tl tham khao

  • Published on
    29-Jun-2015

  • View
    381

  • Download
    3

Embed Size (px)

Transcript

2.1. Khi qut chung v KLN v nhim KLN trong mi trng t. Thut ng kim loi nng dng ch bt k nguyn t kim loi no c khi lng ring ln (d 5g/cm3) v th hin c tnh nng thp. Cc nguyn t KLN l thnh phn t nhin ca v tri t. Cc nguyn t ny khng th b thot bin hay ph hu. Mt lng nh cc nguyn t KLN ny i vo c th thng qua thc n, nc ung v khng kh. Mt vi nguyn t KLN ng vai tr nh cc nguyn t cn thit cho vic duy tr qu trnh trao i cht ca c th con ngi chng hn nh km (Zn), ng (Cu) v selen (Se). Tuy nhin nng cao chng vn c th gy c cho c th ngi v sinh vt [3]. Theo nh ngha ca t chc Y t th gii (WHO) th: nhim mi trng l s a vo mi trng cc cht thi nguy hi hoc nng lng n mc nh hng tiu cc n i sng sinh vt, sc kho con ngi hoc lm suy thoi cht lng mi trng [10].Nh vy, nhim mi trng t do KLN c xem l tt c cc hin tng nhim bn mi trng t bi cc KLN v hp cht ca chng, gy nh hng trc tip hoc gin tip n i sng ca sinh vt v con ngi. 2.2. Hin trng nhim mi trng t do KLN. t c th b bc mu, nhim bn, mt kh nng canh tc do nhng tp qun phn khoa hc, do cc hot ng nng nghip vi nhng phng thc canh tc khc nhau v do cc hot ng sn xut cng nghip thng qua thi cc cht thi rn, lng, kh khng hp l vo t. nhim mi trng t lin quan cht ch n s xut hin ca cht thi trong qu trnh sn xut. 2.2.1. Hin trng nhim mi trng t do KLN trn Th Gii. Cht lng mi trng ni chung, mi trng t ni ring ang c c th gii quan tm. Pht trin x hi phi i i vi bo v mi trng , ang v s l mc tiu chung ca mi quc gia. Mi nm th gii mt i 25 t tn t mt do b ra tri, xi mn. Khong 2 t ha t canh tc v t trng c trn Th Gii v ang b suy thoi do s dng t thiu khoa hc hoc khng c quy hoch. nhiu ni t b xi mn, sa mc ho, phn ho, mn ho khng cn kh nng canh tc. Trc sc p v gia tng dn s trn ton cu, tng sn lng lng thc p ng yu cu ngi nng dn lm dng phn bn ho hc, ho cht bo v thc vt tng nng sut cy trng l mt trong nhng nguyn nhn gy nhim t v nc. Ngoi

ra, s pht trin cng nghip, mng li giao thng v th ho lm cho t, nc, khng kh ni ring v mi trng ni chung ca chng ta b nhim KLN. Theo thng k ca cc t chc Mi Trng Th Gii, hng nm cc con sng ca Chu a ra bin khong 50% cht cn lng, c ti 70% trong s chy vo Thi Bnh Dng khng c x l. Hn 40% nhim trong khu vc bt ngun t cng nghip, nng nghip, sinh hot, th v giao thng vn ti. Tnh hnh nhim xy ra hu ht cc nc ang pht trin. Hn 90% cht thi, nc thi t cc nc ny c trc tip vo cc con sng, cnh ng m khng qua x l [1]. Ti n , nng cc cht nh Pb, Cd v Hg cao bt thng c pht hin nhiu lch sng Thane thuc b bin thnh ph Bom Bay, cc trm quan trc ngoi khi cng c bo co c cha Pb vi hm lng ng k. Pakistan, ngi ta cng pht hin thy nng ng k KLN trong nc v cc cn lng vng ven b khu vc sng Indus [24]. Ti Anh, hm lng Cd lp t mt xung quanh vng khai thc Zn ln ti 336 ppm. M, nhng vng t ln cn cc nh my ch bin kim loi, hm lng Cd t n con s khng l l 1500 ppm. Khi nghin cu hm lng KLN trong t Ria of Ortigueira, Ty Ban Nha, tc gi X. L. Otero v cng s (2000) [25], nhn thy hm lng Ni v Cr c bit cao trong tng t mt ca vng Esteiro (1930 mg/kg v 582 mg/kg) l do nh hng ca bn thi t hot ng khai thc ca m Sepentin gn . Hm lng Cu v Zn mc thp hn. Mt s vng thuc cc nc nh an Mch, Nht, Anh v Ailen c hm lng Pb trong t cao hn 100 ppm phn nh tnh trng nhim Pb. Trong khi hm lng Pb Alaska li kh thp ch khong 20 ppm trn lp t mt. Ngy nay vi tc pht trin mnh m ca cng nghip v hnh thnh nhiu thnh ph ln, vn nhim mi trng ngy cng nghim trng. Khi t cc nh my, t xe c gii thi ra lm nhim bu kh quyn. Nc thi t cc nh my, khu dn c th lm nhim ngun nc. Trong nng nghip s dng ngy mt nhiu cc ho cht bo v thc vt v mt s loi phn ho hc lm nhim nghim trng ngun ti nguyn t. Cc nguyn t KLN nh: Cu, Pb, Zn, Cd, Hg, Cr, As thng cha trong ph thi ca ngnh luyn kim mu, sn xut t. Khi nc thi cha 13 mg Cu/l, 10 mg Pb/l, 1 mg Zn/l gy nhim t nghim trng. Hm lng Cd trong t Thu S c th ln ti 3 mg/kg trong vng 20 30 nm ti. Tnh di ng gy c ca cc KLN cn ph thuc vo nhiu yu t nh:

