and Cd mainly associated with the Fe and Mn oxide fraction (52 and 61% respectively). Interestingly, Cd was also significantly associated with the exchangeable fraction (29%). Non-sludge amended soil (RFU) containing heavy metal levels below stated UK guidelines showed all metals were predominantly associated with the residual fraction (50-100%). Anaerobically digested dewatered sludge (ADDS - OX) contained Cu and Zn above UK guideline levels with Ni, Cu, Zn and Cd predominantly associated with the residual fraction (54-100%), and Pb with the organic fraction (53%). Comprehensive greenhouse trials, using perennial ryegrass and white clover, were conducted with clinoptilolite addition to both sludge-amended soils (RFC and RFU/OX) and non-sludge amended soils (RFU and commercial peat-CP). Increasing zeolite additions to the sludge-amended soils did not significantly affect the heavy metal mobility or bioavailability of Cu, Zn and Pb. A reduction in soil Ni was observed for the sludge-amended soil (RFC) for both crops and a reduction in soil Cd for ryegrass only. This may be due to difficulties in measuring low levels of these heavy metals in the collected media. Zeolite additions significantly increased the concentration of both Na and K in all soils, suggesting possible ion exchange occurring in these soils with components other than those studied, e. g. anions, organic complexes, etc. Increasing zeolite addition to soils also resulted in plant morphological changes, such as, signs of wilting, yellowing of leaves. These plants were generally unhealthy compared with non-zeolite addition crops. This research confirms the potential negative environmental implications of using particular natural zeolites as 'in-situ' remediation materials. Thus Na-dominated zeolites should be treated with caution in the remediation of metal-contaminated media. However, there is some commercial potential in using zeolites (natural and conditioned) provided that the selected zeolite 'in-situ' with the contaminated media are considered in combination and are 'fit-for-purpose' in the long term.