s thay i in th oxy ho kh, pH, s lng mui v cc phc cht c kh nng ho tan nhng KLN trong t [24]. 2.2.2. Hin trng nhim mi trng t do KLN Vit Nam. a. nhim KLN do hot ng cng nghip v th ho. Ngun pht tn cc KLN gy nhim mi trng trc ht phi k n sn xut cng nghip nh cc ngnh cng nghip c s dng xt, clo, cng nghip s dng than v du m l cc ngun thi ch, thu ngn v cacdimi Trong khi , cc nguyn nhn tch ng KLN gy nhim mi trng mt phn l do tc ng trc tip t ngun thi, mt phn l do qu trnh qun l v x l cc ngun thi cha cht ch, khng c coi trng gin tip gy nhim mi trng. Chnh v vy m vic tm ra cc bin php phng v khc phc nhim KLN l vn rt cn thit. H Ni l th ln ca c nc, s tp trung dn s v cc hot ng cng nghip khin cho th l ni c nguy c nhim ln nht. Theo s liu iu tra nm 2001 ca Trung tm K thut Mi trng th v khu cng nghip H Ni, lng rc thi nguy hi ngun gc cng nghip ca H Ni dao ng t 13.000 tn/nm n 20.000 tn/nm. Trong khi lng cht thi c thnh phn cht d n mn l 2.272,95 tn (chim 18.80%), cht c c tn cao l 2.562,98 tn (chim 20,91%). Do hin nay thnh ph cha xy dng xong h thng x l v chn lp cht thi rn nguy hi nn phn ln ngun cht thi rn cng nghip u b chn ln vi cc loi cht thi khc ti bi rc Nam Sn (Sc Sn) v bi rc Ty M (T Lim). Cc bi cht thi ny u khng c h thng chng thm, k thut vn hnh khng m bo v khng c h thng x l nc rc dn n tnh trng nc rc c thi trc tip vo cc h thng thu vc xung quanh (ti bi rc Ty M nc rc c thi trc tip ra Sng Nhu), hoc ngm xung ngun nc ngm gy nh hng nghim trng n cht lng mi trng t v nc [15]. Theo Cng ty Mi trng th H Ni (URENCO) tnh trung bnh ni thnh H Ni thi ra khong 1.455 tn rc thi/ngy, khng k rc xy dng. Thc t Cng ty mi ch thu gom v vn chuyn c 1.200 tn rc/ngy (bng 82,5% tng lng rc thi). Lng rc cn li khng c thu gom, tn ng cc ao h, ng xm, knh mng, theo dng nc ma chy trn gy nhim t [8]. Cng vi H Ni, thnh ph H Ch Minh cng l a phng c nguy c nhim cao, theo iu tra ca tc gi Nguyn Ngc Qunh v cng s (2002) [14] thnh ph c hn 28.500 c s sn xut cng nghip v tiu

th cng nghip, phn ln cha c h thng x l nc thi. Nc thi t cc c s sn xut cha qua x l x trc tip qua cc knh rch, vo cc vng sn xut nng nghip, gy nhim mi trng t v ngun nc ti nng nghip. Kt qu phn tch hin trng nhim KLN khu vc pha Nam thnh ph H Ch Minh cho thy: hm lng Cu, Zn, Pb, Hg v Cr trong t trng la chu nh hng trc tip ca nc thi t cm cng nghip pha Nam thnh ph u tng ng hoc cao hn ngng cho php (TCVN 7209:2002) i vi t s dng cho mc ch nng nghip. Trong hm lng Cd dao ng t 2,1 23,5 ppm (vt qu TCCP); hm lng Cu t 9,2 55,4 ppm (tng ng v c du hiu vt ngng cho php); hm lng Zn t 70 353 ppm, gi tr cao nht ti im Bnh M l 353 ppm vt qu TCCP 1,76 ln; hm lng Pb t 14 85 ppm (vt qu TCCP 1,2 ln ti im Long Thi). Cc s liu ny chng t t y b nhim Cd (nh hng r n hm lng protein, amylaza, trng lng ht la) v c du hiu ca nhim Pb, Zn, Cr. Theo L Huy B [14], vng t trng la chu nh hng ca cm cng nghip pha Bc thnh ph H Ch Minh (Th c, Qun 2, Qun 9) c nguy c nhim Zn cao. C th, hm lng Zn Qun 2 l 162 390 ppm v Qun 9 l 356 679 ppm (vt TCVN 7209:2002 t 1,62 6,79 ln) [17]. Ti ng Bng Sng Cu Long, kt qu kho st hm lng Cd trong t la ca Nguyn Hu On v Ng Ngc Hng (2004) [13] cho thy: hm lng Cd dao ng t 0,01 0,56 ppm, cn thp hn TCCP ca t nng nghip Vit Nam rt nhiu. Tuy nhin trn nhm t ph sa ngt c s v canh tc cao hn so vi trn nhm t nhim mn (3 4 v/nm so vi 1 2 v/nm) th vic s dng phn ln cng nhiu hn, do lm tng lng Cd a vo trong t. Bn cnh , qu trnh canh tc s dng nhiu phn bn ho hc lm t chua, kt hp vi nguy c nhim mn cao ca vng ny s gia tng s hp th Cd vo cy trng v chui thc n. Nguyn Ngc Nng (2003) [12] cho rng, hm lng Cd dao ng t 0,46 1,05 ppm trong t Bc Cn v t 0,78 1,59 ppm trong t Thi Nguyn; hm lng Pb dao ng t 1,78 3,12 ppm vi t Bc Cn, t 1,97 4,45 ppm t Thi Nguyn; hm lng As dao ng t 1,25 2,98 ppm t Bc Cn v t 1,88 5,12 ppm Thi Nguyn. Hm lng cc nguyn t ny trong t cng ln i vi cc vng gn Th, khu Cng Nghip v khu dn c tp trung. Tuy hm lng cc nguyn t KLN cha vt qu TCCP nhng hm lng Cd, Pb, As li kh cao trong vi loi t vng ven thnh ph Thi Nguyn.

Cc tc gi V nh Tun v Phm Quang H (2004) [19] khi nghin cu hm lng KLN trong t trng rau Thanh Tr v T Lim cho thy: Cu t 21,88 n 53,88 ppm; Zn t 74,45 n 98,35 ppm; Pb t 19,53 n 34, 28 ppm; Cd t 0,03 n 0,70 ppm; As t 0,02 n 0,044 ppm. Tt c cc gi tr ny thp hn mc cho php v KLN i vi t nng nghip theo TCVN (7209:2002), c bit hm lng As rt thp. V Cr v Hg: Cr dao ng t 1,65 32,28 ppm; Hg dao ng t 0,01 0,05 ppm, cc gi tr ny u thp hn ngng cho php ca Canada (1997) (Theo ngng cho php ca Cr l 64ppm v Hg l 0,6 ppm). b. nhim KLN do hot ng nng nghip. Trong qu trnh sn xut nng nghip, con ngi lm tng ng k cc nguyn t KLN trong t. Cc loi thuc bo v thc vt thng c cha cc KLN nh: As, Pb, Hg. Cc loi phn bn ha hc c bit l do cc chngThiobacillus sp. m nhn. Thiobacillus ferrooxidans l loi duy nht c kh nng tng trng t dng trong qu trnh loi b st sulfua. Torma v Sakaguchi (1978) [26] thy rng tc oxy ha cc sulfua kim loi c tng theo tnh tan ca chng, th t nh sau: NiS > CoS > ZnS > CdS > CuS > Cu2S. Qu trnh metyl ha Pb, As, Hg l rt ph bin, c bit l Hg. S gii phng metyl Hg va c to thnh vo mi trng l c ch loi b c hi ca Hg i vi cc VSV. Bn cnh metyl Hg c s ph thuc ln vo pH, n ch tn ti bn vng trong gii hn pH hp (5,5 6,5); Ngc li metyl As li bn vng trong khong rng ca pH (3,5 7,5). Nhng nghin cu gn y cho rng qu trinh metyl ha xy ra mnh cc trm tch y vi s tham gia tch cc ca cc vi khun kh sulfat. Do vy s tch t ca SO42- s thc y qu trnh hnh thnh cc metyl Hg. S kh metyl ha cng c xut hin c trong iu kin thong kh cng nh him kh. mn ca t khng c kh nng ngn cn qu trnh metyl ha Hg. Cc hp cht hu c lm tng kh nng di ng ca kim loi c th c sinh ra t qu trnh trao i cht ca VSV. Vi khun v nm sn sinh cc loi axit hu c khc nhau. Vi khun c xu hng sinh ra nhiu axit hu c n gin nh : formic, axetic, propionic v butyric. Nm sinh nhng axit hu c khng bay hi nh: oxalic, fumaric, succinic, lactic, malnic (Burekhard v cng s, 1995; Fox, 1995) [15]. Ngoi ra, cc axit thm: ferunic, vanilic, syringic, phydroxybenzoic, p-hydrocynamic cng c sinh ra bi cc VSV (Burekhard v cng s, 1995) [15]. Nhng hp cht hu c ny dn ti nhng nh hng khc nhau i vi kh nng di ng ca kim loi.

Cc axit hu c phn t nh c